{"id":109973,"date":"2024-07-16T07:00:00","date_gmt":"2024-07-16T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=109973"},"modified":"2024-06-24T10:49:03","modified_gmt":"2024-06-24T17:49:03","slug":"20240716-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20240716-00\/?p=109973","title":{"rendered":"Creating an already-completed asynchronous activity in C++\/WinRT, part 6"},"content":{"rendered":"

Last time, we finished making generalized Make\u00adCompleted<\/code> and Make\u00adFailed<\/code> functions<\/a> for creating already-completed asynchronous activities in C++\/WinRT. We left off with the observation that it’s annoying having to write out the template type parameter all the time:<\/p>\n

winrt::Windows::Foundation::IAsyncOperationWithProgress<bool, WidgetSaveProgress>\r\n    SaveChanges()\r\n{\r\n    \/\/ The widget is immutable, so there are no changes to save.\r\n    return MakeCompleted<\r\n        winrt::Windows::Foundation::IAsyncOperationWithProgress<\r\n            bool, WidgetSaveProgress>>(true);\r\n}\r\n<\/pre>\n
We can avoid the extra typing by returning a proxy object and let the conversion operator tell us what interface the caller wants.<\/p>\n
template<typename... Result>\r\nstruct async_completed\r\n{\r\n    static_assert(sizeof...(Result) <= 1);\r\n    std::tuple<std::decay_t<Result>...> m_result;\r\n\r\n    async_completed(Result&&... result) :\r\n        m_result(std::forward<Result>(result)...) {}\r\n\r\n    template<typename Async>\r\n    Async as<\/a>() {\r\n        if constexpr (sizeof...(Result) == 0) {\r\n            co_return;\r\n        } else {\r\n            co_return std::move(std::get<0>(m_result));\r\n        }\r\n    }\r\n\r\n    template<typename Async>\r\n    operator Async() { return as<Async>(); }\r\n};\r\n\r\ntemplate<typename Error>\r\nstruct async_failed\r\n{\r\n    std::decay_t<Error> m_error;\r\n\r\n    async_failed(Error&& error) :\r\n        m_error(std::forward<Error>(error)) {}\r\n\r\n    template<typename Async>\r\n    operator Async() {\r\n        (void) co_await winrt::get_cancellation_token();\r\n        throw std::move(m_error);\r\n    }\r\n};\r\n<\/pre>\n
Now you can let the proxy convert to the desired type.<\/p>\n
winrt::Windows::Foundation::IAsyncAction\r\n    SaveAsync()\r\n{\r\n    return async_completed();\r\n}\r\n\r\nwinrt::Windows::Foundation::IAsyncOperation<bool>\r\n    SaveAsync()\r\n{\r\n    return async_completed(false);\r\n}\r\n\r\nwinrt::Windows::Foundation::IAsyncAction\r\n    SaveAsync()\r\n{\r\n    return async_failed(winrt::hresult_access_denied());\r\n}\r\n<\/pre>\n
Now, for cases like this, the async_completed<\/code> and async_failed<\/code> are awfully awkward-looking. The more natural way to write the methods would be<\/p>\n
winrt::Windows::Foundation::IAsyncAction\r\n    SaveAsync()\r\n{\r\n    co_return;\r\n}\r\n\r\nwinrt::Windows::Foundation::IAsyncOperation<bool>\r\n    SaveAsync()\r\n{\r\n    co_return false;\r\n}\r\n\r\nwinrt::Windows::Foundation::IAsyncAction\r\n    SaveAsync()\r\n{\r\n    (void) co_await winrt::get_cancellation_token();\r\n    throw winrt::hresult_access_denied();\r\n}\r\n<\/pre>\n
But sometimes you might want to create a failed or completed asynchronous activity outside of a coroutine, and for those cases, async_completed<\/code> and async_failed<\/code> might be handy.<\/p>\n
But wait, we’re firing up the coroutine infrastructure just to return a value or throw an exception. That feels like overkill. We’ll look into a direct implementation next time.<\/p>\n","protected":false},"excerpt":{"rendered":"
Let the conversion do the talking.<\/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-109973","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Let the conversion do the talking.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109973","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=109973"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109973\/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=109973"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=109973"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=109973"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}