# std::function

From cppreference.com
< cpp‎ | utility‎ | functional

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

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) | `T` if sizeof...(Args)==1 and `T` is the first and only type in `Args...`
`first_argument_type`(deprecated in C++17) | `T1` if sizeof...(Args)==2 and `T1` is the first of the two types in `Args...`
`second_argument_type`(deprecated in C++17) | `T2` if sizeof...(Args)==2 and `T2` is the second of the two types in `Args...`

### Member functions

(constructor) | constructs a new `std::function` instance (public member function)
(destructor) | destroys a `std::function` instance (public member function)
operator= | assigns a new target (public member function)
swap | swaps the contents (public member function)
assign (until C++17) | assigns a new target (public member function)
operator bool | checks if a valid target is contained (public member function)
operator() | invokes the target (public member function)

##### Target access

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

### Non-member functions

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

### Helper classes

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

### 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);

        // 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
    num_: 314159
    314161
    314162
    18

### See also

bad_function_call (C++11) | the exception thrown when invoking an empty std::function (class)
mem_fn (C++11) | creates a function object out of a pointer to a member (function template)

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