Last time, we figured out how to create an already-completed asynchronous activity in C++/WinRT. Today we’ll try to generalize it to cover the four kinds of Windows Runtime asynchronous activities.

One way to do this is to write four different functions for each category, similar to how we dealt with cv-qualifiers before we had deducing this.

winrt::Windows::Foundation::IAsyncAction
MakeCompletedAsyncAction()
{
    co_return;
}

template<typename Progress>
winrt::Windows::Foundation::IAsyncActionWithProgress<Progress>
MakeCompletedAsyncActionWithProgress()
{
    co_return;
}

template<typename Result, typename Progress>
winrt::Windows::Foundation::IAsyncOperation<Result>
MakeCompletedAsyncOperation(Result result)
{
    co_return result;
}

template<typename Result, typename Progress>
winrt::Windows::Foundation::IAsyncOperationWithProgress<Result, Progress>
MakeCompletedAsyncOperationWithProgress(Result result)
{
    co_return result;
}

// Sample usage:

winrt::Windows::Foundation::IAsyncOperation<int>
GetHeightAsync()
{
    return MakeCompletedAsyncOperation(42);
}

winrt::Windows::Foundation::
    IAsyncOperationWithProgress<int, HeightProgress>
GetHeightAsync()
{
    return MakeCompletedAsyncOperationWithProgress<
        int, HeightProgress>(42);
}

Explicit specialization is required for the WithProgress versions, since there is no opportunity to deduce the progress type.

We could combine the four flavors into a single function, though this means that specialization is mandatory.

template<typename Async, typename... Result>
Async MakeCompleted(Result... result)
{
    if constexpr (sizeof...(Result) == 0) {
        co_return;
    } else {
        static_assert(sizeof...(Result) == 1);
        co_return std::get<0>(
            std::forward_as_tuple(result...));
    }
}

We use a trick in Make­Completed by formally accepting any number of arguments, although we check inside the function body that it is zero or one. In the case where there is one parameter, we use the forward_as_tuple + get technique to pull a single item from a parameter pack.

Next time, we’ll try to write MakeFailed.