{"id":103968,"date":"2020-07-08T07:00:00","date_gmt":"2020-07-08T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=103968"},"modified":"2020-07-08T08:18:46","modified_gmt":"2020-07-08T15:18:46","slug":"20200708-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20200708-00\/?p=103968","title":{"rendered":"Cancelling a Windows Runtime asynchronous operation, part 6: C++\/WinRT-generated asynchronous operations"},"content":{"rendered":"

Last time, we learned that C++\/WinRT defers to the underlying asynchronous operation to report the cancellation in whatever way it sees fit<\/a>. Today, we’ll look at the case that the asynchronous operation was generated by the C++\/WinRT library.<\/p>\n

When you invoke the Cancel()<\/code> on a C++\/WinRT asynchronous operation, this is the code that runs:<\/p>\n

struct promise_base : ...\r\n{\r\n    ...\r\n\r\n    void Cancel() noexcept\r\n    {\r\n        winrt::delegate<> cancel;\r\n        {\r\n            slim_lock_guard const guard(m_lock);\r\n            if (m_status == AsyncStatus::Started)\r\n            {\r\n                m_status = AsyncStatus::Canceled;<\/span>\r\n                cancel = std::move(m_cancel);\r\n            }\r\n        }\r\n        if (cancel)\r\n        {\r\n            cancel();\r\n        }\r\n    }\r\n};\r\n<\/pre>\n
The promise transitions into the Canceled<\/code>, and if the coroutine had registered a cancellation callback, it is invoked.<\/p>\n
Whenever the coroutine associated with the promise performs a co_await<\/code>, the await_transform<\/code> kicks in (sorry, I haven’t explained this yet, but trust me), and that’s where the C++\/WinRT library gets a chance to abandon the operation:<\/p>\n
template <typename Expression>\r\nExpression&& await_transform(Expression&& expression)\r\n{\r\n    if (Status() == AsyncStatus::Canceled)\r\n    {\r\n        throw winrt::hresult_canceled();\r\n    }<\/span>\r\n    return std::forward<Expression>(expression);\r\n}\r\n<\/pre>\n
The thrown hresult_canceled<\/code> exception is captured into the operation for later rethrowing.<\/p>\n
The C++\/WinRT library also checks for cancellation when the coroutine runs to completion:<\/p>\n
struct promise_type final : ...\r\n{\r\n    ...\r\n\r\n    void return_void()\r\n    {\r\n        ...\r\n        if (this->m_status == AsyncStatus::Started)\r\n        {\r\n            this->m_status = AsyncStatus::Completed;\r\n        }\r\n        else\r\n        {\r\n            WINRT_ASSERT(this->m_status == AsyncStatus::Canceled);\r\n            this->m_exception = make_exception_ptr(winrt::hresult_canceled());<\/span>\r\n        }\r\n        ...\r\n    }\r\n};\r\n<\/pre>\n
If the operation has been cancelled, then we manufacture a fake hresult_canceled<\/code> exception and save it in the m_exception<\/code>.<\/p>\n
So we see that whether the operation’s cancellation is detected by await_transform<\/code> or by return_void<\/code> (or return_value<\/code> for coroutines that produce a value), we end up with an hresult_canceled<\/code> exception stashed in the operation.<\/p>\n
And it is this exception that comes back out when somebody asks for the result of the asynchronous activity:<\/p>\n
struct promise_type final : ...\r\n{\r\n    ...\r\n\r\n    void GetResults()\r\n    {\r\n        ...\r\n        if (this->m_status == AsyncStatus::Completed)\r\n        {\r\n            return;\r\n        }\r\n        this->rethrow_if_failed();<\/span>\r\n        ...\r\n    }\r\n\r\n    void rethrow_if_failed() const\r\n    {\r\n        if (m_status == AsyncStatus::Error || m_status == AsyncStatus::Canceled)\r\n        {\r\n            std::rethrow_exception(m_exception);\r\n        }\r\n    }\r\n};\r\n<\/pre>\n
If the operation was canceled, then we reach rethrow_if_failed<\/code> which rethrows the captured exception, which we saw earlier is going to be an hresult_canceled<\/code>.<\/p>\n
But C++\/WinRT is not the only source of IAsync\u00adAction<\/code> and IAsync\u00adOperation<\/code> objects. Next time, we’ll look at another major source: WRL.<\/p>\n","protected":false},"excerpt":{"rendered":"
It came from inside the building.<\/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-103968","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
It came from inside the building.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103968","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=103968"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/103968\/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=103968"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=103968"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=103968"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}