{"id":109256,"date":"2024-01-10T07:00:00","date_gmt":"2024-01-10T15:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=109256"},"modified":"2024-01-10T06:34:23","modified_gmt":"2024-01-10T14:34:23","slug":"20240110-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20240110-00\/?p=109256","title":{"rendered":"In C++\/WinRT, how can I await multiple coroutines and capture the results?, part 1"},"content":{"rendered":"

The winrt::when_all<\/code> function in C++\/WinRT is not a complicated beast. It just awaits each of its parameters. Some time ago, we looked at how it works and how you could use it to await all the coroutines in a container or range<\/a>. But all of those examples just throw away the results. What if we also want to gather the results?<\/p>\n

Here’s what doesn’t work:<\/p>\n

auto op1 = DoSomething1Async();\r\nauto op2 = DoSomething2Async();\r\n\r\nco_await winrt::when_all(op1, op2);\r\n\r\n\/\/ These don't return proper results.\r\nauto result1 = op1.GetResults();\r\nauto result2 = op2.GetResults();\r\n<\/pre>\n
The problem is that the co_await<\/code> inside when_all<\/code> calls GetResults()<\/code> in order to produce the result of the co_await<\/code>. And GetResults()<\/code> produces valid results only the first time it is called after completion.<\/p>\n
One way to solve this problem is to use a different awaiter whose await_resume()<\/code> doesn’t call GetResults()<\/code>, leave it to the caller to fetch the results when they are ready.<\/p>\n
template<typename Async>\r\nstruct simple_async_awaiter\r\n{\r\n    simple_async_awaiter(Async const& async) : async(async) {}\r\n\r\n    Async const& async;\r\n\r\n    bool await_ready() const noexcept { return false; }\r\n\r\n    void await_suspend(std::coroutine_handle<> handle) const {\r\n        async.Completed([handle](auto&&...) { handle(); });\r\n    }\r\n\r\n    void await_resume() const noexcept {\r\n        \/* return async.GetResults();<\/span> *\/<\/span>\r\n    }\r\n};\r\n<\/pre>\n
We return void<\/code> from await_resume()<\/code>, which means that co_await<\/code> returns void<\/code>, and you can then fetch the results by calling GetResults()<\/code>. (We also don’t bother preserving the COM apartment context, and we don’t propagate cancellation, but the point here was to show that we skipped the GetResults()<\/code> inside await_resume()<\/code>.)<\/p>\n
But this is an awful lot of work, particularly because you have to replicate  all of the stuff that C++\/WinRT does behind the scenes in its awaiter<\/a>. Maybe we can find something simpler.<\/p>\n
We’ll continue our exploration next time.<\/p>\n","protected":false},"excerpt":{"rendered":"
Using a custom awaiter to suppress the GetResults()<\/CODE>.<\/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-109256","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Using a custom awaiter to suppress the GetResults()<\/CODE>.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109256","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=109256"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109256\/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=109256"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=109256"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=109256"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}