{"id":10109,"date":"2024-09-30T10:14:19","date_gmt":"2024-09-30T10:14:19","guid":{"rendered":"https:\/\/www.modernescpp.com\/?p=10109"},"modified":"2025-07-04T14:25:43","modified_gmt":"2025-07-04T14:25:43","slug":"reflection-in-c26-metafunctions","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/reflection-in-c26-metafunctions\/","title":{"rendered":"Reflection in C++26: Metafunctions"},"content":{"rendered":"\n

Reflection offers many metafunctions that run at compile time. <\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

The metafunctions are declared as consteval<\/a><\/code>. consteval <\/code>creates a so-called immediate function. Each invocation of an immediate function creates a compile-time constant. To say it more directly. A consteval <\/code>(immediate) function is executed at compile-time.<\/p>\n\n\n\n

Read more about consteval <\/code>and constinit <\/code>in my previous post: Two new Keywords in C++20: consteval and constinit<\/a>.<\/p>\n\n\n\n

What we Know so far:<\/h2>\n\n\n\n
 constexpr auto<\/span> r =<\/span> ^<\/span>int<\/span>;\n    typename<\/span>[:<\/span>r:<\/span>] x =<\/span> 42<\/span>;       \/\/ Same as: int x = 42;<\/span>\n    typename<\/span>[:^<\/span>char<\/span>:<\/span>] c =<\/span> '*'<\/span>;  \/\/ Same as: char c = '*';<\/span>\n<\/pre><\/div>\n\n\n\n
The reflection operator (^<\/strong><\/code>) produces out of the grammatical element (C++ element) a reflection value. The type of the reflection value std::meta::info.<\/code> The reflection value (the reflection) can be an argument of the metafunctions or the splicers ([: refl :]<\/strong><\/code>). The splicers create grammatical elements.<\/p>\n\n\n\n
 So reflection value can be defined as follows:<\/p>\n\n\n\n
namespace<\/span> std {\n  namespace<\/span> meta {\n    using<\/span> info =<\/span> decltype(^::<\/span>);\n  }\n}\n<\/pre><\/div>\n\n\n\n
So, what is missing? Of course, the metafunctions. I will present you all of them for two reasons:<\/p>\n\n\n\n