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## 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;

      // target access
      template<typename Functor> Functor* target();
      template<typename Functor> const Functor* target() const;
      template<typename Functor> bool contains(const Functor&) 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>&);

    // comparison operators
    template<typename Signature, typename Allocator, typename Functor>
      bool operator==(const function<Signature, Allocator>&, Functor);
    template<typename Signature, typename Allocator, typename Functor>
      bool operator==(Functor, const function<Signature, Allocator>&);
    template<typename Signature, typename Allocator, typename Functor>
      bool operator==(const function<Signature, Allocator>&,
                      reference_wrapper<Functor>);
    template<typename Signature, typename Allocator, typename Functor>
      bool operator==(reference_wrapper<Functor>,
                      const function<Signature, Allocator>&);
    template<typename Signature1, typename Allocator1, typename Signature2,
             typename Allocator2>
      void operator==(const function<Signature1, Allocator1>&,
                      const function<Signature2, Allocator2>&);
    template<typename Signature, typename Allocator, typename Functor>
      bool operator!=(const function<Signature, Allocator>&, Functor);
    template<typename Signature, typename Allocator, typename Functor>
      bool operator!=(Functor, const function<Signature, Allocator>&);
    template<typename Signature, typename Allocator, typename Functor>
      bool operator!=(const function<Signature, Allocator>&,
                      reference_wrapper<Functor>);
    template<typename Signature, typename Allocator, typename Functor>
      bool operator!=(reference_wrapper<Functor>,
                      const function<Signature, Allocator>&);
    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

1.     function();

   Postconditions: `this->empty()`
   Throws: Will not throw.

2.     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.

3.     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.

4.     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.

5.     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.

6.     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.

7.     ~function();

   Effects: If `!this->empty()`, destroys the target of `this`.

### `function` modifiers

1.     void swap(const function& f);

   Effects: Interchanges the targets of `*this` and `f`.
   Throws: Will not throw.

2.     void clear();

   Postconditions: `this->empty()`
   Throws: Will not throw.

### `function` capacity

1.     bool empty() const;

   Returns: `false` if `this` has a target, and `true` otherwise.
   Throws: Will not throw.

2.     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.

3.     bool operator!() const;

   Returns: `this->empty()`
   Throws: Will not throw.

### `function` target access

1.     template<typename Functor> Functor* target();
       template<typename Functor> const Functor* target() const;

   Returns: If `this` stores a target of type `Functor`, returns the address of the target. Otherwise, returns the NULL pointer.
   Throws: Will not throw.

2.     template<typename Functor> bool contains(const Functor& f) const;

   Returns: `true` if `this->target<Functor>()` is non-NULL and `function_equal(*(this->target<Functor>()), f)`

### `function` invocation

1.     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

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

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

### `function` comparison operators

1.     template<typename Signature, typename Allocator, typename Functor>
         bool operator==(const function<Signature, Allocator>& f, Functor g);
       template<typename Signature, typename Allocator, typename Functor>
         bool operator==(Functor g, const function<Signature, Allocator>& f);
       template<typename Signature, typename Allocator, typename Functor>
         bool operator==(const function<Signature, Allocator>& f,
                         reference_wrapper<Functor> g);
       template<typename Signature, typename Allocator, typename Functor>
         bool operator==(reference_wrapper<Functor> g,
                         const function<Signature, Allocator>& f);
       template<typename Signature1, typename Allocator1, typename Signature2,
                typename Allocator2>
         void operator==(const function<Signature1, Allocator1>& f1,
                         const function<Signature2, Allocator2>& f2);

   Returns: True when `f` stores an object of type `Functor` and one of the following conditions applies:

   - `g` is of type `reference_wrapper<Functor>` and `f.target<Functor>() == g.get_pointer()`.
   - `g` is not of type `reference_wrapper<Functor>` and `function_equals(*(f.target<Functor>()), g)`.
   Notes: `function` objects are not EqualityComparable.
   Rationale: The `safe_bool` conversion opens a loophole whereby two `function` instances can be compared via `==`, although this is not feasible to implement. The undefined `void operator==` closes the loophole and ensures a compile-time or link-time error.

2.     template<typename Signature, typename Allocator, typename Functor>
         bool operator!=(const function<Signature, Allocator>& f, Functor g);
       template<typename Signature, typename Allocator, typename Functor>
         bool operator!=(Functor g, const function<Signature, Allocator>& f);
       template<typename Signature, typename Allocator, typename Functor>
         bool operator!=(const function<Signature, Allocator>& f,
                         reference_wrapper<Functor> g);
       template<typename Signature, typename Allocator, typename Functor>
         bool operator!=(reference_wrapper<Functor> g,
                         const function<Signature, Allocator>& f);
       template<typename Signature1, typename Allocator1, typename Signature2,
                typename Allocator2>
         void operator!=(const function<Signature1, Allocator1>& f1,
                         const function<Signature2, Allocator2>& f2);

   Returns: True when `f` does not store an object of type `Functor` or it stores an object of type `Functor` and one of the following conditions applies:

   - `g` is of type `reference_wrapper<Functor>` and `f.target<Functor>() != g.get_pointer()`.
   - `g` is not of type `reference_wrapper<Functor>` and `!function_equals(*(f.target<Functor>()), g)`.
   Notes: `function` objects are not EqualityComparable.
   Rationale: The `safe_bool` conversion opens a loophole whereby two `function` instances can be compared via `!=`, although this is not feasible to implement. The undefined `void operator!=` closes the loophole and ensures a compile-time or link-time error.

| Copyright © 2001-2004 Douglas Gregor