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Defined in header <functional> | ||
template<class...> class move_only_function;// not defined | (1) | (since C++23) |
template<class R,class...Args> class move_only_function<R(Args...)>; | (2) | (since C++23) |
Class templatestd::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)Callabletarget — 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 thetarget ofstd::move_only_function. If astd::move_only_function contains no target, it is calledempty. Unlikestd::function, invoking anemptystd::move_only_function results in undefined behavior.
std::move_only_functions supports every possible combination ofcv-qualifiers (not includingvolatile),ref-qualifiers, andnoexcept-specifiers provided in its template parameter. These qualifiers and specifier (if any) are added to itsoperator().
std::move_only_function satisfies the requirements ofMoveConstructible andMoveAssignable, but does not satisfyCopyConstructible orCopyAssignable.
Contents |
| Type | Definition |
result_type | R |
constructs a newstd::move_only_function object(public member function)[edit] | |
destroys astd::move_only_function object(public member function)[edit] | |
| replaces or destroys the target (public member function)[edit] | |
swaps the targets of twostd::move_only_function objects(public member function)[edit] | |
checks if thestd::move_only_function has a target(public member function)[edit] | |
| invokes the target (public member function)[edit] |
| specializes thestd::swap algorithm (function)[edit] | |
(C++23) | compares astd::move_only_function withnullptr(function)[edit] |
Implementations may store a callable object of small size within thestd::move_only_function object. Such small object optimization is effectively required for function pointers andstd::reference_wrapper specializations, and can only be applied to typesT for whichstd::is_nothrow_move_constructible_v<T> istrue.
If astd::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 alsostd::function Notes.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_move_only_function | 202110L | (C++23) | std::move_only_function |
#include <functional>#include <future>#include <iostream> int main(){std::packaged_task<double()> packaged_task([](){return3.14159;}); 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
(C++11) | copyable wrapper of any copy constructible callable object (class template)[edit] |
(C++26) | non-owning wrapper of any callable object (class template)[edit] |
(C++26) | copyable wrapper of any copy constructible callable object that supports qualifiers in a given call signature (class template)[edit] |
