{"id":107755,"date":"2023-01-27T07:00:00","date_gmt":"2023-01-27T15:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=107755"},"modified":"2023-01-26T20:23:13","modified_gmt":"2023-01-27T04:23:13","slug":"20230127-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20230127-00\/?p=107755","title":{"rendered":"Inside C++\/WinRT: Apartment switching: Unwinding the stack"},"content":{"rendered":"

Last time, we found a case where we could avoid calling IContext\u00adCallback::Context\u00adCallback()<\/code><\/a>, thereby reducing stack usage.<\/p>\n

Another case where we can avoid having to call IContext\u00adCallback::Context\u00adCallback()<\/code> is the case where we are resuming on the multi-threaded apartment (MTA). In that case, we can just resume on a new threadpool thread, since threadpool tasks run in the implicit multi-threaded apartment. This is an even better way of reducing stack usage, because it releases the original thread entirely.<\/p>\n

In order to know what kind of apartment we captured, we’ll have to remember the apartment type as well as the IContext\u00adCallback<\/code>. We’ll keep both of them in a new structure:<\/p>\n

struct resume_apartment_context\r\n{\r\n    resume_apartment_context() = default;\r\n    resume_apartment_context(std::nullptr_t) :\r\n        m_context(nullptr) {}\r\n\r\n    bool valid() const noexcept\r\n    {\r\n        return m_context != nullptr;\r\n    }\r\n\r\n    com_ptr<IContextCallback> m_context =\r\n        capture<IContextCallback>(WINRT_IMPL_CoGetObjectContext);\r\n    int32_t m_context_type = get_apartment_type().first;<\/span>\r\n};\r\n<\/pre>\n
When switching apartments, we add a check to see if we are switching to the MTA. If so, then we just queue the coroutine contination directly to a threadpool thread.<\/p>\n
inline auto resume_apartment(\r\n    resume_apartment_context<\/span> const& context,\r\n    coroutine_handle<> handle)\r\n{\r\n    WINRT_ASSERT(context.valid());\r\n    if (context.m_context<\/span> ==\r\n        capture<IContextCallback>(WINRT_IMPL_CoGetObjectContext))\r\n    {\r\n        handle();\r\n    }\r\n    else if (context.m_context_type == 1 \/* APTTYPE_MTA \/)\r\n    {\r\n        resume_background(handle);\r\n    }\r\n    else<\/span> if (is_sta_thread())\r\n    {\r\n        resume_apartment_on_threadpool(context.m_context<\/span>, handle);\r\n    }\r\n    else\r\n    {\r\n        resume_apartment_sync(context.m_context<\/span>, handle);\r\n    }\r\n}\r\n<\/pre>\n
The resume_background()<\/code> function resumes the coroutine on a threadpool thread:<\/p>\n
inline void __stdcall resume_background_callback(\r\n    void*, void* context) noexcept\r\n{\r\n    coroutine_handle<>::from_address(context)();\r\n};\r\n\r\ninline auto resume_background(coroutine_handle<> handle)\r\n{\r\n    submit_threadpool_callback(\r\n        resume_background_callback, handle.address());\r\n}\r\n<\/pre>\n
The apartment_context<\/code> holds this structure instead of just the IContext\u00adCallback<\/code>.<\/p>\n
struct apartment_context\r\n{\r\n    apartment_context() = default;\r\n    apartment_context(std::nullptr_t) : context(nullptr) { }\r\n\r\n    operator bool() const noexcept { return context.valid()<\/span>; }\r\n    bool operator!() const noexcept { return !context.valid()<\/span>; }\r\n\r\n    resume_apartment_context context;<\/span>\r\n};\r\n<\/pre>\n
And the Windows Runtime awaiter captures it into the lambda:<\/p>\n
template <typename Async>\r\nstruct await_adapter\r\n{\r\n    await_adapter(Async const& async) : async(async) { }\r\n\r\n    Async const& async;\r\n\r\n    bool await_ready() const noexcept\r\n    {\r\n        return false;\r\n    }\r\n\r\n    void await_suspend(std::experimental::coroutine_handle<> handle) const\r\n    {\r\n        auto extend_lifetime = async;\r\n        async.Completed([\r\n            handle,\r\n            context = resume_apartment_context()<\/span>\r\n        ](auto&& ...)\r\n        {\r\n            resume_apartment(context.context, handle);\r\n        });\r\n    }\r\n\r\n    auto await_resume() const\r\n    {\r\n        return async.GetResults();\r\n    }\r\n};\r\n<\/pre>\n
Next time, we’ll look at another feature of C++\/WinRT that needs to be added to our apartment switching framework.<\/p>\n","protected":false},"excerpt":{"rendered":"
Reducing stack usage.<\/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-107755","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Reducing stack usage.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/107755","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=107755"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/107755\/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=107755"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=107755"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=107755"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}