![\"fern](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2018\/10\/fern-821293_1280.jpg\")
<\/p>\nLet me continue with the rules for defining concepts in the guidelines. The first of the three remaining rules in this post is quite sophisticated.<\/p>\n
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Here are the rules for today:<\/p>\n
The explanation of the first rules is relatively concise. Maybe, too concise.<\/p>\n<\/p>\n
This is the reason for this rule from the guidelines: “Two concepts requiring the same syntax but having different semantics leads to ambiguity unless the programmer differentiates them.”<\/p>\n
Let’s assume; I defined the is_contiguous<\/span> trait. In this case, I can use it to distinguish a random access iterator RA_iter<\/span> from a contiguous iterator Contiguous_iter<\/span>.<\/p>\n <\/p>\n I can even use a tag class such as is_contiguous into a concept. Now, I have a more straightforward expression of my idea of contiguous iterator Contiguous_iter<\/span>.<\/p>\n <\/p>\n Okay, let me explain two key terms: traits and tag dispatching.<\/p>\n Traits are class templates that extract properties from a generic type. <\/p>\n The following program presents a type that satisfies the specific trait for each of the 14 primary type categories of the type-traits library. The primary type categories are complete and don\u2019t overlap. So each type is a member of a type category. If you check a type category for your type, the request is independent of the const<\/span> or volatile<\/span> qualifiers.<\/p>\n <\/p>\n I don’t want to bore you to death. Therefore, there is only the output of the line (1).<\/p>\n And here is the output of line (2).<\/p>\n <\/p>\n Tag dispatching enables it to choose a function based on the properties of its types. The decision takes place at compile time, and traits which I explained in the last paragraph are used. <\/p>\n\ntemplate<\/span><<\/span>typename<\/span> I><\/span> \/\/ iterator providing random access<\/span>\r\nconcept bool<\/span> RA_iter =<\/span> ...;\r\n\r\ntemplate<\/span><<\/span>typename<\/span> I><\/span> \/\/ iterator providing random access to contiguous data<\/span>\r\nconcept bool<\/span> Contiguous_iter =<\/span>\r\n RA_iter<<\/span>I><\/span> &&<\/span> is_contiguous<<\/span>I>::<\/span>value; \/\/ using is_contiguous trait<\/span>\r\n<\/pre>\n<\/div>\n\ntemplate<\/span><<\/span>typename<\/span> I><\/span> concept Contiguous =<\/span> is_contiguous<<\/span>I>::<\/span>value;\r\n\r\ntemplate<\/span><<\/span>typename<\/span> I><\/span>\r\nconcept bool<\/span> Contiguous_iter =<\/span> RA_iter<<\/span>I><\/span> &&<\/span> Contiguous<<\/span>I><\/span>;\r\n<\/pre>\n<\/div>\nTraits<\/h3>\n
\n\/\/ traitsPrimary.cpp<\/span>\r\n\r\n#include <iostream><\/span>\r\n#include <type_traits><\/span>\r\n\r\nusing<\/span> namespace<\/span> std;\r\n\r\ntemplate<\/span> <<\/span>typename<\/span> T><\/span>\r\nvoid<\/span> getPrimaryTypeCategory(){\r\n\r\n cout <<<\/span> boolalpha <<<\/span> endl;\r\n\r\n cout <<<\/span> \"is_void<T>::value: \"<\/span> <<<\/span> is_void<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_integral<T>::value: \"<\/span> <<<\/span> is_integral<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_floating_point<T>::value: \"<\/span> <<<\/span> is_floating_point<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_array<T>::value: \"<\/span> <<<\/span> is_array<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_pointer<T>::value: \"<\/span> <<<\/span> is_pointer<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_null_pointer<T>::value: \"<\/span> <<<\/span> is_null_pointer<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_member_object_pointer<T>::value: \"<\/span> <<<\/span> is_member_object_pointer<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_member_function_pointer<T>::value: \"<\/span> <<<\/span> is_member_function_pointer<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_enum<T>::value: \"<\/span> <<<\/span> is_enum<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_union<T>::value: \"<\/span> <<<\/span> is_union<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_class<T>::value: \"<\/span> <<<\/span> is_class<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_function<T>::value: \"<\/span> <<<\/span> is_function<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_lvalue_reference<T>::value: \"<\/span> <<<\/span> is_lvalue_reference<<\/span>T>::<\/span>value <<<\/span> endl;\r\n cout <<<\/span> \"is_rvalue_reference<T>::value: \"<\/span> <<<\/span> is_rvalue_reference<<\/span>T>::<\/span>value <<<\/span> endl;\r\n\r\n cout <<<\/span> endl;\r\n\r\n}\r\n\r\nint<\/span> main(){\r\n \r\n getPrimaryTypeCategory<<\/span>void<\/span>><\/span>(); \/\/ (1)<\/span>\r\n getPrimaryTypeCategory<<\/span>short<\/span>><\/span>(); \/\/ (1)<\/span>\r\n getPrimaryTypeCategory<<\/span>double<\/span>><\/span>();\r\n getPrimaryTypeCategory<<\/span>int<\/span> []><\/span>();\r\n getPrimaryTypeCategory<<\/span>int<\/span>*><\/span>();\r\n getPrimaryTypeCategory<<\/span>std::nullptr_t<\/span>><\/span>();\r\n struct<\/span> A{\r\n int<\/span> a;\r\n int<\/span> f<\/span>(double<\/span>){return<\/span> 2011<\/span>;}\r\n };\r\n getPrimaryTypeCategory<<\/span>int<\/span> A::*><\/span>();\r\n getPrimaryTypeCategory<<\/span>int<\/span> (A::*<\/span>)(double<\/span>)><\/span>();\r\n enum<\/span> E{\r\n e=<\/span> 1<\/span>,\r\n };\r\n getPrimaryTypeCategory<<\/span>E><\/span>();\r\n union<\/span> U{\r\n int<\/span> u;\r\n };\r\n getPrimaryTypeCategory<<\/span>U><\/span>();\r\n getPrimaryTypeCategory<<\/span>string><\/span>();\r\n getPrimaryTypeCategory<<\/span>int<\/span> *<\/span> (double<\/span>)><\/span>();\r\n getPrimaryTypeCategory<<\/span>int<\/span>&><\/span>(); \/\/ (2) <\/span>\r\n getPrimaryTypeCategory<<\/span>int<\/span>&&><\/span>(); \/\/ (2)<\/span>\r\n \r\n}\r\n<\/pre>\n<\/div>\n![\"traitsPrimaryFixed1\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2018\/10\/traitsPrimaryFixed1.png\")
<\/p>\n![\"traitsPrimaryFixed2\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2018\/10\/traitsPrimaryFixed2.png\")
<\/p>\nTag Dispatching<\/h3>\n