{"id":1981,"date":"2014-02-12T13:00:17","date_gmt":"2014-02-12T13:00:17","guid":{"rendered":"http:\/\/writeasync.net\/?p=1981"},"modified":"2014-02-11T09:58:56","modified_gmt":"2014-02-11T09:58:56","slug":"porting-inputqueue-to-c","status":"publish","type":"post","link":"http:\/\/writeasync.net\/?p=1981","title":{"rendered":"Porting InputQueue to C++"},"content":{"rendered":"

In the previous post<\/a>, I gave a brief intro to the PPL and the basic constructs for writing async C++ code. In this post, I will discuss the highlights of how I ported the InputQueue sample<\/a> to C++ using the PPL and TDD. The code is available on GitHub: NativeQueueSample commit history<\/a>.<\/p>\n

My C++ TDD tool of choice is the Microsoft Unit Testing Framework for C++<\/a>. The main selling point is its smooth integration with Visual Studio — even the Express edition. You just create a Native Unit Test Project, create a wrapping TEST_CLASS and some TEST_METHOD functions and you’re done. Note that with the Express edition, you cannot set the tests to run automatically after each build. However, you can do as I did and create a Post-Build Event<\/a> to run the tests. You won’t get the nice integration with Test Explorer but I like it better than manually triggering tests. The command line should look something like this: \"$(VSInstallDir)Common7\\IDE\\CommonExtensions\\Microsoft\\TestWindow\\vstest.console.exe\" \"$(TargetPath)\" \/Platform:x64 \/inIsolation<\/code><\/p>\n

The first thing to watch out for when writing unit tests for async C++ code: don’t double fault! I learned this the hard way when the test runner inexplicably hung after running my very first test:<\/p>\n

\r\nTEST_METHOD(Dequeue_completes_after_enqueue)\r\n{\r\n    InputQueue<wstring> queue;\r\n\r\n    task<wstring> task = queue.DequeueAsync();\r\n\r\n    Assert::IsFalse(task.is_done());\r\n\r\n    queue.Enqueue(wstring(L"a"));\r\n\r\n    Assert::IsTrue(task.is_done());\r\n    Assert::AreEqual(wstring(L"a"), task.get());\r\n}\r\n<\/pre>\n
Seems simple enough. However, my implementation code, being just a stub, looked like this:<\/p>\n
\r\nvoid Enqueue(T item)\r\n{\r\n    throw std::runtime_error("Not implemented.");\r\n}\r\n\r\nconcurrency::task<T> DequeueAsync()\r\n{\r\n    return concurrency::task_from_exception<T>(runtime_error("Not implemented."));\r\n}\r\n<\/pre>\n
It turns out that the first Assert::IsFalse<\/code> was failing (expected, since I’m completing DequeueAsync<\/code> immediately with an exception), which ended up throwing an exception (expected for any assertion failure), which ended up unwinding the stack and destroying the task<\/code> object, which ended up failing due to the fact that the original exception was unobserved. To avoid this, I came up with an AssertTaskPending<\/code> helper:<\/p>\n
\r\ntemplate <typename T>\r\ntask<T> & AssertTaskPending(task<T> & task)\r\n{\r\n    if (task.is_done())\r\n    {\r\n        \/\/ Before asserting, force rethrow of exception if task finished with error.\r\n        \/\/ Otherwise, we run the risk of a "double fault" due to an unobserved task\r\n        \/\/ error, which can end up hanging the VS test executor.\r\n        Logger::WriteMessage(L"Task completed unexpectedly.");\r\n        task.get();\r\n    }\r\n\r\n    Assert::IsFalse(task.is_done());\r\n    return task;\r\n}\r\n<\/pre>\n
Ensuring that we call get()<\/code> before asserting will avoid the issue.<\/p>\n
The unit tests ended up looking quite similar to the C# implementation. The actual implementation code, however, had a few minor differences worth pointing out.<\/p>\n
To properly represent the case where there is no pending dequeue operation, I needed to make it possible to have a null<\/code> value for the task_completion_event<\/code>. In modern C++, this suggests the use of smart pointers<\/a> — specifically, unique_ptr<\/code><\/a> since there is only a single owner of the resource (the InputQueue<\/code> itself).<\/p>\n
C++ has deterministic destruction. Unlike the C# implementation where we had to implement IDisposable<\/code> and do some handling of still-active tasks, the chained destruction of InputQueue<\/code>‘s task_completion_event<\/code> conveniently takes care of all of this automatically. Note that tasks that are canceled in this fashion throw task_canceled<\/code><\/a>.<\/p>\n
Despite these minor details, InputQueue<\/code> in C++ is largely identical to its managed counterpart. In the next post, I will talk about some thread-safety considerations and the integration test.<\/p>\n","protected":false},"excerpt":{"rendered":"
In the previous post, I gave a brief intro to the PPL and the basic constructs for writing async C++ code. In this post, I will discuss the highlights of how I ported the InputQueue sample to C++ using the… <\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[21,101,41],"tags":[],"class_list":["post-1981","post","type-post","status-publish","format-standard","hentry","category-async","category-native","category-tdd"],"_links":{"self":[{"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/posts\/1981","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/writeasync.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1981"}],"version-history":[{"count":0,"href":"http:\/\/writeasync.net\/index.php?rest_route=\/wp\/v2\/posts\/1981\/revisions"}],"wp:attachment":[{"href":"http:\/\/writeasync.net\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1981"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/writeasync.net\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1981"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/writeasync.net\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1981"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}