How to wait for multiple C++ coroutines to complete before propagating failure, unhelpful lambda

Last time, we found a solution for waiting for multiple C++ coroutines to complete before propagating failure, but it relied on expansion statements, which weren’t finished in time for C++20. We’ll have to find something that uses only features available in C++20, or even better, works in C++17.¹

The usual way to get expansion-like behavior is to use a lambda and the comma operator:

([&](auto& arg) {
    /* do something */
}(args), ...);

So let’s try applying it to our “wait for all” coroutine function:

template<typename... T>
IAsyncAction when_all_complete(T... asyncs)
{
    std::exception_ptr eptr;

    ([&] (auto& async) {
        try {
            co_await async;
        } catch (...) {
            if (!eptr) {
                eptr = std::current_exception();
            }
        }
    }(asyncs), ...);

    if (eptr) std::rethrow_exception(eptr);
}

Sadly, this doesn’t work because the lambda performs a co_await, and co_await requires that you be in a coroutine.

I guess we have to make the lambda a coroutine.

template<typename... T>
IAsyncAction when_all_complete(T... asyncs)
{
    std::exception_ptr eptr;

    auto each = [&] (auto& async) -> IAsyncAction {
        try {
            co_await async;
        } catch (...) {
            if (!eptr) {
                eptr = std::current_exception();
            }
        }
    };

    (co_await each(asyncs), ...);

    if (eptr) std::rethrow_exception(eptr);
}

One thing you might object to is the fact that we have a lambda coroutine with a capture. This is generally frowned upon, due to the risk of the lambda being destructed while suspended, but it works here because the lambda is not destructed until when_all_complete finishes executing all of its co_awaits. Even if one of the calls to co_await async throws an exception, that exception is caught and saved in eptr, and the overall coroutine completes without an exception.

It looks like we’ve done it, but there’s a catch: The threading model.

In the original when_all, the parameters were each co_awaited directly in the main function, which means that if any of the co_awaited things changed threads, the thread change would remain in effect for the next co_await:

winrt::fire_and_forget example()
{
    co_await winrt::when_all(resume_background(), Something());
}

Inside when_all, this expands into

auto first = resume_background();
auto second = Something();
co_await first;
co_await second;

The co_await first performs a thread switch, and the co_await second executes on the new thread.

This is different from our when_all_complete because that one wraps each co_await inside another IAsyncAction, which changes the threading behavior: In C++/WinRT, co_awaiting an IAsyncAction resumes on the same apartment that started the co_await. Since we wrapped each awaitable inside a lambda that produces an IAsyncAction, each co_await of the lambda will resume back on the original apartment, even if the original awaitable completed in a different apartment.

Here’s a comparison: With expansion statements, we just co_await directly from the function.

IAsyncAction v1(async1, async2)
{
    co_await async1;
    co_await async2;
}

Suppose that async1 completes on a different apartment. (For example, it might be an apartment-switching awaitable, like resume_background.) Then the co_await async1 will complete on the other apartment, and the co_await async2 therefore begins on that other apartment.

On the other hand, with a lambda, we co_await from a lambda, and then co_await the lambda.

IAsyncAction v2(async1, async2)
{
    co_await [](auto& async) -> IAsyncAction {
        co_await async;
    }(async1);

    co_await [](auto& async) -> IAsyncAction {
        co_await async;
    }(async2);
}

First, we invoke the lambda, which does a co_await async1, which completes on some other apartment. However, since we co_await the lambda, and the lambda itself returns an IAsyncAction, the co_await of the lambda will complete back on the original apartment (because that’s how the IAsyncAction awaiter works). Next, we co_await async2, which now begins on the original apartment, not the apartment in which async1 completed.

Next time, we’ll try to get all the co_awaits to happen in the main function, so that we can preserve the apartment-switching behavior of the original when_all function.

Spoiler alert: Next time will be a failure.

¹ Yes, coroutines are not part of C++ until C++20, but the Microsoft Visual Studio compiler lets you opt into coroutine support while in C++17 mode, so “C++17 with optional coroutines” is the current baseline for C++/WinRT.