In C++/WinRT, how can I await multiple coroutines and capture the results?, part 2

Raymond Chen

Instead of replacing the awaiter so it doesn’t retrieve the results, we can go ahead and collect the results, and then return them. The Windows Runtime doesn’t have a convenient way to return a strongly-typed heterogeneous collection. Structures must be declared in metadata, and returning a vector of IInspectables is not strongly-typed.

Fortunately, we can use our friend simple_task, which has since been added to the Windows Implementation Library as wil::task.

template<typename... Results>
wil::task<std::tuple<Results>>
when_all_with_results(
    winrt::Windows::Foundation::IAsyncOperation<Results>... asyncs)
{
    co_return std::make_tuple(co_await asyncs...);
}

auto [result1, result2] =
    co_await when_all_with_results(Do1Async(), Do2Async());

We wish we could have written

template<typename... Asyncs>
wil::task<auto>
when_all_with_results(Asyncs... asyncs)
{
    co_return std::make_tuple(co_await asyncs...);
}

but there is currently no facility in the C++ language for this sort of weirdo template placeholder usage.

The above formulation does limit you to IAsyncOperation<T>, so you cannot use other awaitables with when_all_with_results, like IAsync­Operation­With­Progress<T, P>. Adding IAsync­Operation­With­Progress<T, P> support isn’t so bad, because the result type is available from both IAsync­Operation<T> and IAsync­Operation­With­Progress<T, P> by checking the return type of Get­Result().

template<typename... Asyncs>
wil::task<std::tuple<
    decltype(std::declval<Asyncs>().GetResults())...>>
when_all_with_results(Asyncs... asyncs)
{
    co_return std::make_tuple(co_await asyncs...);
}

Or, taking advantage of trailing return types so we don’t need to go through the hassle of declval:

template<typename... Asyncs>
auto
when_all_with_results(Asyncs... asyncs) ->
    wil::task<std::tuple<
        decltype(asyncs.GetResults())...>>
{
    co_return std::make_tuple(co_await asyncs...);
}

Extending support to other types of awaitables, such as wil::task, means having to fire up a lot of infrastructure to figure out what the co_await return type is.

template<typename T>
using await_result = decltype(std::declval<
        awaiter_finder::type<T>>().await_resume());

template<typename... Asyncs>
wil::task<std::tuple<await_result<Asyncs>...>>
when_all_with_results(Asyncs... async)
{
    co_return std::make_tuple(co_await async...);
}

Great, you solved one problem but introduced at least two new ones.

First problem is that one of these awaitables might produce a C++ reference. This wasn’t a problem with IAsyncOperation, since that never produces a C++ reference, but arbitrary awaitables might do that. Another problem is that one of the async values might be an awaitable that completes with void. You can’t put a void inside a tuple.

We’ll look more closely at these problems next time.

Raymond Chen

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