std::future<T>::valid - cppreference.com

bool valid() const noexcept;

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';
}

See also