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std::move_only_function::operator() - cppreference.com

From cppreference.com

R operator()( Args... args ) /*cv*/ /*ref*/ noexcept(/*noex*/);

 (since C++23)

Invokes the stored callable target with the parameters args. The /*cv*/, /*ref*/, and /*noex*/ parts of operator() are identical to those of the template parameter of std::move_only_function.

Equivalent to return std::invoke_r<R>(/*cv-ref-cast*/(f), std::forward<Args>(args)...);, where f is a cv-unqualified lvalue that denotes the target object of *this, and /*cv-ref-cast*/(f) is equivalent to:

  • f if cv ref is either empty or &, or
  • std::as_const(f) if cv ref is either const or const &, or
  • std::move(f) if cv ref is &&, or
  • std::move(std::as_const(f)) if cv ref is const &&.

The behavior is undefined if *this is empty.

Parameters

args - parameters to pass to the stored callable target

Return value

std::invoke_r<R>(/*cv-ref-cast*/(f), std::forward<Args>(args)...).

Exceptions

Propagates the exception thrown by the underlying function call.

Example

The following example shows how std::move_only_function can passed to other functions by value. Also, it shows how std::move_only_function can store lambdas.

#include <iostream>
#include <functional>

void call(std::move_only_function<int() const> f)  // can be passed by value
{ 
    std::cout << f() << '\n';
}

int normal_function() 
{
    return 42;
}

int main()
{
    int n = 1;
    auto lambda = [&n](){ return n; };
    std::move_only_function<int() const> f = lambda;
    call(std::move(f));

    n = 2;
    call(lambda); 

    f = normal_function; 
    call(std::move(f));
}

Output:

See also

invokes the target
(public member function of std::function<R(Args...)>) [edit]
calls the stored function
(public member function of std::reference_wrapper<T>) [edit]
invokes any Callable object with given arguments and possibility to specify return type(since C++23)
(function template) [edit]