{"id":105061,"date":"2021-04-07T07:00:00","date_gmt":"2021-04-07T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=105061"},"modified":"2021-04-06T21:49:10","modified_gmt":"2021-04-07T04:49:10","slug":"20210407-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20210407-00\/?p=105061\/","title":{"rendered":"C++ coroutines: Accepting types via return_void and return_value"},"content":{"rendered":"

Last time, we built the result holder for our simple_task<\/code> coroutine<\/a> and showed how to move values into the holder and move them back out.<\/p>\n

So now we’re actually going to move them in.<\/p>\n

    template<typename T>\r\n    struct simple_promise : simple_promise_base<T>\r\n    {\r\n        \/\/ \u27e6implement return_value\u27e7 \u2254\r\n        void return_value(T&& value)\r\n        {\r\n            this->set_value(std::forward<T>(value));\r\n        }\r\n\r\n        template<typename Dummy = void>\r\n        std::enable_if_t<!std::is_reference_v<T>, Dummy>\r\n            return_value(T const& value)\r\n        {\r\n            this->set_value(value);\r\n        }\r\n    };\r\n<\/pre>\n
In the case where T<\/code> is not void<\/code>, we have a few different cases to deal with: T<\/code> could be an lvalue reference, an rvalue reference, or a non-reference.<\/p>\n
Let’s write out the possibilities. Let U<\/code> the result of removing all references from T<\/code>.<\/p>\n\n\n\n\n\n\n
 <\/td>\nObject<\/th>\nLvalue reference<\/th>\nRvalue reference<\/th>\n<\/tr>\n
T<\/tt><\/td>\nU<\/tt><\/td>\nU&<\/tt><\/td>\nU&&<\/tt><\/td>\n<\/tr>\n
T&&<\/tt><\/td>\nU&&<\/tt><\/td>\nU&<\/tt><\/td>\nU&&<\/tt><\/td>\n<\/tr>\n
T const&<\/tt><\/td>\nU const&<\/tt><\/td>\nU&<\/tt><\/td>\nU&<\/tt><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
Reference collapsing rules say that adding &&<\/code> to a reference has no effect. Furthermore, references are already immutable, so adding const<\/code> also has no effect. Therefore T const&<\/code> is the same as T&<\/code> when T<\/code> is a reference. The reference collapsing rules say that adding a single &<\/code> converts an rvalue reference to an lvalue reference and leaves lvalue references unchanged.\u00b9<\/p>\n
Okay, so let’s apply the above table to our situation.<\/p>\n
If T<\/code> is not a reference, then we are in the U<\/code> column, and we want both overloads. The first will move the value and the second will copy it.<\/p>\n
If T<\/code> is an lvalue reference, then we are in the U&<\/code> column, so T&&<\/code> and T const&<\/code> are the same thing. The two overloads conflict. We want only the first overload and we should delete the second.<\/p>\n
If T<\/code> is an rvalue reference, then we are in the U&&<\/code> column. We don’t want to accept lvalue references, because they will create an error deep inside set_value<\/code> which will not be obvious to interpret for those unfamiliar with our implementation. Let’s just delete the second overload to convert the error message to a more understandable “cannot bind an lvalue to an rvalue reference.” (This is another example of compiler error message metaprogramming.)<\/p>\n
Putting this all together says that we want to delete the second overload if T<\/code> is any kind of reference. So we use SFINAE to remove it.<\/p>\n
The case where T<\/code> is void<\/code> is much more straightforward:<\/p>\n
    template<>\r\n    struct simple_promise<void> : simple_promise_base<void>\r\n    {\r\n        \/\/ \u27e6implement return_void\u27e7 \u2254\r\n        void return_void()\r\n        {\r\n            this->set_value();\r\n        }\r\n    };\r\n<\/pre>\n
No weird reference shenanigans. There’s only one kind of void, and we want to call set_value<\/code> with nothing.<\/p>\n
Note that we had to put an explicit this-><\/code> prefix on our calls to set_value<\/code> to tell the compiler that set_value<\/code> is a dependent name. Otherwise, two-phase lookup would interpret it as a non-dependent name and try to resolve it in the enclosing scope.<\/p>\n
Next time, we’ll look more closely at the awaiter used when the caller co_await<\/code>s the simple_task<\/code>.<\/p>\n
\u00b9 The way I remember the reference collapsing rules is to rephrase it as “lvalues are sticky.” Once there’s an lvalue reference anywhere in the reference chain, the final result is an lvalue.<\/p>\n","protected":false},"excerpt":{"rendered":"
A little bit of SFINAE tweaking.<\/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-105061","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
A little bit of SFINAE tweaking.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/105061","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=105061"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/105061\/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=105061"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=105061"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=105061"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}