Class template function

boost::function — A generalized function pointer that can be used for callbacks or wrapping function objects.

Synopsis

template<typename Signature,   // Function type R (T1, T2, ..., TN)
         typename Allocator = std::allocator<void> > 
class function : public functionN<R, T1, T2, ..., TN, Allocator> {
public:
  // types
  typedef R         result_type;         
  typedef Allocator allocator_type;      
  typedef T1        argument_type;         // If N == 1
  typedef T1        first_argument_type;   // If N == 2
  typedef T2        second_argument_type;  // If N == 2
  typedef T1        arg1_type;           
  typedef T2        arg2_type;           
     .
     .
     .
  typedef TN        argN_type;           

  // static constants
  static const int arity = N;

  // Lambda library support
  template<typename Args> 
  struct sig {
    // types
    typedef result_type type;
  };

  // construct/copy/destruct
  function();
  function(const functionN&);
  function(const function&);
  template<typename F> function(F);
  function& operator=(const functionN&);
  function& operator=(const function&);
  ~function();

  // modifiers
  void swap(const function&);
  void clear();

  // capacity
  bool empty() const;
  operator safe_bool() const;
  bool operator!() const;

  // invocation
  result_type operator()(arg1_type, arg2_type, ..., argN_type) const;
};

// specialized algorithms
template<typename Signature, typename Allocator> 
  void swap(function<Signature, Allocator>&, function<Signature, Allocator>&);

// undefined operators
template<typename Signature1, typename Allocator1, typename Signature2, 
         typename Allocator2> 
  void operator==(const function<Signature1, Allocator1>&, 
                  const function<Signature2, Allocator2>&);
template<typename Signature1, typename Allocator1, typename Signature2, 
         typename Allocator2> 
  void operator!=(const function<Signature1, Allocator1>&, 
                  const function<Signature2, Allocator2>&);

Description

Class template function is a thin wrapper around the numbered class templates function0, function1, etc. It accepts a function type with N arguments and will will derive from functionN instantiated with the arguments it receives.

The semantics of all operations in class template function are equivalent to that of the underlying functionN object, although additional member functions are required to allow proper copy construction and copy assignment of function objects.

`function` construct/copy/destruct

```
function();
```
Postconditions: this->empty()
Throws: Will not throw.
```
function(const functionN& f);
```
Postconditions: Contains a copy of the f's target, if it has one, or is empty if f.empty().
Throws: Will not throw unless copying the target of f throws.
```
function(const function& f);
```
Postconditions: Contains a copy of the f's target, if it has one, or is empty if f.empty().
Throws: Will not throw unless copying the target of f throws.
```
template<typename F> function(F f);
```
Requires: F is a function object Callable from this.
Postconditions: *this targets a copy of f if f is nonempty, or this->empty() if f is empty.
Throws: Will not throw when f is a stateless function object.
```
function& operator=(const functionN& f);
```
Postconditions: *this targets a copy of f's target, if it has one, or is empty if f.empty()
Throws: Will not throw when the target of f is a stateless function object or a reference to the function object.
```
function& operator=(const function& f);
```
Postconditions: *this targets a copy of f's target, if it has one, or is empty if f.empty()
Throws: Will not throw when the target of f is a stateless function object or a reference to the function object.
```
~function();
```
Effects: If !this->empty(), destroys the target of this.

`function` modifiers

```
void swap(const function& f);
```
Effects: Interchanges the targets of *this and f.
Throws: Will not throw.
```
void clear();
```
Postconditions: this->empty()
Throws: Will not throw.

`function` capacity

```
bool empty() const;
```
Returns: true if this has a target, and false otherwise.
Throws: Will not throw.
```
operator safe_bool() const;
```
Returns: A safe_bool that evaluates false in a boolean context when this->empty(), and true otherwise.
Throws: Will not throw.
```
bool operator!() const;
```
Returns: this->empty()
Throws: Will not throw.

`function` invocation

```
result_type operator()(arg1_type a1, arg2_type a2, ... , argN_type aN) const;
```
Effects: f(a1, a2, ..., aN), where f is the target of *this.
Returns: if R is void, nothing is returned; otherwise, the return value of the call to f is returned.
Throws: bad_function_call if !this->empty(). Otherwise, may through any exception thrown by the target function f.

`function` specialized algorithms

template<typename Signature, typename Allocator> 
  void swap(function<Signature, Allocator>& f1, 
            function<Signature, Allocator>& f2);

Effects: f1.swap(f2)
Throws: Will not throw.

`function` undefined operators

template<typename Signature1, typename Allocator1, typename Signature2, 
         typename Allocator2> 
  void operator==(const function<Signature1, Allocator1>& f1, 
                  const function<Signature2, Allocator2>& f2);

Notes: This function must be left undefined.
Rationale: The safe_bool conversion opens a loophole whereby two function instances can be compared via ==. This undefined void operator == closes the loophole and ensures a compile-time or link-time error.

template<typename Signature1, typename Allocator1, typename Signature2, 
         typename Allocator2> 
  void operator!=(const function<Signature1, Allocator1>& f1, 
                  const function<Signature2, Allocator2>& f2);

Notes: This function must be left undefined.
Rationale: The safe_bool conversion opens a loophole whereby two function instances can be compared via !=. This undefined void operator != closes the loophole and ensures a compile-time or link-time error.


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