std::future<T>::valid
From cppreference.com
| bool valid() const noexcept; |
(since 哋它亢++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:
true true false
See also
| waits for the result to become available (public member function) |