std::move_only_function

template< class... >
class move_only_function; // not defined

template< class R, class... Args >
class move_only_function<R(Args...)>;
template< class R, class... Args >
class move_only_function<R(Args...) noexcept>;
template< class R, class... Args >
class move_only_function<R(Args...) &>;
template< class R, class... Args >
class move_only_function<R(Args...) & noexcept>;
template< class R, class... Args >
class move_only_function<R(Args...) &&>;
template< class R, class... Args >
class move_only_function<R(Args...) && noexcept>;
template< class R, class... Args >
class move_only_function<R(Args...) const>;
template< class R, class... Args >
class move_only_function<R(Args...) const noexcept>;
template< class R, class... Args >
class move_only_function<R(Args...) const &>;
template< class R, class... Args >
class move_only_function<R(Args...) const & noexcept>;
template< class R, class... Args >
class move_only_function<R(Args...) const &&>;
template< class R, class... Args >
class move_only_function<R(Args...) const && noexcept>;

Class template std::move_only_function is a general-purpose polymorphic function wrapper. std::move_only_function objects can store and invoke any constructible (not required to be move constructible) Callable target functions, lambda expressions, bind expressions, or other function objects, as well as pointers to member functions and pointers to member objects.

The stored callable object is called the target of std::move_only_function. If a std::move_only_function contains no target, it is called empty. Unlike std::function, invoking an empty std::move_only_function results in undefined behavior.

std::move_only_functions supports every possible combination of cv-qualifiers (not including volatile), ref-qualifiers, and noexcept-specifiers provided in its template parameter. These qualifiers and specifier (if any) are added to its operator().

std::move_only_function satisfies the requirements of MoveConstructible and MoveAssignable, but does not satisfy CopyConstructible or CopyAssignable.

Member types

Member functions

Non-member functions

Notes

Implementations may store a callable object of small size within the std::move_only_function object. Such small object optimization is effectively required for function pointers and std::reference_wrapper specializations, and can only be applied to types T for which std::is_nothrow_move_constructible_v<T> is true.

If a std::move_only_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. See also std::function Notes.

Example

#include <functional>
#include <future>
#include <iostream>

int main()
{
    std::packaged_task<double()> packaged_task([](){ return 3.14159; });

    std::future<double> future = packaged_task.get_future();

    auto lambda = [task = std::move(packaged_task)]() mutable { task(); };

//  std::function<void()> function = std::move(lambda); // Error
    std::move_only_function<void()> function = std::move(lambda); // OK

    function();

    std::cout << future.get();
}

3.14159

See also