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std::function - cppreference.com

template< class >
class function; /* undefined */

template< class R, class... Args >
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 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.

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.

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.

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

#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) << ";  ";
    std::cout << '\n';
}

Possible output:

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

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