Allocator Access

Overview

The header <boost/core/allocator_access.hpp> provides the class and function templates to simplify allocator use. It provides the same functionality as the C++ standard library std::allocator_traits but with individual templates for each allocator feature.

These facilities also simplify existing libraries by avoiding having to check for BOOST_NO_CXX11_ALLOCATOR and conditionally use std::allocator_traits.

Examples

The following example shows these utilities used in the definition of an allocator-aware container class:

template<class T, class A = boost::default_allocator<T> >
class container
    : boost::empty_value<typename boost::allocator_rebind<A, T>::type> {
public:
    typedef T value_type;
    typedef A allocator_type;
    typedef typename boost::allocator_size_type<A>::type size_type;
    typedef typename boost::allocator_difference_type<A>::type difference_type;
    typedef value_type& reference;
    typedef const value_type& const_reference;
    typedef typename boost::allocator_pointer<A>::type pointer;
    typedef typename boost::allocator_const_pointer<A>::type const_pointer;
    // ...
};

In C++11 or above, aliases such as boost::allocator_pointer_t<A> can be used instead of typename boost::allocator_pointer<A>::type.

Reference

namespace boost {

template<class A>
struct allocator_value_type;

template<class A>
using allocator_value_type_t = typename allocator_value_type<A>::type;

template<class A>
struct allocator_pointer;

template<class A>
using allocator_pointer_t = typename allocator_pointer<A>::type;

template<class A>
struct allocator_const_pointer;

template<class A>
using allocator_const_pointer_t = typename allocator_const_pointer<A>::type;

template<class A>
struct allocator_void_pointer;

template<class A>
using allocator_void_pointer_t = typename allocator_void_pointer<A>::type;

template<class A>
struct allocator_const_void_pointer;

template<class A>
using allocator_const_void_pointer_t =
    typename allocator_const_void_pointer<A>::type;

template<class A>
struct allocator_difference_type;

template<class A>
using allocator_difference_type_t =
    typename allocator_difference_type<A>::type;

template<class A>
struct allocator_size_type;

template<class A>
using allocator_size_type_t = typename allocator_size_type<A>::type;

template<class A>
struct allocator_propagate_on_container_copy_assignment;

template<class A>
using allocator_propagate_on_container_copy_assignment_t =
    typename allocator_propagate_on_container_copy_assignment<A>::type;

template<class A>
struct allocator_propagate_on_container_move_assignment;

template<class A>
using allocator_propagate_on_container_move_assignment_t =
    typename allocator_propagate_on_container_move_assignment<A>::type;

template<class A>
struct allocator_propagate_on_container_swap;

template<class A>
using allocator_propagate_on_container_swap_t =
    typename allocator_propagate_on_container_swap<A>::type;

template<class A>
struct allocator_is_always_equal;

template<class A>
using allocator_is_always_equal_t =
    typename allocator_is_always_equal<A>::type;

template<class A, class T>
struct allocator_rebind;

template<class A, class T>
using allocator_rebind_t = typename allocator_rebind<A, T>::type;

template<class A>
allocator_pointer_t<A> allocator_allocate(A& a, allocator_size_type_t<A> n);

template<class A>
allocator_pointer_t<A> allocator_allocate(A& a, allocator_size_type_t<A> n,
    allocator_const_void_pointer_t<A> h);

template<class A>
void allocator_deallocate(A& a, allocator_pointer_t<A> p,
    allocator_size_type_t<A> n);

template<class A, class T, class... Args>
void allocator_construct(A& a, T* p, Args&&... args);

template<class A, class T>
void allocator_destroy(A& a, T* p);

template<class A>
allocator_size_type_t<A> allocator_max_size(const A& a);

template<class A>
A allocator_select_on_container_copy_construction(const A& a);

} // boost

Types

template<class A> struct allocator_value_type;: The member type is A::value_type.
template<class A> struct allocator_pointer;: The member type is A::pointer if valid, otherwise A::value_type*.
template<class A> struct allocator_const_pointer;: The member type is A::const_pointer if valid, otherwise pointer_traits<allocator_pointer_t<A> >::rebind<const allocator_value_type_t<A> >.
template<class A> struct allocator_void_pointer;: The member type is A::void_pointer if valid, otherwise pointer_traits<allocator_pointer_t<A> >::rebind<void>.
template<class A> struct allocator_const_void_pointer;: The member type is A::const_void_pointer if valid, otherwise pointer_traits<allocator_pointer_t<A> >::rebind<const void>.
template<class A> struct allocator_difference_type;: The member type is A::difference_type if valid, otherwise pointer_traits<allocator_pointer_t<A> >::difference_type.
template<class A> struct allocator_size_type;: The member type is A::size_type if valid, otherwise std::make_unsigned_t<allocator_difference_type_t<A> >.
template<class A> struct allocator_propagate_on_container_copy_assignment;: The member type is A::propagate_on_container_copy_assignment if valid, otherwise std::false_type.
template<class A> struct allocator_propagate_on_container_move_assignment;: The member type is A::propagate_on_container_move_assignment if valid, otherwise std::false_type.
template<class A> struct allocator_propagate_on_container_swap;: The member type is A::propagate_on_container_swap if valid, otherwise std::false_type.
template<class A> struct allocator_is_always_equal;: The member type is A::is_always_equal if valid, otherwise std::is_empty<A>::type.
template<class A, class T> struct allocator_rebind;: The member type is A::rebind<T>::other if valid, otherwise A<T, Args> if this A is A<U, Args>.

Functions

template<class A> allocator_pointer_t<A> allocator_allocate(A& a, allocator_size_type_t<A> n);: Calls a.allocate(n).
template<class A> allocator_pointer_t<A> allocator_allocate(A& a, allocator_size_type_t<A> n, allocator_const_void_pointer_t<A> hint);: Calls a.allocate(n, hint) if valid, otherwise calls a.allocate(n).
template<class A> void allocator_deallocate(A& a, allocator_pointer_t<A> p, allocator_size_type_t<A> n);: Calls a.deallocate(p, n).
template<class A, class T, class... Args> void allocator_construct(A& a, T*p, Args&&... args);: Calls a.construct(p, std::forward<Args>(args)...) if valid, otherwise calls ::new(static_cast<void*>(p)) T(std::forward<Args>(args)...).
template<class A, class T> void allocator_destroy(A& a, T* p);: Calls a.destroy(p) if valid, otherwise calls p->~T().
template<class A> allocator_size_type_t<A> allocator_max_size(const A& a);: Returns a.max_size() if valid, otherwise returns std::numeric_limits<allocator_size_type_t<A> >::max() / sizeof(A::value_type).
template<class A> A allocator_select_on_container_copy_construction(const A& a);: Returns a.select_on_container_copy_construction() if valid, otherwise returns a.

Acknowledgments

Glen Fernandes implemented the allocator access utilities.

Boost C++ Libraries