如何在不设置值的情况下通知 C++ 未来？

Question

I have a thread waiting on a future. In the event where my application is shut down, I would like to join the thread. How do I go about notifying the future without fudging it and setting it to some value?

auto promise = std::promise<std::string>();
...
auto future = promise.get_future();
auto consumer = std::thread([&]
{
   while (future.wait_for(10ms)){
       // do something
       std::cout << future.get().c_str();
   }
});
...
consumer.join();

Answer 1

So std threading primitives are primitives. They are ok for direct use in toy applications, where "shut down early" means "tell the OS to shut down the process".

For more complex applications, you need to build things up from them.

You'll want thread pools instead of using raw threads, you'll probably want continuations of thread tasks, and you'll want a shutdown architecture.

One way to implement a shutdown architecture is to use exceptions. If all of your outstanding future's have promises in a central bit of code, you can set exception on them. Or destroy the promises. Both result in people waiting on the futures to get an exception thrown.

Or you can enforce that all of your promises/futures are actually future<optional<T>> and use "nullopt" to say "we are shutting down". Or use future<expected<T,Error>> .

On the other hand, if your async code is based on continuations (including "continue into a task queue for idle processing on main thread" and "continue into another async task"), you can have a policy for when the source of a continuation has failed. Some continuation systems give the next one something like an std::experimental::expected<Data, Error> . Others hand it a ready-or-error-state std::future<Data> . In other cases, abandoned inputs of continutations cause the continuation to become abandoned automatically.

Shutting down a C++ application is hard, especially if you want to do it on-demand. It basically requires a global event that rolls-down all control paths in the app in a clean way. There isn't an easy answer. There are many easy ways you can get it to sort of work in a way that doesn't seem to cause any immediate problems, like "just detach the thread" or "just call exit ".