{"id":105002,"date":"2021-03-25T07:00:00","date_gmt":"2021-03-25T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=105002"},"modified":"2021-03-26T08:05:00","modified_gmt":"2021-03-26T15:05:00","slug":"20210325-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20210325-00\/?p=105002","title":{"rendered":"Creating a task completion source for a C++ coroutine: Producing nothing"},"content":{"rendered":"

Last time, we created a result_holder<\/code> that can hold a reference<\/a>, and we solved it by using a wrapper. But there’s another type that we can’t put in a result_holder<\/code>, not even with the help of a wrapper. That type is void<\/code>.<\/p>\n

    struct wrapper\r\n    {\r\n        void value; \/\/ not allowed\r\n    };\r\n<\/pre>\n
This doesn’t work because you cannot have an object of type void<\/code>. You might nevertheless want to have a result_holder<\/code> that “holds” a void<\/code>, because that is basically an event: The result is the fact that something happened.<\/p>\n
There are a few ways to work around this problem. One is to redirect void<\/code> to some other type like bool<\/code>, and just ignore the bool<\/code> value. This is the approach that is often used in C# code: A task completion event of void<\/code> is just a task completion event of bool<\/code> where the bool<\/code> is ignored.<\/p>\n
But in C++, we have partial specialization, so we can get all fancy-like.<\/p>\n
    template<typename T>\r\n    struct wrapper\r\n    {\r\n        T value;\r\n        T get_value() { return static_cast<T>(value); }\r\n    };\r\n\r\n    template<>\r\n    struct wrapper<void>\r\n    {\r\n        void get_value() { }\r\n    };\r\n<\/pre>\n
In the case of void<\/code>, we use an empty class. This avoids the trouble of having to initialize a dummy bool<\/code> member, and it opens the door to empty base class optimization, although we won’t take advantage of EBO here. We then add get_value<\/code> methods to extract the value in a uniform way:<\/p>\n