std::future<T>:: valid

Checks if the future refers to a shared state.

This is the case only for futures that were not default-constructed or moved from (i.e. returned by std::promise::get_future() , std::packaged_task::get_future() or std::async() ) until the first time get() or share() is called.

The behavior is undefined if any member function other than the destructor, the move-assignment operator, or valid is called on a future that does not refer to shared state (although implementations are encouraged to throw std::future_error indicating no_state in this case). It is valid to move from a future object for which valid() is false .

Parameters

(none)

Return value

true if * this refers to a shared state, otherwise false .

Example

#include <future>
#include <iostream>
 
int main()
{
    std::promise<void> p;
    std::future<void> f = p.get_future();
 
    std::cout << std::boolalpha;
 
    std::cout << f.valid() << '\n';
    p.set_value();
    std::cout << f.valid() << '\n';
    f.get();
    std::cout << f.valid() << '\n';
}

true
true
false

See also