Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

This is the documentation for a snapshot of the develop branch, built from commit ba911e1383.

Introduction

Smart pointers are objects which store pointers to dynamically allocated (heap) objects. They behave much like built-in C++ pointers except that they automatically delete the object pointed to at the appropriate time. Smart pointers are particularly useful in the face of exceptions as they ensure proper destruction of dynamically allocated objects. They can also be used to keep track of dynamically allocated objects shared by multiple owners.

Conceptually, smart pointers are seen as owning the object pointed to, and thus responsible for deletion of the object when it is no longer needed. As such, they are examples of the "resource acquisition is initialization" idiom described in Bjarne Stroustrup’s "The C++ Programming Language", 3rd edition, Section 14.4, Resource Management.

This library provides six smart pointer class templates:

  • scoped_ptr, used to contain ownership of a dynamically allocated object to the current scope;

  • scoped_array, which provides scoped ownership for a dynamically allocated array;

  • shared_ptr, a versatile tool for managing shared ownership of an object or array;

  • weak_ptr, a non-owning observer to a shared_ptr-managed object that can be promoted temporarily to shared_ptr;

  • intrusive_ptr, a pointer to objects with an embedded reference count;

  • local_shared_ptr, providing shared ownership within a single thread.

shared_ptr and weak_ptr are part of the C++ standard since its 2011 iteration.

In addition, the library contains the following supporting utility functions and classes:

As a general rule, the destructor or operator delete for an object managed by pointers in the library are not allowed to throw exceptions.

scoped_ptr: Scoped Object Ownership

Description

The scoped_ptr class template stores a pointer to a dynamically allocated object. (Dynamically allocated objects are allocated with the C++ new expression.) The object pointed to is guaranteed to be deleted, either on destruction of the scoped_ptr, or via an explicit reset. See the example.

scoped_ptr is a simple solution for simple needs. It supplies a basic "resource acquisition is initialization" facility, without shared-ownership or transfer-of-ownership semantics. Both its name and enforcement of semantics (by being noncopyable) signal its intent to retain ownership solely within the current scope. Because it is noncopyable, it is safer than shared_ptr for pointers which should not be copied.

Because scoped_ptr is simple, in its usual implementation every operation is as fast as for a built-in pointer and it has no more space overhead that a built-in pointer.

scoped_ptr cannot be used in C++ Standard Library containers. Use shared_ptr or std::unique_ptr if you need a smart pointer that can.

scoped_ptr cannot correctly hold a pointer to a dynamically allocated array. See scoped_array for that usage.

The class template is parameterized on T, the type of the object pointed to. Destroying T must not thow exceptions, and T must be complete at the point scoped_ptr<T>::~scoped_ptr is instantiated.

Synopsis

scoped_ptr is defined in <boost/smart_ptr/scoped_ptr.hpp>.

namespace boost {

  template<class T> class scoped_ptr {
  private:

    scoped_ptr(scoped_ptr const&);
    scoped_ptr& operator=(scoped_ptr const&);

    void operator==(scoped_ptr const&) const;
    void operator!=(scoped_ptr const&) const;

  public:

    typedef T element_type;

    explicit scoped_ptr(T * p = 0) noexcept;
    ~scoped_ptr() noexcept;

    void reset(T * p = 0) noexcept;

    T & operator*() const noexcept;
    T * operator->() const noexcept;
    T * get() const noexcept;

    explicit operator bool() const noexcept;

    void swap(scoped_ptr & b) noexcept;
  };

  template<class T> void swap(scoped_ptr<T> & a, scoped_ptr<T> & b) noexcept;

  template<class T>
    bool operator==( scoped_ptr<T> const & p, std::nullptr_t ) noexcept;
  template<class T>
    bool operator==( std::nullptr_t, scoped_ptr<T> const & p ) noexcept;

  template<class T>
    bool operator!=( scoped_ptr<T> const & p, std::nullptr_t ) noexcept;
  template<class T>
    bool operator!=( std::nullptr_t, scoped_ptr<T> const & p ) noexcept;
}

Members

element_type

typedef T element_type;

Provides the type of the stored pointer.

constructor

explicit scoped_ptr(T * p = 0) noexcept;

Constructs a scoped_ptr, storing a copy of p, which must have been allocated via a C++ new expression or be 0. T is not required be a complete type.

destructor

~scoped_ptr() noexcept;

Destroys the object pointed to by the stored pointer, if any, as if by using delete this->get(). T must be a complete type.

reset

void reset(T * p = 0) noexcept;

Deletes the object pointed to by the stored pointer and then stores a copy of p, which must have been allocated via a C++ new expression or be 0.

Since the previous object needs to be deleted, T must be a complete type.

indirection

T & operator*() const noexcept;

Returns a reference to the object pointed to by the stored pointer. Behavior is undefined if the stored pointer is 0.

T * operator->() const noexcept;

Returns the stored pointer. Behavior is undefined if the stored pointer is 0.

get

T * get() const noexcept;

Returns the stored pointer. T need not be a complete type.

conversions

explicit operator bool () const noexcept; // never throws

Returns get() != 0.

Note
On C++03 compilers, the return value is of an unspecified type.

swap

void swap(scoped_ptr & b) noexcept;

Exchanges the contents of the two smart pointers. T need not be a complete type.

Free Functions

swap

template<class T> void swap(scoped_ptr<T> & a, scoped_ptr<T> & b) noexcept;

Equivalent to a.swap(b).

comparisons

template<class T> bool operator==( scoped_ptr<T> const & p, std::nullptr_t ) noexcept;
template<class T> bool operator==( std::nullptr_t, scoped_ptr<T> const & p ) noexcept;

Returns p.get() == nullptr.

template<class T> bool operator!=( scoped_ptr<T> const & p, std::nullptr_t ) noexcept;
template<class T> bool operator!=( std::nullptr_t, scoped_ptr<T> const & p ) noexcept;

Returns p.get() != nullptr.

Example

Here’s an example that uses scoped_ptr.

#include <boost/scoped_ptr.hpp>
#include <iostream>

struct Shoe { ~Shoe() { std::cout << "Buckle my shoe\n"; } };

class MyClass {
    boost::scoped_ptr<int> ptr;
  public:
    MyClass() : ptr(new int) { *ptr = 0; }
    int add_one() { return ++*ptr; }
};

int main()
{
    boost::scoped_ptr<Shoe> x(new Shoe);
    MyClass my_instance;
    std::cout << my_instance.add_one() << '\n';
    std::cout << my_instance.add_one() << '\n';
}

The example program produces the beginning of a child’s nursery rhyme:

1
2
Buckle my shoe

Rationale

The primary reason to use scoped_ptr rather than std::auto_ptr or std::unique_ptr is to let readers of your code know that you intend "resource acquisition is initialization" to be applied only for the current scope, and have no intent to transfer ownership.

A secondary reason to use scoped_ptr is to prevent a later maintenance programmer from adding a function that transfers ownership by returning the auto_ptr, because the maintenance programmer saw auto_ptr, and assumed ownership could safely be transferred.

Think of bool vs int. We all know that under the covers bool is usually just an int. Indeed, some argued against including bool in the C++ standard because of that. But by coding bool rather than int, you tell your readers what your intent is. Same with scoped_ptr; by using it you are signaling intent.

It has been suggested that scoped_ptr<T> is equivalent to std::auto_ptr<T> const. Ed Brey pointed out, however, that reset will not work on a std::auto_ptr<T> const.

Handle/Body Idiom

One common usage of scoped_ptr is to implement a handle/body (also called pimpl) idiom which avoids exposing the body (implementation) in the header file.

The scoped_ptr_example_test.cpp sample program includes a header file, scoped_ptr_example.hpp, which uses a scoped_ptr<> to an incomplete type to hide the implementation. The instantiation of member functions which require a complete type occurs in the scoped_ptr_example.cpp implementation file.

Frequently Asked Questions

  1. Why doesn’t scoped_ptr have a release() member?

    When reading source code, it is valuable to be able to draw conclusions about program behavior based on the types being used. If scoped_ptr had a release() member, it would become possible to transfer ownership of the held pointer, weakening its role as a way of limiting resource lifetime to a given context. Use std::auto_ptr where transfer of ownership is required. (supplied by Dave Abrahams)

scoped_array: Scoped Array Ownership

Description

The scoped_array class template stores a pointer to a dynamically allocated array. (Dynamically allocated arrays are allocated with the C++ new[] expression.) The array pointed to is guaranteed to be deleted, either on destruction of the scoped_array, or via an explicit reset.

The scoped_array template is a simple solution for simple needs. It supplies a basic "resource acquisition is initialization" facility, without shared-ownership or transfer-of-ownership semantics. Both its name and enforcement of semantics (by being noncopyable) signal its intent to retain ownership solely within the current scope. Because it is noncopyable, it is safer than shared_ptr<T[]> for pointers which should not be copied.

Because scoped_array is so simple, in its usual implementation every operation is as fast as a built-in array pointer and it has no more space overhead that a built-in array pointer.

It cannot be used in C++ standard library containers. See shared_ptr<T[]> if scoped_array does not meet your needs.

It cannot correctly hold a pointer to a single object. See scoped_ptr for that usage.

std::vector is an alternative to scoped_array that is a bit heavier duty but far more flexible. boost::array is an alternative that does not use dynamic allocation.

The class template is parameterized on T, the type of the object pointed to.

Synopsis

scoped_array is defined in <boost/smart_ptr/scoped_array.hpp>.

namespace boost {

  template<class T> class scoped_array {
  private:

    scoped_array(scoped_array const &);
    scoped_array & operator=(scoped_array const &);

    void operator==( scoped_array const& ) const;
    void operator!=( scoped_array const& ) const;

  public:

    typedef T element_type;

    explicit scoped_array(T * p = 0) noexcept;
    ~scoped_array() noexcept;

    void reset(T * p = 0) noexcept;

    T & operator[](std::ptrdiff_t i) const noexcept;
    T * get() const noexcept;

    explicit operator bool () const noexcept;

    void swap(scoped_array & b) noexcept;
  };

  template<class T> void swap(scoped_array<T> & a, scoped_array<T> & b) noexcept;

  template<class T>
    bool operator==( scoped_array<T> const & p, std::nullptr_t ) noexcept;
  template<class T>
    bool operator==( std::nullptr_t, scoped_array<T> const & p ) noexcept;

  template<class T>
    bool operator!=( scoped_array<T> const & p, std::nullptr_t ) noexcept;
  template<class T>
    bool operator!=( std::nullptr_t, scoped_array<T> const & p ) noexcept;
}

Members

element_type

typedef T element_type;

Provides the type of the stored pointer.

constructors

explicit scoped_array(T * p = 0) noexcept;

Constructs a scoped_array, storing a copy of p, which must have been allocated via a C++ new[] expression or be 0. T is not required be a complete type.

destructor

~scoped_array() noexcept;

Deletes the array pointed to by the stored pointer. Note that delete[] on a pointer with a value of 0 is harmless. T must be complete, and delete[] on the stored pointer must not throw exceptions.

reset

void reset(T * p = 0) noexcept;

Deletes the array pointed to by the stored pointer and then stores a copy of p, which must have been allocated via a C++ new[] expression or be 0. T must be complete, and delete[] on the stored pointer must not throw exceptions.

subscripting

T & operator[](std::ptrdiff_t i) const noexcept;

Returns a reference to element i of the array pointed to by the stored pointer. Behavior is undefined and almost certainly undesirable if the stored pointer is 0, or if i is less than 0 or is greater than or equal to the number of elements in the array.

get

T * get() const noexcept;

Returns the stored pointer. T need not be a complete type.

conversions

explicit operator bool () const noexcept;

Returns get() != 0.

Note
On C++03 compilers, the return value is of an unspecified type.

swap

void swap(scoped_array & b) noexcept;

Exchanges the contents of the two smart pointers. T need not be a complete type.

Free Functions

swap

template<class T> void swap(scoped_array<T> & a, scoped_array<T> & b) noexcept;

Equivalent to a.swap(b).

comparisons

template<class T>
  bool operator==( scoped_array<T> const & p, std::nullptr_t ) noexcept;
template<class T>
  bool operator==( std::nullptr_t, scoped_array<T> const & p ) noexcept;

Returns p.get() == nullptr.

template<class T>
  bool operator!=( scoped_array<T> const & p, std::nullptr_t ) noexcept;
template<class T>
  bool operator!=( std::nullptr_t, scoped_array<T> const & p ) noexcept;

Returns p.get() != nullptr.

shared_ptr: Shared Ownership

Description

The shared_ptr class template stores a pointer to a dynamically allocated object, typically with a C++ new-expression. The object pointed to is guaranteed to be deleted when the last shared_ptr pointing to it is destroyed or reset.

Code Example 1. Using shared_ptr
shared_ptr<X> p1( new X );
shared_ptr<void> p2( new int(5) );

shared_ptr deletes the exact pointer that has been passed at construction time, complete with its original type, regardless of the template parameter. In the second example above, when p2 is destroyed or reset, it will call delete on the original int* that has been passed to the constructor, even though p2 itself is of type shared_ptr<void> and stores a pointer of type void*.

Every shared_ptr meets the CopyConstructible, MoveConstructible, CopyAssignable and MoveAssignable requirements of the C++ Standard Library, and can be used in standard library containers. Comparison operators are supplied so that shared_ptr works with the standard library’s associative containers.

Because the implementation uses reference counting, cycles of shared_ptr instances will not be reclaimed. For example, if main() holds a shared_ptr to A, which directly or indirectly holds a shared_ptr back to A, A’s use count will be 2. Destruction of the original `shared_ptr will leave A dangling with a use count of 1. Use weak_ptr to "break cycles."

The class template is parameterized on T, the type of the object pointed to. shared_ptr and most of its member functions place no requirements on T; it is allowed to be an incomplete type, or void. Member functions that do place additional requirements (constructors, reset) are explicitly documented below.

shared_ptr<T> can be implicitly converted to shared_ptr<U> whenever T* can be implicitly converted to U*. In particular, shared_ptr<T> is implicitly convertible to shared_ptr<T const>, to shared_ptr<U> where U is an accessible base of T, and to shared_ptr<void>.

shared_ptr is now part of the C++11 Standard, as std::shared_ptr.

Starting with Boost release 1.53, shared_ptr can be used to hold a pointer to a dynamically allocated array. This is accomplished by using an array type (T[] or T[N]) as the template parameter. There is almost no difference between using an unsized array, T[], and a sized array, T[N]; the latter just enables operator[] to perform a range check on the index.

Code Example 2. Using shared_ptr with arrays
shared_ptr<double[1024]> p1( new double[1024] );
shared_ptr<double[]> p2( new double[n] );

Best Practices

A simple guideline that nearly eliminates the possibility of memory leaks is: always use a named smart pointer variable to hold the result of new. Every occurence of the new keyword in the code should have the form:

shared_ptr<T> p(new Y);

It is, of course, acceptable to use another smart pointer in place of shared_ptr above; having T and Y be the same type, or passing arguments to the constructor of Y is also OK.

If you observe this guideline, it naturally follows that you will have no explicit delete statements; try/catch constructs will be rare.

Avoid using unnamed shared_ptr temporaries to save typing; to see why this is dangerous, consider this example:

Code Example 3. Exception-safe and -unsafe use of shared_ptr
void f(shared_ptr<int>, int);
int g();

void ok()
{
    shared_ptr<int> p( new int(2) );
    f( p, g() );
}

void bad()
{
    f( shared_ptr<int>( new int(2) ), g() );
}

The function ok follows the guideline to the letter, whereas bad constructs the temporary shared_ptr in place, admitting the possibility of a memory leak. Since function arguments are evaluated in unspecified order, it is possible for new int(2) to be evaluated first, g() second, and we may never get to the shared_ptr constructor if g throws an exception. See Herb Sutter’s treatment of the issue for more information.

The exception safety problem described above may also be eliminated by using the make_shared or allocate_shared factory functions defined in <boost/smart_ptr/make_shared.hpp>. These factory functions also provide an efficiency benefit by consolidating allocations.

Synopsis

shared_ptr is defined in <boost/smart_ptr/shared_ptr.hpp>.

namespace boost {

  class bad_weak_ptr: public std::exception;

  template<class T> class weak_ptr;

  template<class T> class shared_ptr {
  public:

    typedef /*see below*/ element_type;

    constexpr shared_ptr() noexcept;
    constexpr shared_ptr(std::nullptr_t) noexcept;

    template<class Y> explicit shared_ptr(Y * p);
    template<class Y, class D> shared_ptr(Y * p, D d);
    template<class Y, class D, class A> shared_ptr(Y * p, D d, A a);
    template<class D> shared_ptr(std::nullptr_t p, D d);
    template<class D, class A> shared_ptr(std::nullptr_t p, D d, A a);

    ~shared_ptr() noexcept;

    shared_ptr(shared_ptr const & r) noexcept;
    template<class Y> shared_ptr(shared_ptr<Y> const & r) noexcept;

    shared_ptr(shared_ptr && r) noexcept;
    template<class Y> shared_ptr(shared_ptr<Y> && r) noexcept;

    template<class Y> shared_ptr(shared_ptr<Y> const & r, element_type * p) noexcept;

    template<class Y> shared_ptr(shared_ptr<Y> && r, element_type * p) noexcept;

    template<class Y> explicit shared_ptr(weak_ptr<Y> const & r);

    template<class Y> explicit shared_ptr(std::auto_ptr<Y> & r);
    template<class Y> shared_ptr(std::auto_ptr<Y> && r);

    template<class Y, class D> shared_ptr(std::unique_ptr<Y, D> && r);

    shared_ptr & operator=(shared_ptr const & r) noexcept;
    template<class Y> shared_ptr & operator=(shared_ptr<Y> const & r) noexcept;

    shared_ptr & operator=(shared_ptr const && r) noexcept;
    template<class Y> shared_ptr & operator=(shared_ptr<Y> const && r) noexcept;

    template<class Y> shared_ptr & operator=(std::auto_ptr<Y> & r);
    template<class Y> shared_ptr & operator=(std::auto_ptr<Y> && r);

    template<class Y, class D> shared_ptr & operator=(std::unique_ptr<Y, D> && r);

    shared_ptr & operator=(std::nullptr_t) noexcept;

    void reset() noexcept;

    template<class Y> void reset(Y * p);
    template<class Y, class D> void reset(Y * p, D d);
    template<class Y, class D, class A> void reset(Y * p, D d, A a);

    template<class Y> void reset(shared_ptr<Y> const & r, element_type * p) noexcept;
    template<class Y> void reset(shared_ptr<Y> && r, element_type * p) noexcept;

    T & operator*() const noexcept; // only valid when T is not an array type
    T * operator->() const noexcept; // only valid when T is not an array type

    // only valid when T is an array type
    element_type & operator[](std::ptrdiff_t i) const noexcept;

    element_type * get() const noexcept;

    bool unique() const noexcept;
    long use_count() const noexcept;

    explicit operator bool() const noexcept;

    void swap(shared_ptr & b) noexcept;

    template<class Y> bool owner_before(shared_ptr<Y> const & rhs) const noexcept;
    template<class Y> bool owner_before(weak_ptr<Y> const & rhs) const noexcept;
  };

  template<class T, class U>
    bool operator==(shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator!=(shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator<(shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;

  template<class T> bool operator==(shared_ptr<T> const & p, std::nullptr_t) noexcept;
  template<class T> bool operator==(std::nullptr_t, shared_ptr<T> const & p) noexcept;

  template<class T> bool operator!=(shared_ptr<T> const & p, std::nullptr_t) noexcept;
  template<class T> bool operator!=(std::nullptr_t, shared_ptr<T> const & p) noexcept;

  template<class T> void swap(shared_ptr<T> & a, shared_ptr<T> & b) noexcept;

  template<class T>
    typename shared_ptr<T>::element_type *
      get_pointer(shared_ptr<T> const & p) noexcept;

  template<class T, class U>
    shared_ptr<T> static_pointer_cast(shared_ptr<U> const & r) noexcept;

  template<class T, class U>
    shared_ptr<T> const_pointer_cast(shared_ptr<U> const & r) noexcept;

  template<class T, class U>
    shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const & r) noexcept;

  template<class T, class U>
    shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U> const & r) noexcept;

  template<class E, class T, class Y>
    std::basic_ostream<E, T> &
      operator<< (std::basic_ostream<E, T> & os, shared_ptr<Y> const & p);

  template<class D, class T> D * get_deleter(shared_ptr<T> const & p) noexcept;

  template<class T> bool atomic_is_lock_free( shared_ptr<T> const * p ) noexcept;

  template<class T> shared_ptr<T> atomic_load( shared_ptr<T> const * p ) noexcept;
  template<class T>
    shared_ptr<T> atomic_load_explicit( shared_ptr<T> const * p, int ) noexcept;

  template<class T>
    void atomic_store( shared_ptr<T> * p, shared_ptr<T> r ) noexcept;
  template<class T>
    void atomic_store_explicit( shared_ptr<T> * p, shared_ptr<T> r, int ) noexcept;

  template<class T>
    shared_ptr<T> atomic_exchange( shared_ptr<T> * p, shared_ptr<T> r ) noexcept;
  template<class T>
    shared_ptr<T> atomic_exchange_explicit(
      shared_ptr<T> * p, shared_ptr<T> r, int ) noexcept;

  template<class T>
    bool atomic_compare_exchange(
      shared_ptr<T> * p, shared_ptr<T> * v, shared_ptr<T> w ) noexcept;
  template<class T>
    bool atomic_compare_exchange_explicit(
      shared_ptr<T> * p, shared_ptr<T> * v, shared_ptr<T> w, int, int ) noexcept;
}

Members

element_type

typedef ... element_type;

element_type is T when T is not an array type, and U when T is U[] or U[N].

default constructor

constexpr shared_ptr() noexcept;
constexpr shared_ptr(std::nullptr_t) noexcept;
  • Effects

    Constructs an empty shared_ptr.

    Postconditions

    use_count() == 0 && get() == 0.

pointer constructor

template<class Y> explicit shared_ptr(Y * p);
  • Requires

    Y must be a complete type. The expression delete[] p, when T is an array type, or delete p, when T is not an array type, must be well-formed, well-defined, and not throw exceptions. When T is U[N], Y(*)[N] must be convertible to T*; when T is U[], Y(*)[] must be convertible to T*; otherwise, Y* must be convertible to T*.

    Effects

    When T is not an array type, constructs a shared_ptr that owns the pointer p. Otherwise, constructs a shared_ptr that owns p and a deleter of an unspecified type that calls delete[] p.

    Postconditions

    use_count() == 1 && get() == p. If T is not an array type and p is unambiguously convertible to enable_shared_from_this<V>* for some V, p->shared_from_this() returns a copy of *this.

    Throws

    std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

    Exception safety

    If an exception is thrown, the constructor calls delete[] p, when T is an array type, or delete p, when T is not an array type.

Note
p must be a pointer to an object that was allocated via a C++ new expression or be 0. The postcondition that use count is 1 holds even if p is 0; invoking delete on a pointer that has a value of 0 is harmless.
Note
This constructor is a template in order to remember the actual pointer type passed. The destructor will call delete with the same pointer, complete with its original type, even when T does not have a virtual destructor, or is void.

constructors taking a deleter

template<class Y, class D> shared_ptr(Y * p, D d);
template<class Y, class D, class A> shared_ptr(Y * p, D d, A a);
template<class D> shared_ptr(std::nullptr_t p, D d);
template<class D, class A> shared_ptr(std::nullptr_t p, D d, A a);
  • Requires

    D must be CopyConstructible. The copy constructor and destructor of D must not throw. The expression d(p) must be well-formed, well-defined, and not throw exceptions. A must be an Allocator, as described in section Allocator Requirements [allocator.requirements] of the C++ Standard. When T is U[N], Y(*)[N] must be convertible to T*; when T is U[], Y(*)[] must be convertible to T*; otherwise, Y* must be convertible to T*.

    Effects

    Constructs a shared_ptr that owns the pointer p and the deleter d. The constructors taking an allocator a allocate memory using a copy of a.

    Postconditions

    use_count() == 1 && get() == p. If T is not an array type and p is unambiguously convertible to enable_shared_from_this<V>* for some V, p->shared_from_this() returns a copy of *this.

    Throws

    std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

    Exception safety

    If an exception is thrown, d(p) is called.

Note
When the the time comes to delete the object pointed to by p, the stored copy of d is invoked with the stored copy of p as an argument.
Note
Custom deallocators allow a factory function returning a shared_ptr to insulate the user from its memory allocation strategy. Since the deallocator is not part of the type, changing the allocation strategy does not break source or binary compatibility, and does not require a client recompilation. For example, a "no-op" deallocator is useful when returning a shared_ptr to a statically allocated object, and other variations allow a shared_ptr to be used as a wrapper for another smart pointer, easing interoperability.
Note
The requirement that the copy constructor of D does not throw comes from the pass by value. If the copy constructor throws, the pointer would leak.

copy and converting constructors

shared_ptr(shared_ptr const & r) noexcept;
template<class Y> shared_ptr(shared_ptr<Y> const & r) noexcept;
  • Requires

    Y* should be convertible to T*.

    Effects

    If r is empty, constructs an empty shared_ptr; otherwise, constructs a shared_ptr that shares ownership with r.

    Postconditions

    get() == r.get() && use_count() == r.use_count().

move constructors

shared_ptr(shared_ptr && r) noexcept;
template<class Y> shared_ptr(shared_ptr<Y> && r) noexcept;
  • Requires

    Y* should be convertible to T*.

    Effects

    Move-constructs a shared_ptr from r.

    Postconditions

    *this contains the old value of r. r is empty and r.get() == 0.

aliasing constructor

template<class Y> shared_ptr(shared_ptr<Y> const & r, element_type * p) noexcept;
  • Effects

    constructs a shared_ptr that shares ownership with r and stores p.

    Postconditions

    get() == p && use_count() == r.use_count().

aliasing move constructor

template<class Y> shared_ptr(shared_ptr<Y> && r, element_type * p) noexcept;
  • Effects

    Move-constructs a shared_ptr from r, while storing p instead.

    Postconditions

    get() == p and use_count() equals the old count of r. r is empty and r.get() == 0.

weak_ptr constructor

template<class Y> explicit shared_ptr(weak_ptr<Y> const & r);
  • Requires

    Y* should be convertible to T*.

    Effects

    Constructs a shared_ptr that shares ownership with r and stores a copy of the pointer stored in r.

    Postconditions

    use_count() == r.use_count().

    Throws

    bad_weak_ptr when r.use_count() == 0.

    Exception safety

    If an exception is thrown, the constructor has no effect.

auto_ptr constructors

template<class Y> shared_ptr(std::auto_ptr<Y> & r);
template<class Y> shared_ptr(std::auto_ptr<Y> && r);
  • Requires

    Y* should be convertible to T*.

    Effects

    Constructs a shared_ptr, as if by storing a copy of r.release().

    Postconditions

    use_count() == 1.

    Throws

    std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

    Exception safety

    If an exception is thrown, the constructor has no effect.

unique_ptr constructor

template<class Y, class D> shared_ptr(std::unique_ptr<Y, D> && r);
  • Requires

    Y* should be convertible to T*.

    Effects
    • When r.get() == 0, equivalent to shared_ptr();

    • When D is not a reference type, equivalent to shared_ptr(r.release(), r.get_deleter());

    • Otherwise, equivalent to shared_ptr(r.release(), del), where del is a deleter that stores the reference rd returned from r.get_deleter() and del(p) calls rd(p).

    Throws

    std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

    Exception safety

    If an exception is thrown, the constructor has no effect.

destructor

~shared_ptr() noexcept;
  • Effects
    • If *this is empty, or shares ownership with another shared_ptr instance (use_count() > 1), there are no side effects.

    • Otherwise, if *this owns a pointer p and a deleter d, d(p) is called.

    • Otherwise, *this owns a pointer p, and delete p is called.

assignment

shared_ptr & operator=(shared_ptr const & r) noexcept;
template<class Y> shared_ptr & operator=(shared_ptr<Y> const & r) noexcept;
template<class Y> shared_ptr & operator=(std::auto_ptr<Y> & r);
  • Effects

    Equivalent to shared_ptr(r).swap(*this).

    Returns

    *this.

Note
The use count updates caused by the temporary object construction and destruction are not considered observable side effects, and the implementation is free to meet the effects (and the implied guarantees) via different means, without creating a temporary.
Note

In particular, in the example:

shared_ptr<int> p(new int);
shared_ptr<void> q(p);
p = p;
q = p;

both assignments may be no-ops.

shared_ptr & operator=(shared_ptr && r) noexcept;
template<class Y> shared_ptr & operator=(shared_ptr<Y> && r) noexcept;
template<class Y> shared_ptr & operator=(std::auto_ptr<Y> && r);
template<class Y, class D> shared_ptr & operator=(std::unique_ptr<Y, D> && r);
  • Effects

    Equivalent to shared_ptr(std::move(r)).swap(*this).

    Returns

    *this.

shared_ptr & operator=(std::nullptr_t) noexcept;
  • Effects

    Equivalent to shared_ptr().swap(*this).

    Returns

    *this.

reset

void reset() noexcept;
  • Effects

    Equivalent to shared_ptr().swap(*this).

template<class Y> void reset(Y * p);
  • Effects

    Equivalent to shared_ptr(p).swap(*this).

template<class Y, class D> void reset(Y * p, D d);
  • Effects

    Equivalent to shared_ptr(p, d).swap(*this).

template<class Y, class D, class A> void reset(Y * p, D d, A a);
  • Effects

    Equivalent to shared_ptr(p, d, a).swap(*this).

template<class Y> void reset(shared_ptr<Y> const & r, element_type * p) noexcept;
  • Effects

    Equivalent to shared_ptr(r, p).swap(*this).

template<class Y> void reset(shared_ptr<Y> && r, element_type * p) noexcept;
  • Effects

    Equivalent to shared_ptr(std::move(r), p).swap(*this).

indirection

T & operator*() const noexcept;
  • Requires

    T should not be an array type. The stored pointer must not be 0.

    Returns

    *get().

T * operator->() const noexcept;
  • Requires

    T should not be an array type. The stored pointer must not be 0.

    Returns

    get().

element_type & operator[](std::ptrdiff_t i) const noexcept;
  • Requires

    T should be an array type. The stored pointer must not be 0. i >= 0. If T is U[N], i < N.

    Returns

    get()[i].

get

element_type * get() const noexcept;
  • Returns

    The stored pointer.

unique

bool unique() const noexcept;
  • Returns

    use_count() == 1.

use_count

long use_count() const noexcept;
  • Returns

    The number of shared_ptr objects, *this included, that share ownership with *this, or 0 when *this is empty.

conversions

explicit operator bool() const noexcept;
  • Returns

    get() != 0.

Note
This conversion operator allows shared_ptr objects to be used in boolean contexts, like if(p && p->valid()) {}.
Note
The conversion to bool is not merely syntactic sugar. It allows shared_ptr variables to be declared in conditions when using dynamic_pointer_cast or weak_ptr::lock.
Note
On C++03 compilers, the return value is of an unspecified type.

swap

void swap(shared_ptr & b) noexcept;
  • Effects

    Exchanges the contents of the two smart pointers.

owner_before

template<class Y> bool owner_before(shared_ptr<Y> const & rhs) const noexcept;
template<class Y> bool owner_before(weak_ptr<Y> const & rhs) const noexcept;
  • Effects

    See the description of operator<.

Free Functions

comparison

template<class T, class U>
  bool operator==(shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;
  • Returns

    a.get() == b.get().

template<class T, class U>
  bool operator!=(shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;
  • Returns

    a.get() != b.get().

template<class T> bool operator==(shared_ptr<T> const & p, std::nullptr_t) noexcept;
template<class T> bool operator==(std::nullptr_t, shared_ptr<T> const & p) noexcept;
  • Returns

    p.get() == 0.

template<class T> bool operator!=(shared_ptr<T> const & p, std::nullptr_t) noexcept;
template<class T> bool operator!=(std::nullptr_t, shared_ptr<T> const & p) noexcept;
  • Returns

    p.get() != 0.

template<class T, class U>
  bool operator<(shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;
  • Returns

    An unspecified value such that

    • operator< is a strict weak ordering as described in section [lib.alg.sorting] of the C++ standard;

    • under the equivalence relation defined by operator<, !(a < b) && !(b < a), two shared_ptr instances are equivalent if and only if they share ownership or are both empty.

Note
Allows shared_ptr objects to be used as keys in associative containers.
Note
The rest of the comparison operators are omitted by design.

swap

template<class T> void swap(shared_ptr<T> & a, shared_ptr<T> & b) noexcept;
  • Effects

    Equivalent to a.swap(b).

get_pointer

template<class T>
  typename shared_ptr<T>::element_type *
    get_pointer(shared_ptr<T> const & p) noexcept;
  • Returns

    p.get().

Note
Provided as an aid to generic programming. Used by mem_fn.

static_pointer_cast

template<class T, class U>
  shared_ptr<T> static_pointer_cast(shared_ptr<U> const & r) noexcept;
  • Requires

    The expression static_cast<T*>( (U*)0 ) must be well-formed.

    Returns

    shared_ptr<T>( r, static_cast<typename shared_ptr<T>::element_type*>(r.get()) ).

Caution
The seemingly equivalent expression shared_ptr<T>(static_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

const_pointer_cast

template<class T, class U>
  shared_ptr<T> const_pointer_cast(shared_ptr<U> const & r) noexcept;
  • Requires

    The expression const_cast<T*>( (U*)0 ) must be well-formed.

    Returns

    shared_ptr<T>( r, const_cast<typename shared_ptr<T>::element_type*>(r.get()) ).

dynamic_pointer_cast

template<class T, class U>
    shared_ptr<T> dynamic_pointer_cast(shared_ptr<U> const & r) noexcept;
  • Requires

    The expression dynamic_cast<T*>( (U*)0 ) must be well-formed.

    Returns
    • When dynamic_cast<typename shared_ptr<T>::element_type*>(r.get()) returns a nonzero value p, shared_ptr<T>(r, p);

    • Otherwise, shared_ptr<T>().

reinterpret_pointer_cast

template<class T, class U>
  shared_ptr<T> reinterpret_pointer_cast(shared_ptr<U> const & r) noexcept;
  • Requires

    The expression reinterpret_cast<T*>( (U*)0 ) must be well-formed.

    Returns

    shared_ptr<T>( r, reinterpret_cast<typename shared_ptr<T>::element_type*>(r.get()) ).

operator<<

template<class E, class T, class Y>
  std::basic_ostream<E, T> &
    operator<< (std::basic_ostream<E, T> & os, shared_ptr<Y> const & p);
  • Effects

    os << p.get();.

    Returns

    os.

get_deleter

template<class D, class T>
  D * get_deleter(shared_ptr<T> const & p) noexcept;
  • Returns

    If *this owns a deleter d of type (cv-unqualified) D, returns &d; otherwise returns 0.

Atomic Access

Note
The function in this section are atomic with respect to the first shared_ptr argument, identified by *p. Concurrent access to the same shared_ptr instance is not a data race, if done exclusively by the functions in this section.
template<class T> bool atomic_is_lock_free( shared_ptr<T> const * p ) noexcept;
  • Returns

    false.

Note
This implementation is not lock-free.
template<class T> shared_ptr<T> atomic_load( shared_ptr<T> const * p ) noexcept;
template<class T> shared_ptr<T> atomic_load_explicit( shared_ptr<T> const * p, int ) noexcept;
  • Returns

    *p.

Note
The int argument is the memory_order, but this implementation does not use it, as it’s lock-based and therefore always sequentially consistent.
template<class T>
  void atomic_store( shared_ptr<T> * p, shared_ptr<T> r ) noexcept;
template<class T>
  void atomic_store_explicit( shared_ptr<T> * p, shared_ptr<T> r, int ) noexcept;
  • Effects

    p->swap(r).

template<class T>
  shared_ptr<T> atomic_exchange( shared_ptr<T> * p, shared_ptr<T> r ) noexcept;
template<class T>
  shared_ptr<T> atomic_exchange_explicit(
    shared_ptr<T> * p, shared_ptr<T> r, int ) noexcept;
  • Effects

    p->swap(r).

    Returns

    The old value of *p.

template<class T>
  bool atomic_compare_exchange(
    shared_ptr<T> * p, shared_ptr<T> * v, shared_ptr<T> w ) noexcept;
template<class T>
  bool atomic_compare_exchange_explicit(
    shared_ptr<T> * p, shared_ptr<T> * v, shared_ptr<T> w, int, int ) noexcept;
  • Effects

    If *p is equivalent to *v, assigns w to *p, otherwise assigns *p to *v.

    Returns

    true if *p was equivalent to *v, false otherwise.

    Remarks

    Two shared_ptr instances are equivalent if they store the same pointer value and share ownership.

Example

See shared_ptr_example.cpp for a complete example program. The program builds a std::vector and std::set of shared_ptr objects.

Note that after the containers have been populated, some of the shared_ptr objects will have a use count of 1 rather than a use count of 2, since the set is a std::set rather than a std::multiset, and thus does not contain duplicate entries. Furthermore, the use count may be even higher at various times while push_back and insert container operations are performed. More complicated yet, the container operations may throw exceptions under a variety of circumstances. Getting the memory management and exception handling in this example right without a smart pointer would be a nightmare.

Handle/Body Idiom

One common usage of shared_ptr is to implement a handle/body (also called pimpl) idiom which avoids exposing the body (implementation) in the header file.

The shared_ptr_example2_test.cpp sample program includes a header file, shared_ptr_example2.hpp, which uses a shared_ptr to an incomplete type to hide the implementation. The instantiation of member functions which require a complete type occurs in the shared_ptr_example2.cpp implementation file. Note that there is no need for an explicit destructor. Unlike ~scoped_ptr, ~shared_ptr does not require that T be a complete type.

Thread Safety

shared_ptr objects offer the same level of thread safety as built-in types. A shared_ptr instance can be "read" (accessed using only const operations) simultaneously by multiple threads. Different shared_ptr instances can be "written to" (accessed using mutable operations such as operator= or reset) simultaneously by multiple threads (even when these instances are copies, and share the same reference count underneath.)

Any other simultaneous accesses result in undefined behavior.

Examples:

shared_ptr<int> p(new int(42));
Code Example 4. Reading a shared_ptr from two threads
// thread A
shared_ptr<int> p2(p); // reads p

// thread B
shared_ptr<int> p3(p); // OK, multiple reads are safe
Code Example 5. Writing different shared_ptr instances from two threads
// thread A
p.reset(new int(1912)); // writes p

// thread B
p2.reset(); // OK, writes p2
Code Example 6. Reading and writing a shared_ptr from two threads
// thread A
p = p3; // reads p3, writes p

// thread B
p3.reset(); // writes p3; undefined, simultaneous read/write
Code Example 7. Reading and destroying a shared_ptr from two threads
// thread A
p3 = p2; // reads p2, writes p3

// thread B
// p2 goes out of scope: undefined, the destructor is considered a "write access"
Code Example 8. Writing a shared_ptr from two threads
// thread A
p3.reset(new int(1));

// thread B
p3.reset(new int(2)); // undefined, multiple writes

Starting with Boost release 1.33.0, shared_ptr uses a lock-free implementation on most common platforms.

If your program is single-threaded and does not link to any libraries that might have used shared_ptr in its default configuration, you can #define the macro BOOST_SP_DISABLE_THREADS on a project-wide basis to switch to ordinary non-atomic reference count updates.

(Defining BOOST_SP_DISABLE_THREADS in some, but not all, translation units is technically a violation of the One Definition Rule and undefined behavior. Nevertheless, the implementation attempts to do its best to accommodate the request to use non-atomic updates in those translation units. No guarantees, though.)

You can define the macro BOOST_SP_USE_PTHREADS to turn off the lock-free platform-specific implementation and fall back to the generic pthread_mutex_t-based code.

Frequently Asked Questions

  1. There are several variations of shared pointers, with different tradeoffs; why does the smart pointer library supply only a single implementation? It would be useful to be able to experiment with each type so as to find the most suitable for the job at hand?

    An important goal of shared_ptr is to provide a standard shared-ownership pointer. Having a single pointer type is important for stable library interfaces, since different shared pointers typically cannot interoperate, i.e. a reference counted pointer (used by library A) cannot share ownership with a linked pointer (used by library B.)

  2. Why doesn’t shared_ptr have template parameters supplying traits or policies to allow extensive user customization?

    Parameterization discourages users. The shared_ptr template is carefully crafted to meet common needs without extensive parameterization.

  3. I am not convinced. Default parameters can be used where appropriate to hide the complexity. Again, why not policies?

    Template parameters affect the type. See the answer to the first question above.

  4. Why doesn’t shared_ptr use a linked list implementation?

    A linked list implementation does not offer enough advantages to offset the added cost of an extra pointer. In addition, it is expensive to make a linked list implementation thread safe.

  5. Why doesn’t shared_ptr (or any of the other Boost smart pointers) supply an automatic conversion to T*?

    Automatic conversion is believed to be too error prone.

  6. Why does shared_ptr supply use_count()?

    As an aid to writing test cases and debugging displays. One of the progenitors had use_count(), and it was useful in tracking down bugs in a complex project that turned out to have cyclic-dependencies.

  7. Why doesn’t shared_ptr specify complexity requirements?

    Because complexity requirements limit implementors and complicate the specification without apparent benefit to shared_ptr users. For example, error-checking implementations might become non-conforming if they had to meet stringent complexity requirements.

  8. Why doesn’t shared_ptr provide a release() function?

    shared_ptr cannot give away ownership unless it’s unique() because the other copy will still destroy the object.

    Consider:

    shared_ptr<int> a(new int);
    shared_ptr<int> b(a); // a.use_count() == b.use_count() == 2
    
    int * p = a.release();
    
    // Who owns p now? b will still call delete on it in its destructor.

    Furthermore, the pointer returned by release() would be difficult to deallocate reliably, as the source shared_ptr could have been created with a custom deleter, or may have pointed to an object of a different type.

  9. Why is operator->() const, but its return value is a non-const pointer to the element type?

    Shallow copy pointers, including raw pointers, typically don’t propagate constness. It makes little sense for them to do so, as you can always obtain a non-const pointer from a const one and then proceed to modify the object through it. shared_ptr is "as close to raw pointers as possible but no closer".

weak_ptr: Non-owning Observer

Description

The weak_ptr class template stores a "weak reference" to an object that’s already managed by a shared_ptr. To access the object, a weak_ptr can be converted to a shared_ptr using the shared_ptr constructor taking weak_ptr, or the weak_ptr member function lock. When the last shared_ptr to the object goes away and the object is deleted, the attempt to obtain a shared_ptr from the weak_ptr instances that refer to the deleted object will fail: the constructor will throw an exception of type boost::bad_weak_ptr, and weak_ptr::lock will return an empty shared_ptr.

Every weak_ptr meets the CopyConstructible and Assignable requirements of the C++ Standard Library, and so can be used in standard library containers. Comparison operators are supplied so that weak_ptr works with the standard library’s associative containers.

weak_ptr operations never throw exceptions.

The class template is parameterized on T, the type of the object pointed to.

Compared to shared_ptr, weak_ptr provides a very limited subset of operations since accessing its stored pointer is often dangerous in multithreaded programs, and sometimes unsafe even within a single thread (that is, it may invoke undefined behavior.) Pretend for a moment that weak_ptr had a get member function that returned a raw pointer, and consider this innocent piece of code:

shared_ptr<int> p(new int(5));
weak_ptr<int> q(p);

// some time later

if(int * r = q.get())
{
    // use *r
}

Imagine that after the if, but immediately before r is used, another thread executes the statement p.reset(). Now r is a dangling pointer.

The solution to this problem is to create a temporary shared_ptr from q:

shared_ptr<int> p(new int(5));
weak_ptr<int> q(p);

// some time later

if(shared_ptr<int> r = q.lock())
{
    // use *r
}

Now r holds a reference to the object that was pointed by q. Even if p.reset() is executed in another thread, the object will stay alive until r goes out of scope or is reset. By obtaining a shared_ptr to the object, we have effectively locked it against destruction.

Synopsis

weak_ptr is defined in <boost/smart_ptr/weak_ptr.hpp>.

namespace boost {

  template<class T> class weak_ptr {
  public:

    typedef /*see below*/ element_type;

    weak_ptr() noexcept;

    template<class Y> weak_ptr(shared_ptr<Y> const & r) noexcept;
    weak_ptr(weak_ptr const & r) noexcept;
    template<class Y> weak_ptr(weak_ptr<Y> const & r) noexcept;

    weak_ptr(weak_ptr && r) noexcept;

    ~weak_ptr() noexcept;

    weak_ptr & operator=(weak_ptr const & r) noexcept;
    weak_ptr & operator=(weak_ptr && r) noexcept;
    template<class Y> weak_ptr & operator=(weak_ptr<Y> const & r) noexcept;
    template<class Y> weak_ptr & operator=(shared_ptr<Y> const & r) noexcept;

    long use_count() const noexcept;
    bool expired() const noexcept;

    shared_ptr<T> lock() const noexcept;

    void reset() noexcept;

    void swap(weak_ptr<T> & b) noexcept;

    template<class Y> bool owner_before( weak_ptr<Y> const & r ) const noexcept;
    template<class Y> bool owner_before( shared_ptr<Y> const & r ) const noexcept;
  };

  template<class T, class U>
    bool operator<(weak_ptr<T> const & a, weak_ptr<U> const & b) noexcept;

  template<class T> void swap(weak_ptr<T> & a, weak_ptr<T> & b) noexcept;
}

Members

element_type

typedef ... element_type;

element_type is T when T is not an array type, and U when T is U[] or U[N].

constructors

weak_ptr() noexcept;
  • Effects

    Constructs an empty weak_ptr.

    Postconditions

    use_count() == 0.

template<class Y> weak_ptr(shared_ptr<Y> const & r) noexcept;
weak_ptr(weak_ptr const & r) noexcept;
template<class Y> weak_ptr(weak_ptr<Y> const & r) noexcept;
  • Effects

    If r is empty, constructs an empty weak_ptr; otherwise, constructs a weak_ptr that shares ownership with r as if by storing a copy of the pointer stored in r.

    Postconditions

    use_count() == r.use_count().

weak_ptr(weak_ptr && r) noexcept;
  • Effects

    Constructs a weak_ptr that has the value r held.

    Postconditions

    r is empty.

destructor

~weak_ptr() noexcept;
  • Effects

    Destroys this weak_ptr but has no effect on the object its stored pointer points to.

assignment

weak_ptr & operator=(weak_ptr const & r) noexcept;
weak_ptr & operator=(weak_ptr && r) noexcept;
template<class Y> weak_ptr & operator=(weak_ptr<Y> const & r) noexcept;
template<class Y> weak_ptr & operator=(shared_ptr<Y> const & r) noexcept;
  • Effects

    Equivalent to weak_ptr(r).swap(*this).

Note
The implementation is free to meet the effects (and the implied guarantees) via different means, without creating a temporary.

use_count

long use_count() const noexcept;
  • Returns

    0 if *this is empty; otherwise, the number of shared_ptr objects that share ownership with *this.

expired

bool expired() const noexcept;
  • Returns

    use_count() == 0.

lock

shared_ptr<T> lock() const noexcept;
  • Returns

    expired()? shared_ptr<T>(): shared_ptr<T>(*this).

reset

void reset() noexcept;
  • Effects

    Equivalent to weak_ptr().swap(*this).

swap

void swap(weak_ptr & b) noexcept;
  • Effects

    Exchanges the contents of the two smart pointers.

template<class Y> bool owner_before( weak_ptr<Y> const & r ) const noexcept;
template<class Y> bool owner_before( shared_ptr<Y> const & r ) const noexcept;
  • Returns

    See the description of operator<.

Free Functions

comparison

template<class T, class U>
  bool operator<(weak_ptr<T> const & a, weak_ptr<U> const & b) noexcept;
  • Returns

    An unspecified value such that

    • operator< is a strict weak ordering as described in section [lib.alg.sorting] of the C++ standard;

    • under the equivalence relation defined by operator<, !(a < b) && !(b < a), two weak_ptr instances are equivalent if and only if they share ownership or are both empty.

Note
Allows weak_ptr objects to be used as keys in associative containers.

swap

template<class T> void swap(weak_ptr<T> & a, weak_ptr<T> & b) noexcept;
  • Effects

    Equivalent to a.swap(b).

Frequently Asked Questions

  1. Can an object create a weak_ptr to itself in its constructor?

    No. A weak_ptr can only be created from a shared_ptr, and at object construction time no shared_ptr to the object exists yet. Even if you could create a temporary shared_ptr to this, it would go out of scope at the end of the constructor, and all weak_ptr instances would instantly expire.

    The solution is to make the constructor private, and supply a factory function that returns a shared_ptr:

    class X
    {
    private:
    
        X();
    
    public:
    
        static shared_ptr<X> create()
        {
            shared_ptr<X> px(new X);
            // create weak pointers from px here
            return px;
        }
    };

make_shared: Creating shared_ptr

Description

The function templates make_shared and allocate_shared provide convenient, safe and efficient ways to create shared_ptr objects.

Rationale

Consistent use of shared_ptr can eliminate the need to use an explicit delete, but alone it provides no support in avoiding explicit new. There were repeated requests from users for a factory function that creates an object of a given type and returns a shared_ptr to it. Besides convenience and style, such a function is also exception safe and considerably faster because it can use a single allocation for both the object and its corresponding control block, eliminating a significant portion of shared_ptr construction overhead. This eliminates one of the major efficiency complaints about shared_ptr.

The family of overloaded function templates, make_shared and allocate_shared, were provided to address this need. make_shared uses the global operator new to allocate memory, whereas allocate_shared uses an user-supplied allocator, allowing finer control.

The rationale for choosing the name make_shared is that the expression make_shared<Widget>() can be read aloud and conveys the intended meaning.

Originally the Boost function templates allocate_shared and make_shared were provided for scalar objects only. There was a need to have efficient allocation of array objects. One criticism of class template shared_array was always the lack of a utility like make_shared that uses only a single allocation. When shared_ptr was enhanced to support array types, additional overloads of allocate_shared and make_shared were provided for array types.

Synopsis

make_shared and allocate_shared are defined in <boost/smart_ptr/make_shared.hpp>.

namespace boost {
  // only if T is not an array type
  template<class T, class... Args>
    shared_ptr<T> make_shared(Args&&... args);
  template<class T, class A, class... Args>
    shared_ptr<T> allocate_shared(const A& a, Args&&... args);

  // only if T is an array type of the form U[]
  template<class T>
    shared_ptr<T> make_shared(std::size_t n);
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, std::size_t n);

  // only if T is an array type of the form U[N]
  template<class T>
    shared_ptr<T> make_shared();
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a);

  // only if T is an array type of the form U[]
  template<class T> shared_ptr<T>
    make_shared(std::size_t n, const remove_extent_t<T>& v);
  template<class T, class A> shared_ptr<T>
    allocate_shared(const A& a, std::size_t n, const remove_extent_t<T>& v);

  // only if T is an array type of the form U[N]
  template<class T>
    shared_ptr<T> make_shared(const remove_extent_t<T>& v);
  template<class T, class A>
    shared_ptr<T> allocate_shared(const A& a, const remove_extent_t<T>& v);

  // only if T is not an array type of the form U[]
  template<class T>
    shared_ptr<T> make_shared_noinit();
  template<class T, class A>
    shared_ptr<T> allocate_shared_noinit(const A& a);

  // only if T is an array type of the form U[N]
  template<class T>
    shared_ptr<T> make_shared_noinit(std::size_t n);
  template<class T, class A>
    shared_ptr<T> allocate_shared_noinit(const A& a, std::size_t n);
}

Common Requirements

The common requirements that apply to all make_shared and allocate_shared overloads, unless specified otherwise, are described below.

Requires

A shall be an allocator. The copy constructor and destructor of A shall not throw exceptions.

Effects

Allocates memory for an object of type T or n objects of U (if T is an array type of the form U[] and n is determined by arguments, as specified by the concrete overload). The object is initialized from arguments as specified by the concrete overload. Uses a rebound copy of a (for an unspecified value_type) to allocate memory. If an exception is thrown, the functions have no effect.

Returns

A shared_ptr instance that stores and owns the address of the newly constructed object.

Postconditions

r.get() != 0 and r.use_count() == 1, where r is the return value.

Throws

std::bad_alloc, an exception thrown from A::allocate, or from the initialization of the object.

Remarks
  • Performs no more than one memory allocation. This provides efficiency equivalent to an intrusive smart pointer.

  • When an object of an array type is specified to be initialized to a value of the same type v, this shall be interpreted to mean that each array element of the object is initialized to the corresponding element from v.

  • When an object of an array type is specified to be value-initialized, this shall be interpreted to mean that each array element of the object is value-initialized.

  • When a (sub)object of non-array type U is specified to be initialized to a value v, or constructed from args..., make_shared shall perform this initialization via the expression ::new(p) U(expr) (where expr is v or std::forward<Args>(args)...) respectively) and p has type void* and points to storage suitable to hold an object of type U.

  • When a (sub)object of non-array type U is specified to be initialized to a value v, or constructed from args..., allocate_shared shall perform this initialization via the expression std::allocator_traits<A2>::construct(a2, p, expr) (where expr is v or std::forward<Args>(args)...) respectively), p points to storage suitable to hold an object of type U, and a2 of type A2 is a rebound copy a such that its value_type is U.

  • When a (sub)object of non-array type U is specified to be default-initialized, make_shared_noinit and allocate_shared_noinit shall perform this initialization via the expression ::new(p) U, where p has type void* and points to storage suitable to hold an object of type U.

  • When a (sub)object of non-array type U is specified to be value-initialized, make_shared shall perform this initialization via the expression ::new(p) U(), where p has type void* and points to storage suitable to hold an object of type U.

  • When a (sub)object of non-array type U is specified to be value-initialized, allocate_shared shall perform this initialization via the expression std::allocator_traits<A2>::construct(a2, p), where p points to storage suitable to hold an object of type U and a2 of type A2 is a rebound copy of a such that its value_type is U.

  • Array elements are initialized in ascending order of their addresses.

  • When the lifetime of the object managed by the return value ends, or when the initialization of an array element throws an exception, the initialized elements should be destroyed in the reverse order of their construction.

Note
These functions will typically allocate more memory than the total size of the element objects to allow for internal bookkeeping structures such as the reference counts.

Free Functions

template<class T, class... Args>
  shared_ptr<T> make_shared(Args&&... args);
template<class T, class A, class... Args>
  shared_ptr<T> allocate_shared(const A& a, Args&&... args);
Remarks

These overloads shall only participate in overload resolution when T is not an array type.

Returns

A shared_ptr to an object of type T, constructed from args....

Examples
  • auto p = make_shared<int>();

  • auto p = make_shared<std::vector<int> >(16, 1);

template<class T>
  shared_ptr<T> make_shared(std::size_t n);
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a, std::size_t n);
Remarks

These overloads shall only participate in overload resolution when T is an array type of the form U[].

Returns

A shared_ptr to a sequence of n value-initialized objects of type U.

Examples
  • auto p = make_shared<double[]>(1024);

  • auto p = make_shared<double[][2][2]>(6);

template<class T>
  shared_ptr<T> make_shared();
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a);
Remarks

These overloads shall only participate in overload resolution when T is an array type of the form U[N].

Returns

A shared_ptr to a sequence of N value-initialized objects of type U.

Examples
  • auto p = make_shared<double[1024]>();

  • auto p = make_shared<double[6][2][2]>();

template<class T> shared_ptr<T>
  make_shared(std::size_t n, const remove_extent_t<T>& v);
template<class T, class A> shared_ptr<T>
  allocate_shared(const A& a, std::size_t n, const remove_extent_t<T>& v);
Remarks

These overloads shall only participate in overload resolution when T is an array type of the form U[].

Returns

A shared_ptr to a sequence of n objects of type U, each initialized to v.

Examples
  • auto p = make_shared<double[]>(1024, 1.0);

  • auto p = make_shared<double[][2]>(6, {1.0, 0.0});

  • auto p = make_shared<std::vector<int>[]>(4, {1, 2});

template<class T>
  shared_ptr<T> make_shared(const remove_extent_t<T>& v);
template<class T, class A>
  shared_ptr<T> allocate_shared(const A& a, const remove_extent_t<T>& v);
Remarks

These overloads shall only participate in overload resolution when T is an array type of the form U[N].

Returns

A shared_ptr to a sequence of N objects of type U, each initialized to v.

Examples
  • auto p = make_shared<double[1024]>(1.0);

  • auto p = make_shared<double[6][2]>({1.0, 0.0});

  • auto p = make_shared<std::vector<int>[4]>({1, 2});

template<class T>
  shared_ptr<T> make_shared_noinit();
template<class T, class A>
  shared_ptr<T> allocate_shared_noinit(const A& a);
Remarks

These overloads shall only participate in overload resolution when T is not an array type, or an array type of the U[N].

Returns

A shared_ptr to a default-initialized object of type T, or a sequence of N default-initialized objects of type U, respectively.

Example

auto p = make_shared_noinit<double[1024]>();

template<class T>
  shared_ptr<T> make_shared_noinit(std::size_t n);
template<class T, class A>
  shared_ptr<T> allocate_shared_noinit(const A& a, std::size_t n);
Remarks

These overloads shall only participate in overload resolution when T is an array type of the form U[].

Returns

A shared_ptr to a sequence of n default-initialized objects of type U.

Example

auto p = make_shared_noinit<double[]>(1024);

enable_shared_from_this

Description

The class template enable_shared_from_this is used as a base class that allows a shared_ptr or a weak_ptr to the current object to be obtained from within a member function.

enable_shared_from_this<T> defines two member functions called shared_from_this that return a shared_ptr<T> and shared_ptr<T const>, depending on constness, to this. It also defines two member functions called weak_from_this that return a corresponding weak_ptr.

Example

#include <boost/enable_shared_from_this.hpp>
#include <boost/shared_ptr.hpp>
#include <cassert>

class Y: public boost::enable_shared_from_this<Y>
{
public:

    boost::shared_ptr<Y> f()
    {
        return shared_from_this();
    }
};

int main()
{
    boost::shared_ptr<Y> p(new Y);
    boost::shared_ptr<Y> q = p->f();
    assert(p == q);
    assert(!(p < q || q < p)); // p and q must share ownership
}

Synopsis

enable_shared_from_this is defined in <boost/smart_ptr/enable_shared_from_this.hpp>.

namespace boost {

  template<class T> class enable_shared_from_this {
  private:

    // exposition only
    weak_ptr<T> weak_this_;

  protected:

    enable_shared_from_this() = default;
    ~enable_shared_from_this() = default;

    enable_shared_from_this(const enable_shared_from_this&) noexcept;
    enable_shared_from_this& operator=(const enable_shared_from_this&) noexcept;

  public:

    shared_ptr<T> shared_from_this();
    shared_ptr<T const> shared_from_this() const;

    weak_ptr<T> weak_from_this() noexcept;
    weak_ptr<T const> weak_from_this() const noexcept;
  }
}

Members

enable_shared_from_this(enable_shared_from_this const &) noexcept;
  • Effects

    Default-constructs weak_this_.

Note
weak_this_ is not copied from the argument.
enable_shared_from_this& operator=(enable_shared_from_this const &) noexcept;
  • Returns

    *this.

Note
weak_this_ is unchanged.
template<class T> shared_ptr<T> shared_from_this();
template<class T> shared_ptr<T const> shared_from_this() const;
  • Returns

    shared_ptr<T>(weak_this_).

Note
These members throw bad_weak_ptr when *this is not owned by a shared_ptr.
Note

weak_this_ is initialized by shared_ptr to a copy of itself when it’s constructed by a pointer to *this. For example, in the following code:

class Y: public boost::enable_shared_from_this<Y> {};

int main()
{
    boost::shared_ptr<Y> p(new Y);
}

the construction of p will automatically initialize p->weak_this_ to p.

template<class T> weak_ptr<T> weak_from_this() noexcept;
template<class T> weak_ptr<T const> weak_from_this() const noexcept;
  • Returns

    weak_this_.

make_unique: Creating unique_ptr

Description

The make_unique function templates provide convenient and safe ways to create std::unique_ptr objects.

Rationale

The C++11 standard introduced std::unique_ptr but did not provide any make_unique utility like std::make_shared that provided the same exception safety and facility to avoid writing new expressions. Before it was implemented by some standard library vendors (and prior to the C++14 standard introducing std::make_unique), this library provided it due to requests from users.

This library also provides additional overloads of make_unique for default-initialization, when users do not need or want to incur the expense of value-initialization. The C++ standard does not yet provide this feature with std::make_unique.

Synopsis

make_unique is defined in <boost/smart_ptr/make_unique.hpp>.

namespace boost {
  // only if T is not an array type
  template<class T, class... Args>
    std::unique_ptr<T> make_unique(Args&&... args);

  // only if T is not an array type
  template<class T>
    std::unique_ptr<T> make_unique(remove_reference_t<T>&& v);

  // only if T is an array type of the form U[]
  template<class T>
    std::unique_ptr<T> make_unique(std::size_t n);

  // only if T is not an array type
  template<class T>
    std::unique_ptr<T> make_unique_noinit();

  // only if T is an array type of the form U[]
  template<class T>
    std::unique_ptr<T> make_unique_noinit(std::size_t n);
}

Free Functions

template<class T, class... Args>
  std::unique_ptr<T> make_unique(Args&&... args);
Remarks

These overloads shall only participate in overload resolution when T is not an array type.

Returns

std::unique_ptr<T>(new T(std::forward<Args>(args)...).

Example

auto p = make_unique<int>();

template<class T>
  std::unique_ptr<T> make_unique(remove_reference_t<T>&& v);
Remarks

These overloads shall only participate in overload resolution when T is not an array type.

Returns

std::unique_ptr<T>(new T(std::move(v)).

Example

auto p = make_unique<std::vector<int> >({1, 2});

template<class T>
  std::unique_ptr<T> make_unique(std::size_t n);
Remarks

These overloads shall only participate in overload resolution when T is an array type of the form U[].

Returns

std::unique_ptr<U[]>(new U[n]()).

Example

auto p = make_unique<double[]>(1024);

template<class T>
  std::unique_ptr<T> make_unique_noinit();
Remarks

These overloads shall only participate in overload resolution when T is not an array type.

Returns

std::unique_ptr<T>(new T).

Example

auto p = make_unique_noinit<double[1024]>();

template<class T>
  std::unique_ptr<T> make_unique_noinit(std::size_t n);
Remarks

These overloads shall only participate in overload resolution when T is an array type of the form U[].

Returns

std::unique_ptr<U[]>(new U[n]).

Example

auto p = make_unique_noinit<double[]>(1024);

intrusive_ptr: Managing Objects with Embedded Counts

Description

The intrusive_ptr class template stores a pointer to an object with an embedded reference count. Every new intrusive_ptr instance increments the reference count by using an unqualified call to the function intrusive_ptr_add_ref, passing it the pointer as an argument. Similarly, when an intrusive_ptr is destroyed, it calls intrusive_ptr_release; this function is responsible for destroying the object when its reference count drops to zero. The user is expected to provide suitable definitions of these two functions. On compilers that support argument-dependent lookup, intrusive_ptr_add_ref and intrusive_ptr_release should be defined in the namespace that corresponds to their parameter; otherwise, the definitions need to go in namespace boost. The library provides a helper base class template intrusive_ref_counter which may help adding support for intrusive_ptr to user types.

The class template is parameterized on T, the type of the object pointed to. intrusive_ptr<T> can be implicitly converted to intrusive_ptr<U> whenever T* can be implicitly converted to U*.

The main reasons to use intrusive_ptr are:

  • Some existing frameworks or OSes provide objects with embedded reference counts;

  • The memory footprint of intrusive_ptr is the same as the corresponding raw pointer;

  • intrusive_ptr<T> can be constructed from an arbitrary raw pointer of type T*.

As a general rule, if it isn’t obvious whether intrusive_ptr better fits your needs than shared_ptr, try a shared_ptr-based design first.

Synopsis

intrusive_ptr is defined in <boost/smart_ptr/intrusive_ptr.hpp>.

namespace boost {

  template<class T> class intrusive_ptr {
  public:

    typedef T element_type;

    intrusive_ptr() noexcept;
    intrusive_ptr(T * p, bool add_ref = true);

    intrusive_ptr(intrusive_ptr const & r);
    template<class Y> intrusive_ptr(intrusive_ptr<Y> const & r);

    ~intrusive_ptr();

    intrusive_ptr & operator=(intrusive_ptr const & r);
    template<class Y> intrusive_ptr & operator=(intrusive_ptr<Y> const & r);
    intrusive_ptr & operator=(T * r);

    void reset();
    void reset(T * r);
    void reset(T * r, bool add_ref);

    T & operator*() const noexcept;
    T * operator->() const noexcept;
    T * get() const noexcept;
    T * detach() noexcept;

    explicit operator bool () const noexcept;

    void swap(intrusive_ptr & b) noexept;
  };

  template<class T, class U>
    bool operator==(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator!=(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator==(intrusive_ptr<T> const & a, U * b) noexcept;

  template<class T, class U>
    bool operator!=(intrusive_ptr<T> const & a, U * b) noexcept;

  template<class T, class U>
    bool operator==(T * a, intrusive_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator!=(T * a, intrusive_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator<(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;

  template<class T> void swap(intrusive_ptr<T> & a, intrusive_ptr<T> & b) noexcept;

  template<class T> T * get_pointer(intrusive_ptr<T> const & p) noexcept;

  template<class T, class U>
    intrusive_ptr<T> static_pointer_cast(intrusive_ptr<U> const & r) noexcept;

  template<class T, class U>
    intrusive_ptr<T> const_pointer_cast(intrusive_ptr<U> const & r) noexcept;

  template<class T, class U>
    intrusive_ptr<T> dynamic_pointer_cast(intrusive_ptr<U> const & r) noexcept;

  template<class E, class T, class Y>
    std::basic_ostream<E, T> & operator<< (std::basic_ostream<E, T> & os,
      intrusive_ptr<Y> const & p);
}

Members

element_type

typedef T element_type;

Provides the type of the template parameter T.

constructors

intrusive_ptr() noexcept;
  • Postconditions

    get() == 0.

intrusive_ptr(T * p, bool add_ref = true);
  • Effects

    if(p != 0 && add_ref) intrusive_ptr_add_ref(p);.

    Postconditions

    get() == p.

intrusive_ptr(intrusive_ptr const & r);
template<class Y> intrusive_ptr(intrusive_ptr<Y> const & r);
  • Effects

    if(r.get() != 0) intrusive_ptr_add_ref(r.get());.

    Postconditions

    get() == r.get().

destructor

~intrusive_ptr();
  • Effects

    if(get() != 0) intrusive_ptr_release(get());.

assignment

intrusive_ptr & operator=(intrusive_ptr const & r);
template<class Y> intrusive_ptr & operator=(intrusive_ptr<Y> const & r);
intrusive_ptr & operator=(T * r);
  • Effects

    Equivalent to intrusive_ptr(r).swap(*this).

    Returns

    *this.

reset

void reset();
  • Effects

    Equivalent to intrusive_ptr().swap(*this).

void reset(T * r);
  • Effects

    Equivalent to intrusive_ptr(r).swap(*this).

void reset(T * r, bool add_ref);
  • Effects

    Equivalent to intrusive_ptr(r, add_ref).swap(*this).

indirection

T & operator*() const noexcept;
  • Requirements

    get() != 0.

    Returns

    *get().

T * operator->() const noexcept;
  • Requirements

    get() != 0.

    Returns

    get().

get

T * get() const noexcept;
  • Returns

    the stored pointer.

detach

T * detach() noexcept;
  • Returns

    the stored pointer.

    Postconditions

    get() == 0.

Note
The returned pointer has an elevated reference count. This allows conversion of an intrusive_ptr back to a raw pointer, without the performance overhead of acquiring and dropping an extra reference. It can be viewed as the complement of the non-reference-incrementing constructor.
Caution
Using detach escapes the safety of automatic reference counting provided by intrusive_ptr. It should by used only where strictly necessary (such as when interfacing to an existing API), and when the implications are thoroughly understood.

conversions

explicit operator bool () const noexcept;
  • Returns

    get() != 0.

Note
This conversion operator allows intrusive_ptr objects to be used in boolean contexts, like if (p && p->valid()) {}.
Note
On C++03 compilers, the return value is of an unspecified type.

swap

void swap(intrusive_ptr & b) noexcept;
  • Effects

    Exchanges the contents of the two smart pointers.

Free Functions

comparison

template<class T, class U>
  bool operator==(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;
  • Returns

    a.get() == b.get().

template<class T, class U>
  bool operator!=(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;
  • Returns

    a.get() != b.get().

template<class T, class U>
  bool operator==(intrusive_ptr<T> const & a, U * b) noexcept;
  • Returns

    a.get() == b.

template<class T, class U>
  bool operator!=(intrusive_ptr<T> const & a, U * b) noexcept;
  • Returns

    a.get() != b.

template<class T, class U>
  bool operator==(T * a, intrusive_ptr<U> const & b) noexcept;
  • Returns

    a == b.get().

template<class T, class U>
  bool operator!=(T * a, intrusive_ptr<U> const & b) noexcept;
  • Returns

    a != b.get().

template<class T, class U>
  bool operator<(intrusive_ptr<T> const & a, intrusive_ptr<U> const & b) noexcept;
  • Returns

    std::less<T *>()(a.get(), b.get()).

Note
Allows intrusive_ptr objects to be used as keys in associative containers.

swap

template<class T> void swap(intrusive_ptr<T> & a, intrusive_ptr<T> & b) noexcept;
  • Effects

    Equivalent to a.swap(b).

get_pointer

template<class T> T * get_pointer(intrusive_ptr<T> const & p) noexcept;
  • Returns

    p.get().

Note
Provided as an aid to generic programming. Used by mem_fn.

static_pointer_cast

template<class T, class U>
  intrusive_ptr<T> static_pointer_cast(intrusive_ptr<U> const & r) noexcept;
  • Returns

    intrusive_ptr<T>(static_cast<T*>(r.get())).

const_pointer_cast

template<class T, class U>
  intrusive_ptr<T> const_pointer_cast(intrusive_ptr<U> const & r) noexcept;
  • Returns

    intrusive_ptr<T>(const_cast<T*>(r.get())).

dynamic_pointer_cast

template<class T, class U>
  intrusive_ptr<T> dynamic_pointer_cast(intrusive_ptr<U> const & r) noexcept;
  • Returns

    intrusive_ptr<T>(dynamic_cast<T*>(r.get())).

operator<<

template<class E, class T, class Y>
  std::basic_ostream<E, T> & operator<< (std::basic_ostream<E, T> & os,
    intrusive_ptr<Y> const & p);
  • Effects

    os << p.get();.

    Returns

    os.

intrusive_ref_counter

Description

The intrusive_ref_counter class template implements a reference counter for a derived user’s class that is intended to be used with intrusive_ptr. The base class has associated intrusive_ptr_add_ref and intrusive_ptr_release functions which modify the reference counter as needed and destroy the user’s object when the counter drops to zero.

The class template is parameterized on Derived and CounterPolicy parameters. The first parameter is the user’s class that derives from intrusive_ref_counter. This type is needed in order to destroy the object correctly when there are no references to it left.

The second parameter is a policy that defines the nature of the reference counter. The library provides two such policies: thread_unsafe_counter and thread_safe_counter. The former instructs the intrusive_ref_counter base class to use a counter only suitable for a single-threaded use. Pointers to a single object that uses this kind of reference counter must not be used in different threads. The latter policy makes the reference counter thread-safe, unless the target platform doesn’t support threading. Since in modern systems support for threading is common, the default counter policy is thread_safe_counter.

Synopsis

intrusive_ref_counter is defined in <boost/smart_ptr/intrusive_ref_counter.hpp>.

namespace boost {
  struct thread_unsafe_counter;
  struct thread_safe_counter;

  template<class Derived, class CounterPolicy = thread_safe_counter>
  class intrusive_ref_counter {
  public:
    intrusive_ref_counter() noexcept;
    intrusive_ref_counter(const intrusive_ref_counter& v) noexcept;

    intrusive_ref_counter& operator=(const intrusive_ref_counter& v) noexcept;

    unsigned int use_count() const noexcept;

  protected:
    ~intrusive_ref_counter() = default;
  };

  template<class Derived, class CounterPolicy>
    void intrusive_ptr_add_ref(
      const intrusive_ref_counter<Derived, CounterPolicy>* p) noexcept;

  template<class Derived, class CounterPolicy>
    void intrusive_ptr_release(
      const intrusive_ref_counter<Derived, CounterPolicy>* p) noexcept;
}

Members

Constructors

intrusive_ref_counter() noexcept;
intrusive_ref_counter(const intrusive_ref_counter&) noexcept;
Postconditions

use_count() == 0.

Note
The pointer to the constructed object is expected to be passed to intrusive_ptr constructor, assignment operator or reset method, which would increment the reference counter.

Destructor

~intrusive_ref_counter();
Effects

Destroys the counter object.

Note
The destructor is protected so that the object can only be destroyed through the Derived class.

Assignment

intrusive_ref_counter& operator=(const intrusive_ref_counter& v) noexcept;
Effects

Does nothing, reference counter is not modified.

use_count

unsigned int use_count() const noexcept;
Returns

The current value of the reference counter.

Note
The returned value may not be actual in multi-threaded applications.

Free Functions

intrusive_ptr_add_ref

template<class Derived, class CounterPolicy>
  void intrusive_ptr_add_ref(
    const intrusive_ref_counter<Derived, CounterPolicy>* p) noexcept;
Effects

Increments the reference counter.

intrusive_ptr_release

template<class Derived, class CounterPolicy>
  void intrusive_ptr_release(
    const intrusive_ref_counter<Derived, CounterPolicy>* p) noexcept;
Effects

Decrements the reference counter. If the reference counter reaches 0, calls delete static_cast<const Derived*>(p).

local_shared_ptr: Shared Ownership within a Single Thread

Description

local_shared_ptr is nearly identical to shared_ptr, with the only difference of note being that its reference count is updated with non-atomic operations. As such, a local_shared_ptr and all its copies must reside in (be local to) a single thread (hence the name.)

local_shared_ptr can be converted to shared_ptr and vice versa. Creating a local_shared_ptr from a shared_ptr creates a new local reference count; this means that two local_shared_ptr instances, both created from the same shared_ptr, refer to the same object but don’t share the same count, and as such, can safely be used by two different threads.

Code Example 9. Two local_shared_ptr instances created from a shared_ptr
shared_ptr<X> p1( new X );

local_shared_ptr<X> p2( p1 ); // p2.local_use_count() == 1
local_shared_ptr<X> p3( p1 ); // p3.local_use_count() also 1

Creating the second local_shared_ptr from the first one, however, does lead to the two sharing the same count:

Code Example 10. A local_shared_ptr created from another local_shared_ptr
shared_ptr<X> p1( new X );

local_shared_ptr<X> p2( p1 ); // p2.local_use_count() == 1
local_shared_ptr<X> p3( p2 ); // p3.local_use_count() == 2

Two shared_ptr instances created from the same local_shared_ptr do share ownership:

Code Example 11. Two shared_ptr instances created from a local_shared_ptr
local_shared_ptr<X> p1( new X );

shared_ptr<X> p2( p1 ); // p2.use_count() == 2
shared_ptr<X> p3( p1 ); // p3.use_count() == 3

Here p2.use_count() is 2, because p1 holds a reference, too.

One can think of local_shared_ptr<T> as shared_ptr<shared_ptr<T>>, with the outer shared_ptr using non-atomic operations for its count. Converting from local_shared_ptr to shared_ptr gives you a copy of the inner shared_ptr; converting from shared_ptr wraps it into an outer shared_ptr with a non-atomic use count (conceptually speaking) and returns the result.

Synopsis

local_shared_ptr is defined in <boost/smart_ptr/local_shared_ptr.hpp>.

namespace boost {

  template<class T> class local_shared_ptr {
  public:

    typedef /*see below*/ element_type;

    // constructors

    constexpr local_shared_ptr() noexcept;
    constexpr local_shared_ptr(std::nullptr_t) noexcept;

    template<class Y> explicit local_shared_ptr(Y * p);

    template<class Y, class D> local_shared_ptr(Y * p, D d);
    template<class D> local_shared_ptr(std::nullptr_t p, D d);

    template<class Y, class D, class A> local_shared_ptr(Y * p, D d, A a);
    template<class D, class A> local_shared_ptr(std::nullptr_t p, D d, A a);

    local_shared_ptr(local_shared_ptr const & r) noexcept;
    template<class Y> local_shared_ptr(local_shared_ptr<Y> const & r) noexcept;

    local_shared_ptr(local_shared_ptr && r) noexcept;
    template<class Y> local_shared_ptr(local_shared_ptr<Y> && r) noexcept;

    template<class Y> local_shared_ptr( shared_ptr<Y> const & r );
    template<class Y> local_shared_ptr( shared_ptr<Y> && r );

    template<class Y> local_shared_ptr(local_shared_ptr<Y> const & r, element_type * p) noexcept;
    template<class Y> local_shared_ptr(local_shared_ptr<Y> && r, element_type * p) noexcept;

    template<class Y, class D> local_shared_ptr(std::unique_ptr<Y, D> && r);

    // destructor

    ~local_shared_ptr() noexcept;

    // assignment

    local_shared_ptr & operator=(local_shared_ptr const & r) noexcept;
    template<class Y> local_shared_ptr & operator=(local_shared_ptr<Y> const & r) noexcept;

    local_shared_ptr & operator=(local_shared_ptr const && r) noexcept;
    template<class Y> local_shared_ptr & operator=(local_shared_ptr<Y> const && r) noexcept;

    template<class Y, class D> local_shared_ptr & operator=(std::unique_ptr<Y, D> && r);

    local_shared_ptr & operator=(std::nullptr_t) noexcept;

    // reset

    void reset() noexcept;

    template<class Y> void reset(Y * p);
    template<class Y, class D> void reset(Y * p, D d);
    template<class Y, class D, class A> void reset(Y * p, D d, A a);

    template<class Y> void reset(local_shared_ptr<Y> const & r, element_type * p) noexcept;
    template<class Y> void reset(local_shared_ptr<Y> && r, element_type * p) noexcept;

    // accessors

    T & operator*() const noexcept; // only valid when T is not an array type
    T * operator->() const noexcept; // only valid when T is not an array type

    // only valid when T is an array type
    element_type & operator[](std::ptrdiff_t i) const noexcept;

    element_type * get() const noexcept;

    long local_use_count() const noexcept;

    // conversions

    explicit operator bool() const noexcept;

    template<class Y> operator shared_ptr<Y>() const noexcept;
    template<class Y> operator weak_ptr<Y>() const noexcept;

    // swap

    void swap(local_shared_ptr & b) noexcept;

    // owner_before

    template<class Y> bool owner_before(local_shared_ptr<Y> const & rhs) const noexcept;
  };

  // comparisons

  template<class T, class U>
    bool operator==(local_shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
  template<class T, class U>
    bool operator==(local_shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;
  template<class T, class U>
    bool operator==(shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;

  template<class T, class U>
    bool operator!=(local_shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
  template<class T, class U>
    bool operator!=(local_shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;
  template<class T, class U>
    bool operator!=(shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;

  template<class T> bool operator==(local_shared_ptr<T> const & p, std::nullptr_t) noexcept;
  template<class T> bool operator==(std::nullptr_t, local_shared_ptr<T> const & p) noexcept;

  template<class T> bool operator!=(local_shared_ptr<T> const & p, std::nullptr_t) noexcept;
  template<class T> bool operator!=(std::nullptr_t, local_shared_ptr<T> const & p) noexcept;

  template<class T, class U>
    bool operator<(local_shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;

  // swap

  template<class T> void swap(local_shared_ptr<T> & a, local_shared_ptr<T> & b) noexcept;

  // get_pointer

  template<class T>
    typename local_shared_ptr<T>::element_type *
      get_pointer(local_shared_ptr<T> const & p) noexcept;

  // casts

  template<class T, class U>
    local_shared_ptr<T> static_pointer_cast(local_shared_ptr<U> const & r) noexcept;

  template<class T, class U>
    local_shared_ptr<T> const_pointer_cast(local_shared_ptr<U> const & r) noexcept;

  template<class T, class U>
    local_shared_ptr<T> dynamic_pointer_cast(local_shared_ptr<U> const & r) noexcept;

  template<class T, class U>
    local_shared_ptr<T> reinterpret_pointer_cast(local_shared_ptr<U> const & r) noexcept;

  // stream I/O

  template<class E, class T, class Y>
    std::basic_ostream<E, T> &
      operator<< (std::basic_ostream<E, T> & os, local_shared_ptr<Y> const & p);

  // get_deleter

  template<class D, class T> D * get_deleter(local_shared_ptr<T> const & p) noexcept;
}

Members

element_type

typedef ... element_type;

element_type is T when T is not an array type, and U when T is U[] or U[N].

default constructor

constexpr local_shared_ptr() noexcept;
constexpr local_shared_ptr(std::nullptr_t) noexcept;
  • Effects

    Constructs an empty local_shared_ptr.

    Postconditions

    local_use_count() == 0 && get() == 0.

pointer constructor

template<class Y> explicit local_shared_ptr(Y * p);
  • Effects

    Constructs a local_shared_ptr that owns shared_ptr<T>( p ).

    Postconditions

    local_use_count() == 1 && get() == p.

    Throws

    std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

constructors taking a deleter

template<class Y, class D> local_shared_ptr(Y * p, D d);
template<class D> local_shared_ptr(std::nullptr_t p, D d);
  • Effects

    Constructs a local_shared_ptr that owns shared_ptr<T>( p, d ).

    Postconditions

    local_use_count() == 1 && get() == p.

    Throws

    std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

template<class Y, class D, class A> local_shared_ptr(Y * p, D d, A a);
template<class D, class A> local_shared_ptr(std::nullptr_t p, D d, A a);
  • Effects

    Constructs a local_shared_ptr that owns shared_ptr<T>( p, d, a ).

    Postconditions

    local_use_count() == 1 && get() == p.

    Throws

    std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

copy and converting constructors

local_shared_ptr(local_shared_ptr const & r) noexcept;
template<class Y> local_shared_ptr(local_shared_ptr<Y> const & r) noexcept;
  • Requires

    Y* should be convertible to T*.

    Effects

    If r is empty, constructs an empty local_shared_ptr; otherwise, constructs a local_shared_ptr that shares ownership with r.

    Postconditions

    get() == r.get() && local_use_count() == r.local_use_count().

move constructors

local_shared_ptr(local_shared_ptr && r) noexcept;
template<class Y> local_shared_ptr(local_shared_ptr<Y> && r) noexcept;
  • Requires

    Y* should be convertible to T*.

    Effects

    Move-constructs a local_shared_ptr from r.

    Postconditions

    *this contains the old value of r. r is empty and r.get() == 0.

shared_ptr constructor

template<class Y> local_shared_ptr( shared_ptr<Y> const & r );
template<class Y> local_shared_ptr( shared_ptr<Y> && r );
  • Effects

    Constructs a local_shared_ptr that owns r.

    Postconditions

    local_use_count() == 1. get() returns the old value of r.get().

    Throws

    std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

aliasing constructor

template<class Y> local_shared_ptr(local_shared_ptr<Y> const & r, element_type * p) noexcept;
  • Effects

    constructs a local_shared_ptr that shares ownership with r and stores p.

    Postconditions

    get() == p && local_use_count() == r.local_use_count().

aliasing move constructor

template<class Y> local_shared_ptr(local_shared_ptr<Y> && r, element_type * p) noexcept;
  • Effects

    Move-constructs a local_shared_ptr from r, while storing p instead.

    Postconditions

    get() == p and local_use_count() equals the old count of r. r is empty and r.get() == 0.

unique_ptr constructor

template<class Y, class D> local_shared_ptr(std::unique_ptr<Y, D> && r);
  • Requires

    Y* should be convertible to T*.

    Effects
    • When r.get() == 0, equivalent to local_shared_ptr();

    • Otherwise, constructs a local_shared_ptr that owns shared_ptr<T>( std::move(r) ).

    Throws

    std::bad_alloc, or an implementation-defined exception when a resource other than memory could not be obtained.

    Exception safety

    If an exception is thrown, the constructor has no effect.

destructor

~local_shared_ptr() noexcept;
  • Effects
    • If *this is empty, or shares ownership with another local_shared_ptr instance (local_use_count() > 1), there are no side effects.

    • Otherwise, destroys the owned shared_ptr.

assignment

local_shared_ptr & operator=(local_shared_ptr const & r) noexcept;
template<class Y> local_shared_ptr & operator=(local_shared_ptr<Y> const & r) noexcept;
  • Effects

    Equivalent to local_shared_ptr(r).swap(*this).

    Returns

    *this.

local_shared_ptr & operator=(local_shared_ptr && r) noexcept;
template<class Y> local_shared_ptr & operator=(local_shared_ptr<Y> && r) noexcept;
template<class Y, class D> local_shared_ptr & operator=(std::unique_ptr<Y, D> && r);
  • Effects

    Equivalent to local_shared_ptr(std::move(r)).swap(*this).

    Returns

    *this.

local_shared_ptr & operator=(std::nullptr_t) noexcept;
  • Effects

    Equivalent to local_shared_ptr().swap(*this).

    Returns

    *this.

reset

void reset() noexcept;
  • Effects

    Equivalent to local_shared_ptr().swap(*this).

template<class Y> void reset(Y * p);
  • Effects

    Equivalent to local_shared_ptr(p).swap(*this).

template<class Y, class D> void reset(Y * p, D d);
  • Effects

    Equivalent to local_shared_ptr(p, d).swap(*this).

template<class Y, class D, class A> void reset(Y * p, D d, A a);
  • Effects

    Equivalent to local_shared_ptr(p, d, a).swap(*this).

template<class Y> void reset(local_shared_ptr<Y> const & r, element_type * p) noexcept;
  • Effects

    Equivalent to local_shared_ptr(r, p).swap(*this).

template<class Y> void reset(local_shared_ptr<Y> && r, element_type * p) noexcept;
  • Effects

    Equivalent to local_shared_ptr(std::move(r), p).swap(*this).

indirection

T & operator*() const noexcept;
  • Requires

    T should not be an array type.

    Returns

    *get().

T * operator->() const noexcept;
  • Requires

    T should not be an array type.

    Returns

    get().

element_type & operator[](std::ptrdiff_t i) const noexcept;
  • Requires

    T should be an array type. The stored pointer must not be 0. i >= 0. If T is U[N], i < N.

    Returns

    get()[i].

get

element_type * get() const noexcept;
  • Returns

    The stored pointer.

local_use_count

long local_use_count() const noexcept;
  • Returns

    The number of local_shared_ptr objects, *this included, that share ownership with *this, or 0 when *this is empty.

conversions

explicit operator bool() const noexcept;
  • Returns

    get() != 0.

Note
On C++03 compilers, the return value is of an unspecified type.
template<class Y> operator shared_ptr<Y>() const noexcept;
template<class Y> operator weak_ptr<Y>() const noexcept;
  • Requires

    T* should be convertible to Y*.

    Returns

    a copy of the owned shared_ptr.

swap

void swap(local_shared_ptr & b) noexcept;
  • Effects

    Exchanges the contents of the two smart pointers.

owner_before

template<class Y> bool owner_before(local_shared_ptr<Y> const & rhs) const noexcept;
  • Effects

    See the description of operator<.

Free Functions

comparison

template<class T, class U>
  bool operator==(local_shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
template<class T, class U>
  bool operator==(local_shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;
template<class T, class U>
  bool operator==(shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
  • Returns

    a.get() == b.get().

template<class T, class U>
  bool operator!=(local_shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
template<class T, class U>
  bool operator!=(local_shared_ptr<T> const & a, shared_ptr<U> const & b) noexcept;
template<class T, class U>
  bool operator!=(shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
  • Returns

    a.get() != b.get().

template<class T> bool operator==(local_shared_ptr<T> const & p, std::nullptr_t) noexcept;
template<class T> bool operator==(std::nullptr_t, local_shared_ptr<T> const & p) noexcept;
  • Returns

    p.get() == 0.

template<class T> bool operator!=(local_shared_ptr<T> const & p, std::nullptr_t) noexcept;
template<class T> bool operator!=(std::nullptr_t, local_shared_ptr<T> const & p) noexcept;
  • Returns

    p.get() != 0.

template<class T, class U>
  bool operator<(local_shared_ptr<T> const & a, local_shared_ptr<U> const & b) noexcept;
  • Returns

    An unspecified value such that

    • operator< is a strict weak ordering as described in section [lib.alg.sorting] of the C++ standard;

    • under the equivalence relation defined by operator<, !(a < b) && !(b < a), two local_shared_ptr instances are equivalent if and only if they share ownership or are both empty.

Note
Allows local_shared_ptr objects to be used as keys in associative containers.
Note
The rest of the comparison operators are omitted by design.

swap

template<class T> void swap(local_shared_ptr<T> & a, local_shared_ptr<T> & b) noexcept;
  • Effects

    Equivalent to a.swap(b).

get_pointer

template<class T>
  typename local_shared_ptr<T>::element_type *
    get_pointer(local_shared_ptr<T> const & p) noexcept;
  • Returns

    p.get().

Note
Provided as an aid to generic programming. Used by mem_fn.

static_pointer_cast

template<class T, class U>
  local_shared_ptr<T> static_pointer_cast(local_shared_ptr<U> const & r) noexcept;
  • Requires

    The expression static_cast<T*>( (U*)0 ) must be well-formed.

    Returns

    local_shared_ptr<T>( r, static_cast<typename local_shared_ptr<T>::element_type*>(r.get()) ).

Caution
The seemingly equivalent expression local_shared_ptr<T>(static_cast<T*>(r.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

const_pointer_cast

template<class T, class U>
  local_shared_ptr<T> const_pointer_cast(local_shared_ptr<U> const & r) noexcept;
  • Requires

    The expression const_cast<T*>( (U*)0 ) must be well-formed.

    Returns

    local_shared_ptr<T>( r, const_cast<typename local_shared_ptr<T>::element_type*>(r.get()) ).

dynamic_pointer_cast

template<class T, class U>
    local_shared_ptr<T> dynamic_pointer_cast(local_shared_ptr<U> const & r) noexcept;
  • Requires

    The expression dynamic_cast<T*>( (U*)0 ) must be well-formed.

    Returns
    • When dynamic_cast<typename local_shared_ptr<T>::element_type*>(r.get()) returns a nonzero value p, local_shared_ptr<T>(r, p);

    • Otherwise, local_shared_ptr<T>().

reinterpret_pointer_cast

template<class T, class U>
  local_shared_ptr<T> reinterpret_pointer_cast(local_shared_ptr<U> const & r) noexcept;
  • Requires

    The expression reinterpret_cast<T*>( (U*)0 ) must be well-formed.

    Returns

    local_shared_ptr<T>( r, reinterpret_cast<typename local_shared_ptr<T>::element_type*>(r.get()) ).

operator<<

template<class E, class T, class Y>
  std::basic_ostream<E, T> &
    operator<< (std::basic_ostream<E, T> & os, local_shared_ptr<Y> const & p);
  • Effects

    os << p.get();.

    Returns

    os.

get_deleter

template<class D, class T>
  D * get_deleter(local_shared_ptr<T> const & p) noexcept;
  • Returns

    If *this owns a shared_ptr instance p, get_deleter<D>( p ), otherwise 0.

make_local_shared: Creating local_shared_ptr

Description

The function templates make_local_shared and allocate_local_shared provide convenient, safe and efficient ways to create local_shared_ptr objects. They are analogous to make_shared and allocate_shared for shared_ptr.

Synopsis

make_local_shared and allocate_local_shared are defined in <boost/smart_ptr/make_local_shared.hpp>.

namespace boost {
  // only if T is not an array type
  template<class T, class... Args>
    local_shared_ptr<T> make_local_shared(Args&&... args);
  template<class T, class A, class... Args>
    local_shared_ptr<T> allocate_local_shared(const A& a, Args&&... args);

  // only if T is an array type of the form U[]
  template<class T>
    local_shared_ptr<T> make_local_shared(std::size_t n);
  template<class T, class A>
    local_shared_ptr<T> allocate_local_shared(const A& a, std::size_t n);

  // only if T is an array type of the form U[N]
  template<class T>
    local_shared_ptr<T> make_local_shared();
  template<class T, class A>
    local_shared_ptr<T> allocate_local_shared(const A& a);

  // only if T is an array type of the form U[]
  template<class T>
    local_shared_ptr<T> make_local_shared(std::size_t n,
      const remove_extent_t<T>& v);
  template<class T, class A>
    local_shared_ptr<T> allocate_local_shared(const A& a, std::size_t n,
      const remove_extent_t<T>& v);

  // only if T is an array type of the form U[N]
  template<class T>
    local_shared_ptr<T> make_local_shared(const remove_extent_t<T>& v);
  template<class T, class A>
    local_shared_ptr<T> allocate_local_shared(const A& a,
      const remove_extent_t<T>& v);

  // only if T is not an array type of the form U[]
  template<class T>
    local_shared_ptr<T> make_local_shared_noinit();
  template<class T, class A>
    local_shared_ptr<T> allocate_local_shared_noinit(const A& a);

  // only if T is an array type of the form U[N]
  template<class T>
    local_shared_ptr<T> make_local_shared_noinit(std::size_t n);
  template<class T, class A>
    local_shared_ptr<T> allocate_local_shared_noinit(const A& a,
      std::size_t n);
}

Description

The requirements and effects of these functions are the same as make_shared and allocate_shared, except that a local_shared_ptr is returned.

Generic Pointer Casts

Description

The pointer cast function templates (static_pointer_cast, dynamic_pointer_cast, const_pointer_cast, and reinterpret_pointer_cast) provide a way to write generic pointer castings for raw pointers, std::shared_ptr and std::unique_ptr.

There is test and example code in pointer_cast_test.cpp

Rationale

Boost smart pointers usually overload those functions to provide a mechanism to emulate pointers casts. For example, shared_ptr<T> implements a static pointer cast this way:

template<class T, class U>
  shared_ptr<T> static_pointer_cast(const shared_ptr<U>& p);

Pointer cast functions templates are overloads of static_pointer_cast, dynamic_pointer_cast, const_pointer_cast, and reinterpret_pointer_cast for raw pointers, std::shared_ptr and std::unique_ptr. This way when developing pointer type independent classes, for example, memory managers or shared memory compatible classes, the same code can be used for raw and smart pointers.

Synopsis

The generic pointer casts are defined in <boost/pointer_cast.hpp>.

namespace boost {
  template<class T, class U> T* static_pointer_cast(U* p) noexcept;
  template<class T, class U> T* dynamic_pointer_cast(U* p) noexcept;
  template<class T, class U> T* const_pointer_cast(U* p) noexcept;
  template<class T, class U> T* reinterpret_pointer_cast(U* p) noexcept;

  template<class T, class U> std::shared_ptr<T>
    static_pointer_cast(const std::shared_ptr<U>& p) noexcept;
  template<class T, class U> std::shared_ptr<T>
    dynamic_pointer_cast(const std::shared_ptr<U>& p) noexcept;
  template<class T, class U> std::shared_ptr<T>
    const_pointer_cast(const std::shared_ptr<U>& p) noexcept;
  template<class T, class U> std::shared_ptr<T>
    reinterpret_pointer_cast(const std::shared_ptr<U>& p) noexcept;

  template<class T, class U> std::unique_ptr<T>
    static_pointer_cast(std::unique_ptr<U>&& p) noexcept;
  template<class T, class U> std::unique_ptr<T>
    dynamic_pointer_cast(std::unique_ptr<U>&& p) noexcept;
  template<class T, class U> std::unique_ptr<T>
    const_pointer_cast(std::unique_ptr<U>&& p) noexcept;
  template<class T, class U> std::unique_ptr<T>
    reinterpret_pointer_cast(std::unique_ptr<U>&& p) noexcept;
}

Free Functions

static_pointer_cast

template<class T, class U> T* static_pointer_cast(U* p) noexcept;
Returns

static_cast<T*>(p)

template<class T, class U> std::shared_ptr<T>
  static_pointer_cast(const std::shared_ptr<U>& p) noexcept;
Returns

std::static_pointer_cast<T>(p)

template<class T, class U> std::unique_ptr<T>
  static_pointer_cast(std::unique_ptr<U>&& p) noexcept;
Requires

The expression static_cast<T*>((U*)0) must be well-formed.

Returns

std::unique_ptr<T>(static_cast<typename std::unique_ptr<T>::element_type*>(p.release())).

Caution
The seemingly equivalent expression std::unique_ptr<T>(static_cast<T*>(p.get())) will eventually result in undefined behavior, attempting to delete the same object twice.

dynamic_pointer_cast

template<class T, class U> T* dynamic_pointer_cast(U* p) noexcept;
Returns

dynamic_cast<T*>(p)

template<class T, class U> std::shared_ptr<T>
  dynamic_pointer_cast(const std::shared_ptr<U>& p) noexcept;
Returns

std::dynamic_pointer_cast<T>(p)

template<class T, class U> std::unique_ptr<T>
  dynamic_pointer_cast(std::unique_ptr<U>&& p) noexcept;
Requires
  • The expression static_cast<T*>((U*)0) must be well-formed.

  • T must have a virtual destructor.

Returns
  • When dynamic_cast<typename std::unique_ptr<T>::element_type*>(p.get()) returns a non-zero value, std::unique_ptr<T>(dynamic_cast<typename std::unique_ptr<T>::element_type*>(p.release()));.

  • Otherwise, std::unique_ptr<T>().

const_pointer_cast

template<class T, class U> T* const_pointer_cast(U* p) noexcept;
Returns

const_cast<T*>(p)

template<class T, class U> std::shared_ptr<T>
  const_pointer_cast(const std::shared_ptr<U>& p) noexcept;
Returns

std::const_pointer_cast<T>(p)

template<class T, class U> std::unique_ptr<T>
  const_pointer_cast(std::unique_ptr<U>&& p) noexcept;
Requires

The expression const_cast<T*>((U*)0) must be well-formed.

Returns

std::unique_ptr<T>(const_cast<typename std::unique_ptr<T>::element_type*>(p.release())).

reinterpret_pointer_cast

template<class T, class U> T* reinterpret_pointer_cast(U* p) noexcept;
Returns

reinterpret_cast<T*>(p)

template<class T, class U> std::shared_ptr<T>
  reinterpret_pointer_cast(const std::shared_ptr<U>& p) noexcept;
Returns

std::reinterpret_pointer_cast<T>(p)

template<class T, class U> std::unique_ptr<T>
  reinterpret_pointer_cast(std::unique_ptr<U>&& p) noexcept;
Requires

The expression reinterpret_cast<T*>((U*)0) must be well-formed.

Returns

std::unique_ptr<T>(reinterpret_cast<typename std::unique_ptr<T>::element_type*>(p.release())).

Example

The following example demonstrates how the generic pointer casts help us create pointer independent code.

#include <boost/pointer_cast.hpp>
#include <boost/shared_ptr.hpp>

class base {
public:
  virtual ~base() { }
};

class derived : public base { };

template<class Ptr>
void check_if_it_is_derived(const Ptr& ptr)
{
  assert(boost::dynamic_pointer_cast<derived>(ptr) != 0);
}

int main()
{
  base* ptr = new derived;
  boost::shared_ptr<base> sptr(new derived);

  check_if_it_is_derived(ptr);
  check_if_it_is_derived(sptr);

  delete ptr;
}

pointer_to_other

Description

The pointer_to_other utility provides a way, given a source pointer type, to obtain a pointer of the same type to another pointee type.

There is test/example code in pointer_to_other_test.cpp.

Rationale

When building pointer independent classes, like memory managers, allocators, or containers, there is often a need to define pointers generically, so that if a template parameter represents a pointer (for example, a raw or smart pointer to an int), we can define another pointer of the same type to another pointee (a raw or smart pointer to a float.)

template <class IntPtr> class FloatPointerHolder
{
    // Let's define a pointer to a float

    typedef typename boost::pointer_to_other
        <IntPtr, float>::type float_ptr_t;

    float_ptr_t float_ptr;
};

Synopsis

pointer_to_other is defined in <boost/smart_ptr/pointer_to_other.hpp>.

namespace boost {

  template<class T, class U> struct pointer_to_other;

  template<class T, class U,
    template <class> class Sp>
      struct pointer_to_other< Sp<T>, U >
  {
    typedef Sp<U> type;
  };

  template<class T, class T2, class U,
    template <class, class> class Sp>
      struct pointer_to_other< Sp<T, T2>, U >
  {
    typedef Sp<U, T2> type;
  };

  template<class T, class T2, class T3, class U,
    template <class, class, class> class Sp>
      struct pointer_to_other< Sp<T, T2, T3>, U >
  {
    typedef Sp<U, T2, T3> type;
  };

  template<class T, class U>
    struct pointer_to_other< T*, U >
  {
    typedef U* type;
  };
}

If these definitions are not correct for a specific smart pointer, we can define a specialization of pointer_to_other.

Example

// Let's define a memory allocator that can
// work with raw and smart pointers

#include <boost/pointer_to_other.hpp>

template <class VoidPtr>
class memory_allocator
{
    // Predefine a memory_block

    struct block;

    // Define a pointer to a memory_block from a void pointer
    // If VoidPtr is void *, block_ptr_t is block*
    // If VoidPtr is smart_ptr<void>, block_ptr_t is smart_ptr<block>

    typedef typename boost::pointer_to_other
        <VoidPtr, block>::type block_ptr_t;

    struct block
    {
        std::size_t size;
        block_ptr_t next_block;
    };

    block_ptr_t free_blocks;
};

As we can see, using pointer_to_other we can create pointer independent code.

atomic_shared_ptr

Description

The class template atomic_shared_ptr<T> implements the interface of std::atomic for a contained value of type shared_ptr<T>. Concurrent access to atomic_shared_ptr is not a data race.

Synopsis

atomic_shared_ptr is defined in <boost/smart_ptr/atomic_shared_ptr.hpp>.

namespace boost {

  template<class T> class atomic_shared_ptr {
  private:

    shared_ptr<T> p_; // exposition only

    atomic_shared_ptr(const atomic_shared_ptr&) = delete;
    atomic_shared_ptr& operator=(const atomic_shared_ptr&) = delete;

  public:

    constexpr atomic_shared_ptr() noexcept;
    atomic_shared_ptr( shared_ptr<T> p ) noexcept;

    atomic_shared_ptr& operator=( shared_ptr<T> r ) noexcept;

    bool is_lock_free() const noexcept;

    shared_ptr<T> load( int = 0 ) const noexcept;
    operator shared_ptr<T>() const noexcept;

    void store( shared_ptr<T> r, int = 0 ) noexcept;

    shared_ptr<T> exchange( shared_ptr<T> r, int = 0 ) noexcept;

    bool compare_exchange_weak( shared_ptr<T>& v, const shared_ptr<T>& w, int, int ) noexcept;
    bool compare_exchange_weak( shared_ptr<T>& v, const shared_ptr<T>& w, int = 0 ) noexcept;
    bool compare_exchange_strong( shared_ptr<T>& v, const shared_ptr<T>& w, int, int ) noexcept;
    bool compare_exchange_strong( shared_ptr<T>& v, const shared_ptr<T>& w, int = 0 ) noexcept;

    bool compare_exchange_weak( shared_ptr<T>& v, shared_ptr<T>&& w, int, int ) noexcept;
    bool compare_exchange_weak( shared_ptr<T>& v, shared_ptr<T>&& w, int = 0 ) noexcept;
    bool compare_exchange_strong( shared_ptr<T>& v, shared_ptr<T>&& w, int, int ) noexcept;
    bool compare_exchange_strong( shared_ptr<T>& v, shared_ptr<T>&& w, int = 0 ) noexcept;
  };
}

Members

constexpr atomic_shared_ptr() noexcept;
  • Effects

    Default-initializes p_.

atomic_shared_ptr( shared_ptr<T> p ) noexcept;
  • Effects

    Initializes p_ to p.

atomic_shared_ptr& operator=( shared_ptr<T> r ) noexcept;
  • Effects

    p_.swap(r).

    Returns

    *this.

bool is_lock_free() const noexcept;
  • Returns

    false.

Note
This implementation is not lock-free.
shared_ptr<T> load( int = 0 ) const noexcept;
operator shared_ptr<T>() const noexcept;
  • Returns

    p_.

Note
The int argument is intended to be of type memory_order, but is ignored. This implementation is lock-based and therefore always sequentially consistent.
void store( shared_ptr<T> r, int = 0 ) noexcept;
  • Effects

    p_.swap(r).

shared_ptr<T> exchange( shared_ptr<T> r, int = 0 ) noexcept;
  • Effects

    p_.swap(r).

    Returns

    The old value of p_.

bool compare_exchange_weak( shared_ptr<T>& v, const shared_ptr<T>& w, int, int ) noexcept;
bool compare_exchange_weak( shared_ptr<T>& v, const shared_ptr<T>& w, int = 0 ) noexcept;
bool compare_exchange_strong( shared_ptr<T>& v, const shared_ptr<T>& w, int, int ) noexcept;
bool compare_exchange_strong( shared_ptr<T>& v, const shared_ptr<T>& w, int = 0 ) noexcept;
  • Effects

    If p_ is equivalent to v, assigns w to p_, otherwise assigns p_ to v.

    Returns

    true if p_ was equivalent to v, false otherwise.

    Remarks

    Two shared_ptr instances are equivalent if they store the same pointer value and share ownership.

bool compare_exchange_weak( shared_ptr<T>& v, shared_ptr<T>&& w, int, int ) noexcept;
bool compare_exchange_weak( shared_ptr<T>& v, shared_ptr<T>&& w, int = 0 ) noexcept;
bool compare_exchange_strong( shared_ptr<T>& v, shared_ptr<T>&& w, int, int ) noexcept;
bool compare_exchange_strong( shared_ptr<T>& v, shared_ptr<T>&& w, int = 0 ) noexcept;
  • Effects

    If p_ is equivalent to v, assigns std::move(w) to p_, otherwise assigns p_ to v.

    Returns

    true if p_ was equivalent to v, false otherwise.

    Remarks

    The old value of w is not preserved in either case.

Appendix A: Smart Pointer Programming Techniques

Using incomplete classes for implementation hiding

A proven technique (that works in C, too) for separating interface from implementation is to use a pointer to an incomplete class as an opaque handle:

class FILE;

FILE * fopen(char const * name, char const * mode);
void fread(FILE * f, void * data, size_t size);
void fclose(FILE * f);

It is possible to express the above interface using shared_ptr, eliminating the need to manually call fclose:

class FILE;

shared_ptr<FILE> fopen(char const * name, char const * mode);
void fread(shared_ptr<FILE> f, void * data, size_t size);

This technique relies on shared_ptr’s ability to execute a custom deleter, eliminating the explicit call to fclose, and on the fact that shared_ptr<X> can be copied and destroyed when X is incomplete.

The "Pimpl" idiom

A C++ specific variation of the incomplete class pattern is the "Pimpl" idiom. The incomplete class is not exposed to the user; it is hidden behind a forwarding facade. shared_ptr can be used to implement a "Pimpl":

// file.hpp:

class file
{
private:

    class impl;
    shared_ptr<impl> pimpl_;

public:

    file(char const * name, char const * mode);

    // compiler generated members are fine and useful

    void read(void * data, size_t size);
};

// file.cpp:

#include "file.hpp"

class file::impl
{
private:

    impl(impl const &);
    impl & operator=(impl const &);

    // private data

public:

    impl(char const * name, char const * mode) { ... }
    ~impl() { ... }
    void read(void * data, size_t size) { ... }
};

file::file(char const * name, char const * mode): pimpl_(new impl(name, mode))
{
}

void file::read(void * data, size_t size)
{
    pimpl_->read(data, size);
}

The key thing to note here is that the compiler-generated copy constructor, assignment operator, and destructor all have a sensible meaning. As a result, file is CopyConstructible and Assignable, allowing its use in standard containers.

Using abstract classes for implementation hiding

Another widely used C++ idiom for separating inteface and implementation is to use abstract base classes and factory functions. The abstract classes are sometimes called "interfaces" and the pattern is known as "interface-based programming". Again, shared_ptr can be used as the return type of the factory functions:

// X.hpp:

class X
{
public:

    virtual void f() = 0;
    virtual void g() = 0;

protected:

    ~X() {}
};

shared_ptr<X> createX();

// X.cpp:

class X_impl: public X
{
private:

    X_impl(X_impl const &);
    X_impl & operator=(X_impl const &);

public:

    virtual void f()
    {
      // ...
    }

    virtual void g()
    {
      // ...
    }
};

shared_ptr<X> createX()
{
    shared_ptr<X> px(new X_impl);
    return px;
}

A key property of shared_ptr is that the allocation, construction, deallocation, and destruction details are captured at the point of construction, inside the factory function.

Note the protected and nonvirtual destructor in the example above. The client code cannot, and does not need to, delete a pointer to X; the shared_ptr<X> instance returned from createX will correctly call ~X_impl.

Preventing delete px.get()

It is often desirable to prevent client code from deleting a pointer that is being managed by shared_ptr. The previous technique showed one possible approach, using a protected destructor. Another alternative is to use a private deleter:

class X
{
private:

    ~X();

    class deleter;
    friend class deleter;

    class deleter
    {
    public:

        void operator()(X * p) { delete p; }
    };

public:

    static shared_ptr<X> create()
    {
        shared_ptr<X> px(new X, X::deleter());
        return px;
    }
};

Encapsulating allocation details, wrapping factory functions

shared_ptr can be used in creating C++ wrappers over existing C style library interfaces that return raw pointers from their factory functions to encapsulate allocation details. As an example, consider this interface, where CreateX might allocate X from its own private heap, ~X may be inaccessible, or X may be incomplete:

X * CreateX();
void DestroyX(X *);

The only way to reliably destroy a pointer returned by CreateX is to call DestroyX.

Here is how a shared_ptr-based wrapper may look like:

shared_ptr<X> createX()
{
    shared_ptr<X> px(CreateX(), DestroyX);
    return px;
}

Client code that calls createX still does not need to know how the object has been allocated, but now the destruction is automatic.

Using a shared_ptr to hold a pointer to a statically allocated object

Sometimes it is desirable to create a shared_ptr to an already existing object, so that the shared_ptr does not attempt to destroy the object when there are no more references left. As an example, the factory function:

shared_ptr<X> createX();

in certain situations may need to return a pointer to a statically allocated X instance.

The solution is to use a custom deleter that does nothing:

struct null_deleter
{
    void operator()(void const *) const
    {
    }
};

static X x;

shared_ptr<X> createX()
{
    shared_ptr<X> px(&x, null_deleter());
    return px;
}

The same technique works for any object known to outlive the pointer.

Using a shared_ptr to hold a pointer to a COM Object

Background: COM objects have an embedded reference count and two member functions that manipulate it. AddRef() increments the count. Release() decrements the count and destroys itself when the count drops to zero.

It is possible to hold a pointer to a COM object in a shared_ptr:

shared_ptr<IWhatever> make_shared_from_COM(IWhatever * p)
{
    p->AddRef();
    shared_ptr<IWhatever> pw(p, mem_fn(&IWhatever::Release));
    return pw;
}

Note, however, that shared_ptr copies created from pw will not "register" in the embedded count of the COM object; they will share the single reference created in make_shared_from_COM. Weak pointers created from pw will be invalidated when the last shared_ptr is destroyed, regardless of whether the COM object itself is still alive.

As explained in the mem_fn documentation, you need to #define BOOST_MEM_FN_ENABLE_STDCALL first.

Using a shared_ptr to hold a pointer to an object with an embedded reference count

This is a generalization of the above technique. The example assumes that the object implements the two functions required by intrusive_ptr, intrusive_ptr_add_ref and intrusive_ptr_release:

template<class T> struct intrusive_deleter
{
    void operator()(T * p)
    {
        if(p) intrusive_ptr_release(p);
    }
};

shared_ptr<X> make_shared_from_intrusive(X * p)
{
    if(p) intrusive_ptr_add_ref(p);
    shared_ptr<X> px(p, intrusive_deleter<X>());
    return px;
}

Using a shared_ptr to hold another shared ownership smart pointer

One of the design goals of shared_ptr is to be used in library interfaces. It is possible to encounter a situation where a library takes a shared_ptr argument, but the object at hand is being managed by a different reference counted or linked smart pointer.

It is possible to exploit shared_ptr’s custom deleter feature to wrap this existing smart pointer behind a shared_ptr facade:

template<class P> struct smart_pointer_deleter
{
private:

    P p_;

public:

    smart_pointer_deleter(P const & p): p_(p)
    {
    }

    void operator()(void const *)
    {
        p_.reset();
    }

    P const & get() const
    {
        return p_;
    }
};

shared_ptr<X> make_shared_from_another(another_ptr<X> qx)
{
    shared_ptr<X> px(qx.get(), smart_pointer_deleter< another_ptr<X> >(qx));
    return px;
}

One subtle point is that deleters are not allowed to throw exceptions, and the above example as written assumes that p_.reset() doesn’t throw. If this is not the case, p_.reset(); should be wrapped in a try {} catch(…​) {} block that ignores exceptions. In the (usually unlikely) event when an exception is thrown and ignored, p_ will be released when the lifetime of the deleter ends. This happens when all references, including weak pointers, are destroyed or reset.

Another twist is that it is possible, given the above shared_ptr instance, to recover the original smart pointer, using get_deleter:

void extract_another_from_shared(shared_ptr<X> px)
{
    typedef smart_pointer_deleter< another_ptr<X> > deleter;

    if(deleter const * pd = get_deleter<deleter>(px))
    {
        another_ptr<X> qx = pd->get();
    }
    else
    {
        // not one of ours
    }
}

Obtaining a shared_ptr from a raw pointer

Sometimes it is necessary to obtain a shared_ptr given a raw pointer to an object that is already managed by another shared_ptr instance. Example:

void f(X * p)
{
    shared_ptr<X> px(???);
}

Inside f, we’d like to create a shared_ptr to *p.

In the general case, this problem has no solution. One approach is to modify f to take a shared_ptr, if possible:

void f(shared_ptr<X> px);

The same transformation can be used for nonvirtual member functions, to convert the implicit this:

void X::f(int m);

would become a free function with a shared_ptr first argument:

void f(shared_ptr<X> this_, int m);

If f cannot be changed, but X uses intrusive counting, use make_shared_from_intrusive described above. Or, if it’s known that the shared_ptr created in f will never outlive the object, use a null deleter.

Obtaining a shared_ptr (weak_ptr) to this in a constructor

Some designs require objects to register themselves on construction with a central authority. When the registration routines take a shared_ptr, this leads to the question how could a constructor obtain a shared_ptr to this:

class X
{
public:

    X()
    {
        shared_ptr<X> this_(???);
    }
};

In the general case, the problem cannot be solved. The X instance being constructed can be an automatic variable or a static variable; it can be created on the heap:

shared_ptr<X> px(new X);

but at construction time, px does not exist yet, and it is impossible to create another shared_ptr instance that shares ownership with it.

Depending on context, if the inner shared_ptr this_ doesn’t need to keep the object alive, use a null_deleter as explained here and here. If X is supposed to always live on the heap, and be managed by a shared_ptr, use a static factory function:

class X
{
private:

    X() { ... }

public:

    static shared_ptr<X> create()
    {
        shared_ptr<X> px(new X);
        // use px as 'this_'
        return px;
    }
};

Obtaining a shared_ptr to this

Sometimes it is needed to obtain a shared_ptr from this in a virtual member function under the assumption that this is already managed by a shared_ptr. The transformations described in the previous technique cannot be applied.

A typical example:

class X
{
public:

    virtual void f() = 0;

protected:

    ~X() {}
};

class Y
{
public:

    virtual shared_ptr<X> getX() = 0;

protected:

    ~Y() {}
};

// --

class impl: public X, public Y
{
public:

    impl() { ... }

    virtual void f() { ... }

    virtual shared_ptr<X> getX()
    {
        shared_ptr<X> px(???);
        return px;
    }
};

The solution is to keep a weak pointer to this as a member in impl:

class impl: public X, public Y
{
private:

    weak_ptr<impl> weak_this;

    impl(impl const &);
    impl & operator=(impl const &);

    impl() { ... }

public:

    static shared_ptr<impl> create()
    {
        shared_ptr<impl> pi(new impl);
        pi->weak_this = pi;
        return pi;
    }

    virtual void f() { ... }

    virtual shared_ptr<X> getX()
    {
        shared_ptr<X> px(weak_this);
        return px;
    }
};

The library now includes a helper class template enable_shared_from_this that can be used to encapsulate the solution:

class impl: public X, public Y, public enable_shared_from_this<impl>
{
public:

    impl(impl const &);
    impl & operator=(impl const &);

public:

    virtual void f() { ... }

    virtual shared_ptr<X> getX()
    {
        return shared_from_this();
    }
}

Note that you no longer need to manually initialize the weak_ptr member in enable_shared_from_this. Constructing a shared_ptr to impl takes care of that.

Using shared_ptr as a smart counted handle

Some library interfaces use opaque handles, a variation of the incomplete class technique described above. An example:

typedef void * HANDLE;

HANDLE CreateProcess();
void CloseHandle(HANDLE);

Instead of a raw pointer, it is possible to use shared_ptr as the handle and get reference counting and automatic resource management for free:

typedef shared_ptr<void> handle;

handle createProcess()
{
    shared_ptr<void> pv(CreateProcess(), CloseHandle);
    return pv;
}

Using shared_ptr to execute code on block exit

shared_ptr<void> can automatically execute cleanup code when control leaves a scope.

  • Executing f(p), where p is a pointer:

    shared_ptr<void> guard(p, f);
  • Executing arbitrary code: f(x, y):

    shared_ptr<void> guard(static_cast<void*>(0), bind(f, x, y));

Using shared_ptr<void> to hold an arbitrary object

shared_ptr<void> can act as a generic object pointer similar to void*. When a shared_ptr<void> instance constructed as:

shared_ptr<void> pv(new X);

is destroyed, it will correctly dispose of the X object by executing ~X.

This propery can be used in much the same manner as a raw void* is used to temporarily strip type information from an object pointer. A shared_ptr<void> can later be cast back to the correct type by using static_pointer_cast.

Associating arbitrary data with heterogeneous shared_ptr instances

shared_ptr and weak_ptr support operator< comparisons required by standard associative containers such as std::map. This can be used to non-intrusively associate arbitrary data with objects managed by shared_ptr:

typedef int Data;

std::map<shared_ptr<void>, Data> userData;
// or std::map<weak_ptr<void>, Data> userData; to not affect the lifetime

shared_ptr<X> px(new X);
shared_ptr<int> pi(new int(3));

userData[px] = 42;
userData[pi] = 91;

Using shared_ptr as a CopyConstructible mutex lock

Sometimes it’s necessary to return a mutex lock from a function, and a noncopyable lock cannot be returned by value. It is possible to use shared_ptr as a mutex lock:

class mutex
{
public:

    void lock();
    void unlock();
};

shared_ptr<mutex> lock(mutex & m)
{
    m.lock();
    return shared_ptr<mutex>(&m, mem_fn(&mutex::unlock));
}

Better yet, the shared_ptr instance acting as a lock can be encapsulated in a dedicated shared_lock class:

class shared_lock
{
private:

    shared_ptr<void> pv;

public:

    template<class Mutex> explicit shared_lock(Mutex & m): pv((m.lock(), &m), mem_fn(&Mutex::unlock)) {}
};

shared_lock can now be used as:

shared_lock lock(m);

Note that shared_lock is not templated on the mutex type, thanks to shared_ptr<void>’s ability to hide type information.

Using shared_ptr to wrap member function calls

shared_ptr implements the ownership semantics required from the Wrap/CallProxy scheme described in Bjarne Stroustrup’s article "Wrapping C++ Member Function Calls" (available online at http://www.stroustrup.com/wrapper.pdf). An implementation is given below:

template<class T> class pointer
{
private:

    T * p_;

public:

    explicit pointer(T * p): p_(p)
    {
    }

    shared_ptr<T> operator->() const
    {
        p_->prefix();
        return shared_ptr<T>(p_, mem_fn(&T::suffix));
    }
};

class X
{
private:

    void prefix();
    void suffix();
    friend class pointer<X>;

public:

    void f();
    void g();
};

int main()
{
    X x;

    pointer<X> px(&x);

    px->f();
    px->g();
}

Delayed deallocation

In some situations, a single px.reset() can trigger an expensive deallocation in a performance-critical region:

class X; // ~X is expensive

class Y
{
    shared_ptr<X> px;

public:

    void f()
    {
        px.reset();
    }
};

The solution is to postpone the potential deallocation by moving px to a dedicated free list that can be periodically emptied when performance and response times are not an issue:

vector< shared_ptr<void> > free_list;

class Y
{
    shared_ptr<X> px;

public:

    void f()
    {
        free_list.push_back(px);
        px.reset();
    }
};

// periodically invoke free_list.clear() when convenient

Another variation is to move the free list logic to the construction point by using a delayed deleter:

struct delayed_deleter
{
    template<class T> void operator()(T * p)
    {
        try
        {
            shared_ptr<void> pv(p);
            free_list.push_back(pv);
        }
        catch(...)
        {
        }
    }
};

Weak pointers to objects not managed by a shared_ptr

Make the object hold a shared_ptr to itself, using a null_deleter:

class X
{
private:

    shared_ptr<X> this_;
    int i_;

public:

    explicit X(int i): this_(this, null_deleter()), i_(i)
    {
    }

    // repeat in all constructors (including the copy constructor!)

    X(X const & rhs): this_(this, null_deleter()), i_(rhs.i_)
    {
    }

    // do not forget to not assign this_ in the copy assignment

    X & operator=(X const & rhs)
    {
        i_ = rhs.i_;
    }

    weak_ptr<X> get_weak_ptr() const { return this_; }
};

When the object’s lifetime ends, X::this_ will be destroyed, and all weak pointers will automatically expire.

Appendix B: History and Acknowledgments

Summer 1994

Greg Colvin proposed to the C++ Standards Committee classes named auto_ptr and counted_ptr which were very similar to what we now call scoped_ptr and shared_ptr. In one of the very few cases where the Library Working Group’s recommendations were not followed by the full committee, counted_ptr was rejected and surprising transfer-of-ownership semantics were added to auto_ptr.

October 1998

Beman Dawes proposed reviving the original semantics under the names safe_ptr and counted_ptr, meeting of Per Andersson, Matt Austern, Greg Colvin, Sean Corfield, Pete Becker, Nico Josuttis, Dietmar Kühl, Nathan Myers, Chichiang Wan and Judy Ward. During the discussion, the four new class names were finalized, it was decided that there was no need to exactly follow the std::auto_ptr interface, and various function signatures and semantics were finalized.

Over the next three months, several implementations were considered for shared_ptr, and discussed on the boost.org mailing list. The implementation questions revolved around the reference count which must be kept, either attached to the pointed to object, or detached elsewhere. Each of those variants have themselves two major variants:

  • Direct detached: the shared_ptr contains a pointer to the object, and a pointer to the count.

  • Indirect detached: the shared_ptr contains a pointer to a helper object, which in turn contains a pointer to the object and the count.

  • Embedded attached: the count is a member of the object pointed to.

  • Placement attached: the count is attached via operator new manipulations.

Each implementation technique has advantages and disadvantages. We went so far as to run various timings of the direct and indirect approaches, and found that at least on Intel Pentium chips there was very little measurable difference. Kevlin Henney provided a paper he wrote on "Counted Body Techniques." Dietmar Kühl suggested an elegant partial template specialization technique to allow users to choose which implementation they preferred, and that was also experimented with.

But Greg Colvin and Jerry Schwarz argued that "parameterization will discourage users", and in the end we choose to supply only the direct implementation.

May 1999

In April and May, 1999, Valentin Bonnard and David Abrahams made a number of suggestions resulting in numerous improvements.

September 1999

Luis Coelho provided shared_ptr::swap and shared_array::swap.

November 1999

Darin Adler provided operator ==, operator !=, and std::swap and std::less specializations for shared types.

May 2001

Vladimir Prus suggested requiring a complete type on destruction. Refinement evolved in discussions including Dave Abrahams, Greg Colvin, Beman Dawes, Rainer Deyke, Peter Dimov, John Maddock, Vladimir Prus, Shankar Sai, and others.

January 2002

Peter Dimov reworked all four classes, adding features, fixing bugs, splitting them into four separate headers, and adding weak_ptr.

March 2003

Peter Dimov, Beman Dawes and Greg Colvin proposed shared_ptr and weak_ptr for inclusion in the Standard Library via the first Library Technical Report (known as TR1). The proposal was accepted and eventually went on to become a part of the C++ standard in its 2011 iteration.

July 2007

Peter Dimov and Beman Dawes proposed a number of enhancements to shared_ptr as it was entering the working paper that eventually became the C++11 standard.

November 2012

Glen Fernandes provided implementations of make_shared and allocate_shared for arrays. They achieve a single allocation for an array that can be initialized with constructor arguments or initializer lists as well as overloads for default initialization and no value initialization.

Peter Dimov aided this development by extending shared_ptr to support arrays via the syntax shared_ptr<T[]> and shared_ptr<T[N]>.

April 2013

Peter Dimov proposed the extension of shared_ptr to support arrays for inclusion into the standard, and it was accepted.

February 2014

Glen Fernandes updated make_shared and allocate_shared to conform to the specification in C++ standard paper N3870, and implemented make_unique for arrays and objects.

Peter Dimov and Glen Fernandes updated the scalar and array implementations, respectively, to resolve C++ standard library defect 2070.

February 2017

Glen Fernandes rewrote allocate_shared and make_shared for arrays for a more optimal and more maintainable implementation.

June 2017

Peter Dimov and Glen Fernandes rewrote the documentation in Asciidoc format.

Peter Dimov added atomic_shared_ptr and local_shared_ptr.

Appendix C: shared_array (deprecated)

Note
This facility is deprecated because a shared_ptr to T[] or T[N] is now available, and is superior in every regard.

Description

The shared_array class template stores a pointer to a dynamically allocated array. (Dynamically allocated array are allocated with the C++ new[] expression.) The object pointed to is guaranteed to be deleted when the last shared_array pointing to it is destroyed or reset.

Every shared_array meets the CopyConstructible and Assignable requirements of the C++ Standard Library, and so can be used in standard library containers. Comparison operators are supplied so that shared_array works with the standard library’s associative containers.

Normally, a shared_array cannot correctly hold a pointer to an object that has been allocated with the non-array form of new. See shared_ptr for that usage.

Because the implementation uses reference counting, cycles of shared_array instances will not be reclaimed. For example, if main holds a shared_array to A, which directly or indirectly holds a shared_array back to A, the use count of A will be 2. Destruction of the original shared_array will leave A dangling with a use count of 1.

A shared_ptr to a std::vector is an alternative to a shared_array that is a bit heavier duty but far more flexible.

The class template is parameterized on T, the type of the object pointed to. shared_array and most of its member functions place no requirements on T; it is allowed to be an incomplete type, or void. Member functions that do place additional requirements (constructors, reset) are explicitly documented below.

Synopsis

namespace boost {

  template<class T> class shared_array {
  public:
    typedef T element_type;

    explicit shared_array(T* p = 0);
    template<class D> shared_array(T* p, D d);
    shared_array(const shared_array& v) noexcept;

    ~shared_array() noexcept;

    shared_array& operator=(const shared_array& v) noexcept;

    void reset(T* p = 0);
    template<class D> void reset(T* p, D d);

    T& operator[](std::ptrdiff_t n) const noexcept;
    T* get() const noexcept;

    bool unique() const noexcept;
    long use_count() const noexcept;

    explicit operator bool() const noexcept;

    void swap(shared_array<T>& v) noexcept;
  };

  template<class T> bool
    operator==(const shared_array<T>& a, const shared_array<T>& b) noexcept;
  template<class T> bool
    operator!=(const shared_array<T>& a, const shared_array<T>& b) noexcept;
  template<class T> bool
    operator<(const shared_array<T>& a, const shared_array<T>& b) noexcept;

  template<class T>
    void swap(shared_array<T>& a, shared_array<T>& b) noexcept;
}

Members

element_type

typedef T element_type;
Type

Provides the type of the stored pointer.

Constructors

explicit shared_array(T* p = 0);
Effects

Constructs a shared_array, storing a copy of p, which must be a pointer to an array that was allocated via a C++ new[] expression or be 0. Afterwards, the use count is 1 (even if p == 0; see ~shared_array).

Requires

T is a complete type.

Throws

std::bad_alloc. If an exception is thrown, delete[] p is called.

template<class D> shared_array(T* p, D d);
Effects

Constructs a shared_array, storing a copy of p and of d. Afterwards, the use count is 1. When the the time comes to delete the array pointed to by p, the object d is used in the statement d(p).

Requires
  • T is a complete type.

  • The copy constructor and destructor of D must not throw.

  • Invoking the object d with parameter p must not throw.

Throws

std::bad_alloc. If an exception is thrown, d(p) is called.

shared_array(const shared_array& v) noexcept;
Effects

Constructs a shared_array, as if by storing a copy of the pointer stored in v. Afterwards, the use count for all copies is 1 more than the initial use count.

Requires

T is a complete type.

Destructor

~shared_array() noexcept;
Effects

Decrements the use count. Then, if the use count is 0, deletes the array pointed to by the stored pointer. Note that delete[] on a pointer with a value of 0 is harmless.

Assignment

shared_array& operator=(const shared_array& v) noexcept;
Effects

Constructs a new shared_array as described above, then replaces this shared_array with the new one, destroying the replaced object.

Requires

T is a complete type.

Returns

*this.

reset

void reset(T* p = 0);
Effects

Constructs a new shared_array as described above, then replaces this shared_array with the new one, destroying the replaced object.

Requires

T is a complete type.

Throws

std::bad_alloc. If an exception is thrown, delete[] p is called.

template<class D> void reset(T* p, D d);
Effects

Constructs a new shared_array as described above, then replaces this shared_array with the new one, destroying the replaced object.

Requires
  • T is a complete type.

  • The copy constructor of D must not throw.

Throws

std::bad_alloc. If an exception is thrown, d(p) is called.

Indexing

T& operator[](std::ptrdiff_t n) const noexcept;
Returns

A reference to element n of the array pointed to by the stored pointer. Behavior is undefined and almost certainly undesirable if the stored pointer is 0, or if n is less than 0 or is greater than or equal to the number of elements in the array.

Requires

T is a complete type.

get

T* get() const noexcept;
Returns

The stored pointer.

unique

bool unique() const noexcept;
Returns

true if no other shared_array is sharing ownership of the stored pointer, false otherwise.

use_count

long use_count() const noexcept;
Returns

The number of shared_array objects sharing ownership of the stored pointer.

Conversions

explicit operator bool() const noexcept;
Returns

get() != 0.

Requires

T is a complete type.

swap

void swap(shared_array<T>& b) noexcept;
Effects

Exchanges the contents of the two smart pointers.

Free Functions

Comparison

template<class T> bool
  operator==(const shared_array<T>& a, const shared_array<T>& b) noexcept;
template<class T> bool
  operator!=(const shared_array<T>& a, const shared_array<T>& b) noexcept;
template<class T> bool
  operator<(const shared_array<T>& a, const shared_array<T>& b) noexcept;
Returns

The result of comparing the stored pointers of the two smart pointers.

Note
The operator< overload is provided to define an ordering so that shared_array objects can be used in associative containers such as std::map. The implementation uses std::less<T*> to perform the comparison. This ensures that the comparison is handled correctly, since the standard mandates that relational operations on pointers are unspecified (5.9 [expr.rel] paragraph 2) but std::less on pointers is well-defined (20.3.3 [lib.comparisons] paragraph 8).

swap

template<class T>
  void swap(shared_array<T>& a, shared_array<T>& b) noexcept;
Returns

a.swap(b).

Requires

T is a complete type.

This documentation is

  • Copyright 1999 Greg Colvin

  • Copyright 1999 Beman Dawes

  • Copyright 2002 Darin Adler

  • Copyright 2003-2017 Peter Dimov

  • Copyright 2005, 2006 Ion Gaztañaga

  • Copyright 2008 Frank Mori Hess

  • Copyright 2012-2017 Glen Fernandes

  • Copyright 2013 Andrey Semashev

and is distributed under the Boost Software License, Version 1.0.