Synthesizing a when_all coroutine from pieces you already have

Raymond Chen


C++/WinRT provides a helper function that takes a bunch of awaitable objects and produces an IAsyncAction that completes when all of the awaitable objects have completed.

It has a very simple definition.

template <typename... T>
Windows::Foundation::IAsyncAction when_all(T... async)
    (co_await async, ...);

Let’s take this apart.

The opening template<typename... T> says that this is a template that takes an arbitrary number of type parameters.

The function prototype is for a function which takes a parameter list of T... async. This means that you can pass as many parameters as you like, of whatever type you prefer, and they are accepted by value. The parameter list is given the name async.

The body is (co_await async, ...). This is a fold expression. If async... represents the parameter list async₁, async₂, async₃, async₄, then

(co_await async, ...)

expands to

(co_await async₁, co_await async₂, co_await async₃, co_await async₄)

Usually, fold expressions are used with operators like + or ||:

(v + ...)

expands to

(v₁ + v₂ + v₃ + v₄)


(is_even(v) || ...)

expands to

(is_even(v₁) || is_even(v₂) || is_even(v₃) || is_even(v₄))

for example.¹ Here, we’re using the comma operator not for anything interesting; it’s just a way to execute a bunch of stuff.

The end result of this all is that if you write when_all(x, y, z), this becomes

Windows::Foundation::IAsyncAction when_all(X x, Y y Z z)
  (co_await x, co_await y, co_await z);

This produce a coroutine which awaits x, then throws the result away; then awaits y, then throws the result away; and finally awaits z, then throws the result away. And then the coroutine is finished.

Mid-article bonus chatter: There are some flaws in the above function. We’ll look at them next time. End of bonus chatter.

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

It reminds me of the old Sesame Street sketch where Grover has no trouble counting blocks, but when asked to count some oranges, Grover freezes up. “I know how to count blocks, but I do not know how to count oranges!”

I have to confess that as I child, I didn’t get the joke.

Anyway, we saw how to count blocks (await every object in a parameter list). We just need to count oranges (await every object in a vector).

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

We can try to wrap this up in a function:

template<typename T>
IAsyncAction when_all(T const& container)
  for (auto&& v : container) co_await v;

This doesn’t work because there is an ambiguity in the case where there is one parameter. Are you trying to await all of the awaitables in a list of length 1? Or is the parameter a container, and you want to await all objects within it?

I’ll say that if the single parameter has a method named begin whose return type is not void, then it’s a container. (I could try to do better by also accepting a free function begin, but I’m feeling lazy.)

template<typename T>
auto when_all(T&& container) ->
  std::enable_if_t<sizeof(container.begin()) >= 0, IAsyncAction>
  for (auto&& v : container) co_await v;

I’m using sizeof as a way to create a constant true value from a dependent type, so it can be tested with std::enable_if_t. We know that the container’s iterator must be a complete type because we’re going to use it in the for loop.

We might also want to support a range expressed as two input iterators.²

template<typename Iter>
    typename std::iterator_traits<Iter>::iterator_category,
    std::input_iterator_tag>, IAsyncAction>
when_all(Iter begin, Iter end)
  for (; begin != end; ++begin) co_await *begin;

In all of these cases, you need to make sure to keep the container or range alive until after the co_await when_all(...) completes.

Whatever way you come up with to express a collection of awaitable objects, you can write a function that accepts that collection and awaits each object in the collection.

Go ahead and count oranges.

¹ More precisely, they expand to

(v₁ + (v₂ + (v₃ + v₄)))


(is_even(v₁) || (is_even(v₂) || (is_even(v₃) || is_even(v₄))))

If you want the left-associative version, then you need to put the ellipsis on the left.

(... + v)
(... || is_even(v))

² For extra flexibility, we could implicitly convert the second argument to match the first.

// C++17
when_all(Iter begin, std::enable_if_t<true, Iter> end)

// C++20
when_all(Iter begin, std::type_identity_t<Iter> end)

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    Irony I was looking for this method yesterday (co_awaiting a vector of independent tasks). I found this code in the WinRT repo… but it’s not exposed in the currently shipping WindowsSDK; I assume it will ship with 16.8 or 16.9.

    Extra credit: Any reason there can’t be a ranges version if __cpp_lib_ranges is defined?