![\"TypeTraits\"](\"https:\/\/www.modernescpp.com\/wp-content\/uploads\/2019\/01\/TypeTraits.PNG\")
<\/h2>\nThe type-traits library supports type checks, type comparisons, and type modifications at compile time. Right! Today, I write about type modifications at compile time.<\/p>\n
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<\/h2>\nType modification is the domain of template metaprogramming <\/a>and, therefore, for the type-traits library.<\/p>\n Maybe, you are curious about what is possible at compile time. A lot! Here are the most exciting metafunctions:<\/p>\n <\/p>\n To get an int<\/span> from int<\/span> or const int<\/span>, you must ask for the type with ::type<\/span>.<\/p>\n <\/p>\n <\/p>\n Since C++14, you can use _t<\/span> to get the type, such as for std::remove_const_t:<\/span><\/p>\n <\/p>\n <\/p>\n To get an idea of how valuable these metafunctions from the type-traits library are, here are a few use cases. Here is std::move<\/span> in one line.<\/p>\n Maybe, you are not convinced about the benefit of the type traits library. Let me end my story with the type-traits with their primary goals: correctness and optimization.<\/p>\n<\/p>\n Correctness means, on one hand, that you can use the type-traits libraries to implement concepts such as Integral, SignedIntegral<\/span>, and UnsignedIntegral.<\/span><\/p>\n <\/p>\n But it also means you can use them to make your algorithm safer. I used in my previous post, More and More Safe, <\/a>the functions std::is_integral, std::conditional, std::common_type,<\/span> and std::enable_if<\/span> from the type-traits library to make the generic gcd<\/span> algorithm successively safer.<\/p>\nType Modifications<\/h3>\n
\n\/\/ const-volatile modifications:<\/span>\r\nremove_const;\r\nremove_volatile;\r\nremove_cv;\r\nadd_const;\r\nadd_volatile;\r\nadd_cv;\r\n \r\n\/\/ reference modifications:<\/span>\r\nremove_reference;\r\nadd_lvalue_reference;\r\nadd_rvalue_reference;\r\n\r\n\/\/ sign modifications:<\/span>\r\nmake_signed;\r\nmake_unsigned;\r\n \r\n\/\/ pointer modifications:<\/span>\r\nremove_pointer;\r\nadd_pointer;\r\n \r\n\/\/ other transformations:<\/span>\r\ndecay;\r\nenable_if;\r\nconditional;\r\ncommon_type;\r\nunderlying_type;\r\n<\/pre>\n<\/div>\n\nint<\/span> main<\/span>(){\r\n \r\n std::<\/span>is_same<<\/span>int<\/span>, std::<\/span>remove_const<<\/span>int<\/span>>::<\/span>type>::<\/span>value; \/\/ true<\/span>\r\n std::<\/span>is_same<<\/span>int<\/span>, std::<\/span>remove_const<<\/span>const<\/span> int<\/span>>::<\/span>type>::<\/span>value; \/\/ true<\/span>\r\n \r\n}\r\n<\/pre>\n<\/div>\n\nint<\/span> main<\/span>(){\r\n \r\n std::<\/span>is_same<<\/span>int<\/span>, std::<\/span>remove_const_t<\/span><<\/span>int<\/span>>>::<\/span>value; \/\/ true<\/span>\r\n std::<\/span>is_same<<\/span>int<\/span>, std::<\/span>remove_const_t<\/span><<\/span>const<\/span> int<\/span>>>::<\/span>value; \/\/ true<\/span>\r\n}\r\n<\/pre>\n<\/div>\n\n
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static_cast<\/span><<\/span>std::<\/span>remove_reference<<\/span>decltype(arg)>::<\/span>type&&><\/span>(arg);<\/pre>\n<\/li>\n<\/ul>\n<\/li>\n<\/code> <\/span>\n\n
\n\nstd::invoke<\/span><\/span><\/a>(<\/span>decay_copy(<\/span>std::forward<\/span><\/span><\/a><<\/span>Function><\/span>(<\/span>f)<\/span>)<\/span>, \r\n decay_copy(<\/span>std::forward<\/span><\/span><\/a><<\/span>Args><\/span>(<\/span>args)<\/span>)<\/span>...)<\/span>;<\/span><\/pre>\n<\/div>\n<\/div>\n<\/li>\n<\/ul>\n<\/li>\n
<\/span><\/li>\nCorrectness<\/h3>\n
\ntemplate<\/span> <<\/span>class<\/span> T<\/span>><\/span>\r\nconcept bool<\/span> Integral() {\r\n return<\/span> is_integral<<\/span>T>::<\/span>value;\r\n}\r\n\r\ntemplate<\/span> <<\/span>class<\/span> T<\/span>><\/span>\r\nconcept bool<\/span> SignedIntegral() {\r\n return<\/span> Integral<<\/span>T><\/span>() &&<\/span> is_signed<<\/span>T>::<\/span>value;\r\n}\r\n\r\ntemplate<\/span> <<\/span>class<\/span> T<\/span>><\/span>\r\nconcept bool<\/span> UnsignedIntegral() {\r\n return<\/span> Integral<<\/span>T><\/span>() &&<\/span> !<\/span>SignedIntegral<<\/span>T><\/span>();\r\n}\r\n<\/pre>\n<\/div>\n