{"id":109959,"date":"2024-07-08T07:00:00","date_gmt":"2024-07-08T14:00:00","guid":{"rendered":"https:\/\/devblogs.microsoft.com\/oldnewthing\/?p=109959"},"modified":"2024-06-24T10:07:25","modified_gmt":"2024-06-24T17:07:25","slug":"20240708-00","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20240708-00\/?p=109959","title":{"rendered":"What’s the point of std::monostate<\/CODE>? You can’t do anything with it!"},"content":{"rendered":"

C++17 introduced std::monostate<\/code>, and I used it as a placeholder to represent the results of a coroutine that produces nothing<\/a>. In the comments, Neil Rashbrook asked what you are expected to do with a std::monostate<\/code><\/a>, seeing as has no members and only trivial member functions.<\/p>\n

The answer is “nothing”.<\/p>\n

The purpose of std::monostate<\/code> is to be a dummy type that does nothing. All instances are considered equal to each other. It is basically this:<\/p>\n

struct monostate {};\r\n\/\/ plus relational operators and a hash specialization\r\n<\/pre>\n
You can see it in  libcxx (LLVM\/clang)<\/a>,  libstdc++ (gcc)<\/a>, and  stl (msvc)<\/a>.<\/p>\n
But what’s the point of a class that does nothing, and which you can do nothing with?<\/p>\n
You don’t use monostate<\/code> because you want to do something. You use monostate<\/code> when you don’t want to do anything<\/i>.<\/p>\n
Its original purpose was to be used as the initial type in a std::variant<\/code> to allow it to be default-constructed in an empty state.<\/p>\n
struct Widget\r\n{\r\n    \/\/ no default constructor\r\n    Widget(std::string const& id);\r\n\r\n    \u27e6 other members \u27e7\r\n};\r\n\r\nstruct PortListener\r\n{\r\n    \/\/ default constructor has unwanted side effects\r\n    PortListener(int port = 80);\r\n\r\n    \u27e6 other members \u27e7\r\n};\r\n\r\nstd::variant<Widget, PortListener> thingie; \/\/ can't do this\r\n<\/pre>\n
The std::variant<\/code>‘s default constructor default-constructs its first alternative. Since Widget<\/code> doesn’t have a default constructor, you can’t put it first. And Port\u00adListener<\/code>‘s default constructor has unwanted side effects, so we don’t want to put it first.<\/p>\n
Enter std::monostate<\/code>. You can put that guy first.<\/p>\n
std::variant<std::monostate, Widget, PortListener> thingie;\r\n<\/pre>\n
The thingie<\/code> default-constructs into a monostate<\/code>. What can you do with a monostate<\/code>? Nothing! Its job is just to be a dummy type that you can use when you are forced to provide a default-constructible type but don’t want to.<\/p>\n
In the case of a std::variant<\/code>, you can think of inserting std::monostate<\/code> as a way to add an “empty” state to a variant<\/code>,\u00b9 saving you the trouble of having to create a std::optional<std::variant<...>><\/code>. You can treat the std::monostate<\/code> as the “empty” state.<\/p>\n
In our usage of std::monostate<\/code>, we used it as just a dummy type that stands in for void<\/code>.\u00b2<\/p>\n
\u00b9 More precisely, a way to add another<\/i> “empty” state to a variant<\/code>. There is already an “empty” state for a std::variant<\/code>, known as valueless_by_exception<\/code>. This state is wacky, though, so you want to avoid it as much as possible. Another topic for another day.<\/p>\n
\u00b2 Bonus reading:  Regular Void<\/a>, which has stalled.<\/p>\n","protected":false},"excerpt":{"rendered":"
Not doing anything with it is exactly the point.<\/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-109959","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Not doing anything with it is exactly the point.<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109959","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=109959"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/109959\/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=109959"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=109959"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=109959"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}