In C++/WinRT, how can I await multiple coroutines and capture the results?, part 1

Raymond Chen

The winrt::when_all function in C++/WinRT is not a complicated beast. It just awaits each of its parameters. Some time ago, we looked at how it works and how you could use it to await all the coroutines in a container or range. But all of those examples just throw away the results. What if we also want to gather the results?

Here’s what doesn’t work:

auto op1 = DoSomething1Async();
auto op2 = DoSomething2Async();

co_await winrt::when_all(op1, op2);

// These don't return proper results.
auto result1 = op1.GetResults();
auto result2 = op2.GetResults();

The problem is that the co_await inside when_all calls GetResults() in order to produce the result of the co_await. And GetResults() produces valid results only the first time it is called after completion.

One way to solve this problem is to use a different awaiter whose await_resume() doesn’t call GetResults(), leave it to the caller to fetch the results when they are ready.

template<typename Async>
struct simple_async_awaiter
{
    simple_async_awaiter(Async const& async) : async(async) {}

    Async const& async;

    bool await_ready() const noexcept { return false; }

    void await_suspend(std::coroutine_handle<> handle) const {
        async.Completed([handle](auto&&...) { handle(); });
    }

    void await_resume() const noexcept {
        /* return async.GetResults(); */
    }
};

We return void from await_resume(), which means that co_await returns void, and you can then fetch the results by calling GetResults(). (We also don’t bother preserving the COM apartment context, and we don’t propagate cancellation, but the point here was to show that we skipped the GetResults() inside await_resume().)

But this is an awful lot of work, particularly because you have to replicate all of the stuff that C++/WinRT does behind the scenes in its awaiter. Maybe we can find something simpler.

We’ll continue our exploration next time.

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