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std::function

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< cpp‎ | utility‎ | functional
 
 
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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 CopyConstructible Callable target -- functions (via pointers thereto), 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.

Contents

[edit] 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...

[edit] Member functions

constructs a new std::function instance
(public member function) [edit]
destroys a std::function instance
(public member function) [edit]
assigns a new target
(public member function) [edit]
swaps the contents
(public member function) [edit]
(removed in C++17)
assigns a new target
(public member function) [edit]
checks if a target is contained
(public member function) [edit]
invokes the target
(public member function) [edit]
Target access
obtains the typeid of the stored target
(public member function) [edit]
obtains a pointer to the stored target
(public member function) [edit]

[edit] Non-member functions

specializes the std::swap algorithm
(function template) [edit]
(removed in C++20)
compares a std::function with nullptr
(function template) [edit]

[edit] Helper classes

specializes the std::uses_allocator type trait
(class template specialization) [edit]

[edit] Deduction guides(since C++17)

[edit] Notes

Care should be taken when a std::function, whose result type is a reference, is initialized from a lambda expression without a trailing-return-type. Due to the way auto deduction works, such lambda expression will always return a prvalue. Hence, the resulting reference will usually bind to a temporary whose lifetime ends when std::function::operator() returns.

(until C++23)

If a std::function returning a reference is initialized from a function or function object returning a prvalue (including a lambda expression without a trailing-return-type), the program is ill-formed because binding the returned reference to a temporary object is forbidden.

(since C++23)
std::function<const int&()> F([]{ return 42; }); // Error since C++23: can't bind
                                                 // the returned reference to a temporary
int x = F(); // Undefined behavior until C++23: the result of F() is a dangling reference
 
std::function<int&()> G([]()->int& { static int i{0x2A}; return i; }); // OK
 
std::function<const int&()> H([i{052}]->const int& { return i; }); // OK

[edit] Example

#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);
 
    auto factorial = [](int n) {
        // store a lambda object to emulate "recursive lambda"; aware of extra overhead
        std::function<int(int)> fac = [&](int n){ return (n < 2) ? 1 : n*fac(n-1); };
        // note that "auto fac = [&](int n){...};" does not work in recursive calls
        return fac(n);
    };
    for (int i{5}; i != 8; ++i) { std::cout << i << "! = " << factorial(i) << ";  "; }
}

Possible output:

-9
42
31337
314160
314160
num_: 314159
314161
314162
18
5! = 120;  6! = 720;  7! = 5040;

[edit] See also

wraps callable object of any type with specified function call signature
(class template) [edit]
the exception thrown when invoking an empty std::function
(class) [edit]
(C++11)
creates a function object out of a pointer to a member
(function template) [edit]