NAME

std::future::valid - std::future::valid

Synopsis

bool valid() const noexcept; (since C++11)

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

// Run this code

#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';
}

Output: