How can I await multiple awaitables/IAsyncActions in C++/WinRT (`Promise.all` equivalent)?

Question

Is there an equivalent to JavaScript's Promise.all in C++ or C++/WinRT for awaitables (or just Windows.Foundation.IAsyncAction )?

For example, I am trying to gather multiple IAsyncActions and continue when all of them have completed. For now I'm starting each action and then awaiting them one by one in a simple loop:

winrt::IAsyncAction AwaitMultiple(std::vector<winrt::IAsyncAction> actions)
{
    for (const auto& action : actions)
    {
        co_await action;
    }
}

// ... somewhere else...

winrt::IAsyncAction MyFunctionAsync()
{
    std::vector<winrt::IAsyncAction> actions{};
    actions.push_back(/* some function that generates an IAsyncAction */);
    actions.push_back(/* some function that generates an IAsyncAction */);
    actions.push_back(/* some function that generates an IAsyncAction */);
    // ... etc...

    co_await AwaitMultiple(actions);

    // ... do other stuff...
}

Is there a better way to do this? Is there a language way?

Answer 1

As is often the case, there is indeed a C++/WinRT function for almost this case: winrt::when_all(T... async) (it is not currently documented on Advanced concurrency and asynchrony with C++/WinRT ).

If you have modern C++/WinRT, you can await multiple actions at once:

winrt::IAsyncAction MyFunctionAsync()
{
    const auto myAction = /* function that generates an IAsyncAction */;
    const auto myAction2 = /* function that generates an IAsyncAction */;
    const auto myAction3 = /* function that generates an IAsyncAction */;

    // magic!
    co_await winrt::when_all(myAction, myAction2, myAction3);
}

This is similar to the Parallel Pattern Library (PPL) when_all .

This doesn't solve the std::vector<winrt::IAsyncAction> above, but luckily Raymond Chen has some posts about that:

A customer wanted to know how they could pass a std::vector of IAsyncAction objects to the when_all function.

 std::vector<IAsyncAction> actions = get_actions(); for (auto&& action: actions) co_await action;

[... some more content about trying to wrap that in an overload for winrt::when_all ...]

Synthesizing a when_all coroutine from pieces you already have

---

See also:

• https://docs.microsoft.com/en-us/windows/uwp/cpp-and-winrt-apis/news#new-when_all-and-when_any-async-helpers
• Original PR for when_all
• More relevant Raymond Chen blog posts:
  • Rough edges in the when_all coroutine, part 1: Empty parameter list
  • Rough edges in the when_all coroutine, part 2: Overloaded comma operator