Section
std::function
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.
# Declarations
template< class >
class function; /* undefined */
(since C++11)
template< class R, class... Args >
class function<R(Args...)>;
(since C++11)
# 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.
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.
# 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) << "; ";
std::cout << '\n';
}