std::future::future (3) Linux Manual Page

std::future<T>::~future – std::future<T>::~future

Synopsis

~future(); (since C++11)
Releases any shared state. This means

*if the return object or provider holds the last reference to its shared state, the shared state is destroyed;
and

    *the return object or
    provider gives up its reference to its shared state;
and

* these actions will not block for the shared state to become ready, except that it may block if all of the following are true: the shared state was created by a call to std::async, the shared state is not yet ready, and this was the last reference to the shared state. (since C++14)