What is STD future?

std::future::valid Returns whether the future object is currently associated with a shared state. For default-constructed future objects, this function returns false (unless move-assigned a valid future ).

On reaching the end of main , the std::future<int> object held in result is destroyed , which will wait for the task to finish, and then discard the result, cleaning up any resources used.

To wait for a thread use the std::thread::join() function . This function makes the current thread wait until the thread identified by *this has finished executing.

– The promise object is the asynchronous provider and is expected to set a value for the shared state at some point. – The future object is an asynchronous return object that can retrieve the value of the shared state, waiting for it to be ready, if necessary.

get() is not blocking .

The returned std::future<> blocks on destruction until the value is available . However the normal destructor of std::future<> does not block: ~future();

The class template std::future provides a mechanism to access the result of asynchronous operations : An asynchronous operation (created via std::async, std::packaged_task, or std::promise) can provide a std::future object to the creator of that asynchronous operation.