{"id":10130,"date":"2024-10-07T13:53:28","date_gmt":"2024-10-07T13:53:28","guid":{"rendered":"https:\/\/www.modernescpp.com\/?p=10130"},"modified":"2025-07-04T14:16:59","modified_gmt":"2025-07-04T14:16:59","slug":"reflection-in-c26-metafunctions-for-enums-and-classes","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/reflection-in-c26-metafunctions-for-enums-and-classes\/","title":{"rendered":"Reflection in C++26: Metafunctions for Enums and Classes"},"content":{"rendered":"\n

Today, I continue my journey through reflection in C++26 and play with enums and classes.<\/p>\n\n\n\n

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

The names of the metafunctions to access an Enum or a Class member are often identical.<\/p>\n\n\n\n

Access the Members of an Enum<\/h2>\n\n\n\n

The following program is based on one by Daveed Vandevoorde. Daveed is one of the fathers of reflection, and he used this example in his presentation, “Reflections on C++ Reflection.<\/a>“<\/p>\n\n\n\n

The program iterates through an Enum and displays its name and value for each enumerator.<\/p>\n\n\n\n

\/\/ daveed.cpp<\/span>\n\n#include <array><\/span>\n#include <experimental\/meta><\/span>\n#include <iostream><\/span>\n\ntemplate<\/span><<\/span>typename<\/span> E><\/span> \nstruct<\/span> enum_item { \n  std::<\/span>string_view name; \n  E value;\n};\n\ntemplate<\/span><<\/span>typename<\/span> E><\/span> \nconsteval auto<\/span> get_enum_data() {\n  std::<\/span>array<<\/span>enum_item<<\/span>E><\/span>, std::<\/span>meta::<\/span>enumerators_of(^<\/span>E).size()><\/span> result;\n  int<\/span> k =<\/span> 0<\/span>;\n  for<\/span> (auto<\/span> mem:<\/span> std::<\/span>meta::<\/span>enumerators_of(^<\/span>E))\n    result[k++<\/span>] =<\/span> enum_item<<\/span>E><\/span>{ std::<\/span>meta::<\/span>identifier_of(mem), std::<\/span>meta::<\/span>extract<<\/span>E><\/span>(mem) };\n  return<\/span> result;\n}\n\nenum<\/span> MyEnum { \n  x, \n  y, \n  e =<\/span> -<\/span>1<\/span>, \n  z =<\/span> 99<\/span> \n};\n\nint<\/span> main<\/span>() {\n\n  std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n  std::<\/span>cout <<<\/span> "members of "<\/span> <<<\/span> std::<\/span>meta::<\/span>identifier_of(^<\/span>MyEnum) <<<\/span> '\\n'<\/span>;\n  for<\/span> (auto<\/span> x:<\/span> get_enum_data<<\/span>MyEnum><\/span>()) {\n    std::<\/span>cout <<<\/span> "  "<\/span> <<<\/span> x.name <<<\/span> " = "<\/span> <<<\/span> (long<\/span>)x.value <<<\/span> '\\n'<\/span>;\n  }\n\n  std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n}\n<\/pre><\/div>\n\n\n\n
The provided code snippet demonstrates the use of experimental metaprogramming features in C++ to introspect and manipulate enumeration types at compile time. The code begins by including the necessary headers: <array><\/code> for array support, <experimental\/meta><\/code> for metaprogramming utilities, and <iostream><\/code> for input-output operations.<\/p>\n\n\n\n
The enum_item<\/code> struct template is defined to hold information about an enumeration item. It contains two members: name<\/code>, a std::string_view<\/code> representing the name of the enumerator, and value<\/code>, which holds the enumerator’s value of type E<\/code><\/a>.<\/p>\n\n\n\n
The get_enum_data<\/code> function template is marked as consteval<\/code><\/a>, meaning it is evaluated at compile time. This function generates an array of enum_item<\/code> structs for a given enumeration type E<\/code>. It uses the std::meta::enumerators_of<\/code> function to retrieve the enumerators of the enumeration type and iterates over them. For each enumerator, it creates an enum_item<\/code> with the enumerator’s name and value, using std::meta::identifier_of<\/code> to get the name and std::meta::extract<\/code> to get the value. The resulting array is then returned.<\/p>\n\n\n\n
The MyEnum<\/code> enumeration is defined with four enumerators: x<\/code>, y<\/code>, e<\/code> (explicitly set to -1<\/code>), and z (explicitly set to 99<\/code>). This enumeration is used as an example to demonstrate the metaprogramming capabilities.<\/p>\n\n\n\n
In the main<\/code> function, the code first prints a newline for formatting purposes. It then prints the name of the enumeration type MyEnum<\/code> using std::meta::identifier_of<\/code>. Next, it calls get_enum_data<MyEnum>()<\/code> to retrieve the array of enum_item<\/code> structs for MyEnum<\/code> and iterates over this array. For each enum_item<\/code>, it prints the name and value of the enumerator. The value is cast to long<\/code> for consistent output formatting. Finally, another newline is printed for formatting.<\/p>\n\n\n\n
Overall, this code showcases how compile-time reflection can be used to introspect enumeration types and generate useful metadata, such as enumerator names and values, which can then be used at runtime.<\/p>\n\n\n\n
Here’s the output of the program: <\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
 My Small Experiment<\/h3>\n\n\n\n
This explanation was created by Microsoft’s KI tool Copilot<\/a>. Honestly, I was pretty impressed because the feature I described is brand new.<\/p>\n\n\n\n
Access the Members of a Class<\/h2>\n\n\n\n
The following program demonstrates two methods for accessing class members: by index and by name.<\/p>\n\n\n\n
<\/p>\n\n\n\n
\/\/ reflectionClass.cpp<\/span>\n\n#include <experimental\/meta><\/span>\n#include <iostream><\/span>\n\nstruct<\/span> Base { \n    int<\/span> i{}; \n    void<\/span> inc<\/span>(int<\/span>&<\/span> j){ j++<\/span>; }\n};\n\nconsteval auto<\/span> number<\/span>(int<\/span> n) {\n  \/\/return std::meta::nonstatic_data_members_of(^Base)[n];<\/span>\n  return<\/span> std::<\/span>meta::<\/span>members_of(^<\/span>Base)[n];\n}\n\n\nconsteval auto<\/span> named<\/span>(std::<\/span>string_view name) {\n  for<\/span> (std::<\/span>meta::<\/span>info field :<\/span> std::<\/span>meta::<\/span>members_of(^<\/span>Base)) {\n    if<\/span> (std::<\/span>meta::<\/span>has_identifier(field) &&<\/span> std::<\/span>meta::<\/span>identifier_of(field) ==<\/span> name)\n      return<\/span> field;\n  } \n  return<\/span> std::<\/span>meta::<\/span>info{};\n}\n\n\nint<\/span> main<\/span>() { \n\n  std::<\/span>cout <<<\/span> '\\n'<\/span>;\n\n  Base base;\n  base.[:<\/span>number(0<\/span>):<\/span>] =<\/span> 1<\/span>;  \n  \/\/ base.[:member_number(10):] = 1;  Error<\/span>\n  std::<\/span>cout <<<\/span> "base.i= "<\/span> <<<\/span> base.i <<<\/span> '\\n'<\/span>;\n  base.[:<\/span>number(1<\/span>):<\/span>](base.i);\n  std::<\/span>cout <<<\/span> "base.i= "<\/span> <<<\/span> base.i <<<\/span> '\\n'<\/span>;\n\n  std::<\/span>cout <<<\/span> '\\n'<\/span>;\n \n  base.[:<\/span>named("i"<\/span>):<\/span>] =<\/span> 3<\/span>;\n  std::<\/span>cout <<<\/span> "base.i= "<\/span> <<<\/span> base.i <<<\/span> '\\n'<\/span>;\n  base.[:<\/span>named("inc"<\/span>):<\/span>](base.i);\n  std::<\/span>cout <<<\/span> "base.i= "<\/span> <<<\/span> base.i <<<\/span> '\\n'<\/span>;\n\n}\n<\/pre><\/div>\n\n\n\n
Base <\/code>is the class I want to reflect on. The metafunctions number <\/code>and named<\/code> provide me with the necessary information.<\/p>\n\n\n\n
    \n
  • number<\/code>: std::meta::members_of(^Base)[n]<\/code> returns the member n <\/code>of Base<\/code>. On the contrary, number<\/code>: std::meta::nonstatic_data_members_of(^Base)[n]<\/code> returns the non-static data member n <\/code>of Base. The reflection library also has a metafunction for getting all static data members of a class: std::meta::static_data_members_of(^Base)[n]<\/code> <\/li>\n\n\n\n
  • named<\/code>: iterates through all members of Base<\/code> and returns the member with the name name<\/code>: std::meta::identifier_of(field) == name<\/code><\/li>\n<\/ul>\n\n\n\n
    Let me jump to the main <\/code>program. The member i <\/code>of Base <\/code>is incremented. Either by assigning the value or invoking the member function inc<\/code>. Using the wrong index or name gives a compile-time error:[:member_number(10):] = 1<\/code>)<\/p>\n\n\n\n
     Finally, here’s the output of the program.<\/p>\n\n\n\n
    \"\"<\/figure>\n\n\n\n
    What’s Next?<\/h2>\n\n\n\n
    In my next post, I will continue to play with reflection in C++26.<\/p>\n","protected":false},"excerpt":{"rendered":"
    Today, I continue my journey through reflection in C++26 and play with enums and classes. The names of the metafunctions to access an Enum or a Class member are often identical. Access the Members of an Enum The following program is based on one by Daveed Vandevoorde. Daveed is one of the fathers of reflection, […]<\/p>\n","protected":false},"author":21,"featured_media":10083,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[559],"tags":[560],"class_list":["post-10130","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-c26-blog","tag-reflection"],"_links":{"self":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10130","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/users\/21"}],"replies":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/comments?post=10130"}],"version-history":[{"count":15,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10130\/revisions"}],"predecessor-version":[{"id":10854,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/posts\/10130\/revisions\/10854"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media\/10083"}],"wp:attachment":[{"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/media?parent=10130"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/categories?post=10130"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.modernescpp.com\/index.php\/wp-json\/wp\/v2\/tags?post=10130"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}