{"id":5837,"date":"2020-01-24T08:00:20","date_gmt":"2020-01-24T08:00:20","guid":{"rendered":"https:\/\/www.modernescpp.com\/index.php\/c-20-define-the-concept-equal-and-ordering\/"},"modified":"2023-06-26T09:55:55","modified_gmt":"2023-06-26T09:55:55","slug":"c-20-define-the-concept-equal-and-ordering","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/c-20-define-the-concept-equal-and-ordering\/","title":{"rendered":"C++20: Define the Concepts Equal and Ordering"},"content":{"rendered":"

In my last post<\/a>, I defined the concept Equal<\/span>. Now, I go one step further and use the concept Equal<\/span> to define the concept Ordering<\/span>.<\/p>\n

<\/p>\n

 <\/p>\n

\"TimelineCpp20Concepts\"<\/p>\n

Here is a short reminder of where I ended with my last post. I defined the concept of Equal<\/span> and a function areEqual<\/span> to use it.<\/p>\n

<\/p>\n

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template<\/span><<\/span>typename T<\/span>><\/span>\r\nconcept<\/span> Equal<\/span> =<\/span>\r\n    requires(T<\/span> a, T<\/span> b) {\r\n        { a ==<\/span> b } -><\/span> bool;\r\n        { a !=<\/span> b } -><\/span> bool;\r\n};\r\n\r\n\r\nbool<\/span> areEqual(Equal<\/span> auto fir, Equal<\/span> auto sec) {                       \r\n  return fir ==<\/span> sec;\r\n}\r\n<\/pre>\n<\/div>\n
 <\/p>\n
My Wrong Usage of the Concept Equal<\/span><\/h3>\n
I used the concept of Equal<\/span> in my last post in the wrong way. The concept Equal<\/span> requires that a<\/span> and b<\/span> have the same type but the function areEqual<\/span> allows fir and sec to be different types that both support the concept Equal<\/span>. Using a constrained template parameter instead of placeholder syntax solves the issue:<\/p>\n
 <\/p>\n
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template<\/span> <<\/span>Equal<\/span> T<\/span>><\/span>\r\nbool<\/span> areEqual(T<\/span> fir, T<\/span> sec) {\r\n    fir ==<\/span> sec;\r\n}\r\n<\/pre>\n<\/div>\n
 <\/p>\n
Now, fir<\/span> and sec<\/span> must have the same type. <\/p>\n
Thanks a lot to Corentin Jabot for pointing out this inconsistency. <\/p>\n
Additionally, the concept Equal<\/span> should not check if the equal and unequal operator returns a bool<\/span> but something implicitly or explicitly convertible to a bool<\/span>. Here we are. <\/p>\n
 <\/p>\n
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template<\/span><<\/span>typename T<\/span>><\/span>\r\nconcept<\/span> Equal<\/span> =<\/span>\r\n    requires(T<\/span> a, T<\/span> b) {\r\n        { a ==<\/span> b } -><\/span> std::<\/span>convertible_to<<\/span>bool><\/span>;\r\n        { a !=<\/span> b } -><\/span> std::<\/span>convertible_to<<\/span>bool><\/span>;\r\n};\r\n<\/pre>\n<\/div>\n
 <\/p>\n
I have to add. std::convertible_to<\/a> <\/span>is a concept and requires the header <concepts><\/span>.  <\/p>\n
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template<\/span> <<\/span>class<\/span> From<\/span>, class<\/span> To<\/span>><\/span>\r\nconcept<\/span> convertible_to =<\/span>\r\n  std::<\/span>is_convertible_v<<\/span>From<\/span>, To<\/span>><\/span> &&<\/span>\r\n  requires(From<\/span> (&<\/span>f)()<\/span>) {\r\n    static_cast<<\/span>To<\/span>><\/span>(f()<\/span>);\r\n  };\r\n<\/pre>\n<\/div>\n
 <\/p>\n
The C++ 20 standard has already defined two concepts for equality comparing:<\/p>\n