{"id":109259,"date":"2024-01-11T07:00:00","date_gmt":"2024-01-11T15:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=109259"},"modified":"2024-01-11T06:44:17","modified_gmt":"2024-01-11T14:44:17","slug":"20240111-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20240111-00\/?p=109259","title":{"rendered":"In C++\/WinRT, how can I await multiple coroutines and capture the results?, part 2"},"content":{"rendered":"

Instead of replacing the awaiter so it doesn’t retrieve the results<\/a>, 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 IInspectable<\/code>s is not strongly-typed.<\/p>\n

Fortunately, we can use our friend simple_task<\/code><\/a>, which has since been added to the Windows Implementation Library<\/a> as wil::task<\/code>.<\/p>\n

template<typename... Results>\r\nwil::task<std::tuple<Results>>\r\nwhen_all_with_results(\r\n    winrt::Windows::Foundation::IAsyncOperation<Results>... asyncs)\r\n{\r\n    co_return std::make_tuple(co_await asyncs...);\r\n}\r\n\r\nauto [result1, result2] =\r\n    co_await when_all_with_results(Do1Async(), Do2Async());\r\n<\/pre>\n
We wish we could have written<\/p>\n
template<typename... Asyncs>\r\nwil::task<auto<\/span>>\r\nwhen_all_with_results(Asyncs... asyncs)\r\n{\r\n    co_return std::make_tuple(co_await asyncs...);\r\n}\r\n<\/pre>\n
but there is currently no facility in the C++ language for this sort of weirdo template placeholder usage.<\/p>\n
The above formulation does limit you to IAsyncOperation<T><\/code>, so you cannot use other awaitables with when_all_with_results<\/code>, like IAsync\u00adOperation\u00adWith\u00adProgress<T, P><\/code>. Adding IAsync\u00adOperation\u00adWith\u00adProgress<T, P><\/code> support isn’t so bad, because the result type is available from both IAsync\u00adOperation<T><\/code> and IAsync\u00adOperation\u00adWith\u00adProgress<T, P><\/code> by checking the return type of Get\u00adResult()<\/code>.<\/p>\n
template<typename... Asyncs>\r\nwil::task<std::tuple<\r\n    decltype(std::declval<Asyncs>().GetResults())...>>\r\nwhen_all_with_results(Asyncs... asyncs)\r\n{\r\n    co_return std::make_tuple(co_await asyncs...);\r\n}\r\n<\/pre>\n
Or, taking advantage of trailing return types so we don’t need to go through the hassle of declval<\/code>:<\/p>\n
template<typename... Asyncs>\r\nauto\r\nwhen_all_with_results(Asyncs... asyncs) ->\r\n    wil::task<std::tuple<\r\n        decltype(asyncs.GetResults())...>>\r\n{\r\n    co_return std::make_tuple(co_await asyncs...);\r\n}\r\n<\/pre>\n
Extending support to other types of awaitables, such as wil::task<\/code>, means having to fire up a lot of infrastructure to  figure out what the co_await<\/code> return type is<\/a>.<\/p>\n
template<typename T>\r\nusing await_result = decltype(std::declval<\r\n        awaiter_finder::type<T>>().await_resume());\r\n\r\ntemplate<typename... Asyncs>\r\nwil::task<std::tuple<await_result<Asyncs>...>>\r\nwhen_all_with_results(Asyncs... async)\r\n{\r\n    co_return std::make_tuple(co_await async...);\r\n}\r\n<\/pre>\n
Great, you solved one problem but introduced at least two new ones.<\/p>\n
First problem is that one of these awaitables might produce a C++ reference. This wasn’t a problem with IAsyncOperation<\/code>, since that never produces a C++ reference, but arbitrary awaitables might do that. Another problem is that one of the async<\/code> values might be an awaitable that completes with void<\/code>. You can’t put a void<\/code> inside a tuple.<\/p>\n
We’ll look more closely at these problems next time.<\/p>\n","protected":false},"excerpt":{"rendered":"
Wrapping the results and returning them in a tuple.<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-109259","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Wrapping the results and returning them in a tuple.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109259","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=109259"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109259\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=109259"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=109259"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=109259"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}