std::function (3) - Linux Manuals

std::function: std::function

NAME

std::function - std::function

Synopsis

Defined in header <functional>
template< class > (since C++11)
class function; /* undefined */
template< class R, class... Args > (since C++11)
class function<R(Args...)>;

Class template std::function is a general-purpose polymorphic function wrapper. Instances of std::function can store, copy, and invoke any Callable target -- functions, lambda_expressions, bind_expressions, or other function objects, as well as pointers to member functions and pointers to data members.
The stored callable object is called the target of std::function. If a std::function contains no target, it is called empty. Invoking the target of an empty std::function results in std::bad_function_call exception being thrown.
std::function satisfies the requirements of CopyConstructible and CopyAssignable.

Member types

Type Definition
result_type R
argument_type(deprecated in C++17)(removed in C++20) T if sizeof...(Args)==1 and T is the first and only type in Args...
first_argument_type(deprecated in C++17)(removed in C++20) T1 if sizeof...(Args)==2 and T1 is the first of the two types in Args...
second_argument_type(deprecated in C++17)(removed in C++20) T2 if sizeof...(Args)==2 and T2 is the second of the two types in Args...

Member functions

              constructs a new std::function instance
constructor (public member function)
              destroys a std::function instance
destructor (public member function)
              assigns a new target
operator= (public member function)
              swaps the contents
swap (public member function)

assign assigns a new target
              (public member function)
(until C++17)
              checks if a valid target is contained
operator_bool (public member function)
              invokes the target
operator() (public member function)

Target access

              obtains the typeid of the stored target
target_type (public member function)
              obtains a pointer to the stored target
target (public member function)

Non-member functions

std::swap(std::function) specializes the std::swap algorithm
                         (function template)
(C++11)
                         compares a std::function with nullptr
operator== (function template)
operator!=

Helper classes

std::uses_allocator<std::function> specializes the std::uses_allocator type trait
                                   (class template specialization)
(C++11) (until C++17)

Deduction_guides(since C++17)

Example

// Run this code

  #include <functional>
  #include <iostream>

  struct Foo {
      Foo(int num) : num_(num) {}
      void print_add(int i) const { std::cout << num_+i << '\n'; }
      int num_;
  };

  void print_num(int i)
  {
      std::cout << i << '\n';
  }

  struct PrintNum {
      void operator()(int i) const
      {
          std::cout << i << '\n';
      }
  };

  int main()
  {
      // store a free function
      std::function<void(int)> f_display = print_num;
      f_display(-9);

      // store a lambda
      std::function<void()> f_display_42 = []() { print_num(42); };
      f_display_42();

      // store the result of a call to std::bind
      std::function<void()> f_display_31337 = std::bind(print_num, 31337);
      f_display_31337();

      // store a call to a member function
      std::function<void(const Foo&, int)> f_add_display = &Foo::print_add;
      const Foo foo(314159);
      f_add_display(foo, 1);
      f_add_display(314159, 1);

      // store a call to a data member accessor
      std::function<int(Foo const&)> f_num = &Foo::num_;
      std::cout << "num_: " << f_num(foo) << '\n';

      // store a call to a member function and object
      using std::placeholders::_1;
      std::function<void(int)> f_add_display2 = std::bind( &Foo::print_add, foo, _1 );
      f_add_display2(2);

      // store a call to a member function and object ptr
      std::function<void(int)> f_add_display3 = std::bind( &Foo::print_add, &foo, _1 );
      f_add_display3(3);

      // store a call to a function object
      std::function<void(int)> f_display_obj = PrintNum();
      f_display_obj(18);
  }

Output:

  -9
  42
  31337
  314160
  314160
  num_: 314159
  314161
  314162
  18