std::move_only_function(3)

NAME
std::move_only_function - std::move_only_function

Synopsis
   Defined in header <functional>
   template< class... > (1) (since C++23)
   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>; (2) (since C++23)
   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,
   ref-qualifiers, and noexcept-specifiers not including volatile 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
   Type Definition
   result_type R

Member functions
   constructor constructs a new std::move_only_function object
   (C++23) (public member function)
   destructor destroys a std::move_only_function object
   (C++23) (public member function)
   operator= replaces or destroys the target
   (C++23) (public member function)
   swap swaps the targets of two std::move_only_function objects
   (C++23) (public member function)
   operator bool checks if the std::move_only_function has a target
   (C++23) (public member function)
   operator() invokes the target
   (C++23) (public member function)

Non-member functions
   swap(std::move_only_function) overloads the std::swap algorithm
   (C++23) (function)
   operator== compares a std::move_only_function with nullptr
   (C++23) (function)

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.
        Feature-test macro Value Std Feature
   __cpp_lib_move_only_function 202110L (C++23) std::move_only_function

Example
// Run this code
 #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();
 }

Output:
 3.14159

See also
   function wraps callable object of any copy constructible type with specified
   (C++11) function call signature
            (class template)