{"id":10081,"date":"2024-09-23T10:56:02","date_gmt":"2024-09-23T10:56:02","guid":{"rendered":"https:\/\/www.modernescpp.com\/?p=10081"},"modified":"2025-07-04T14:37:33","modified_gmt":"2025-07-04T14:37:33","slug":"reflection-in-c26","status":"publish","type":"post","link":"https:\/\/www.modernescpp.com\/index.php\/reflection-in-c26\/","title":{"rendered":"Reflection in C++26"},"content":{"rendered":"\n

After the search into the breadth starts today, the search into the depth: reflection.<\/p>\n\n\n\n

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

Reflection<\/h2>\n\n\n\n

Reflection <\/strong>is the ability of a program to examine, introspect, and modify its structure and behavior. <\/p>\n\n\n\n

Reflection in C++ is more. Here are two statements from the proposal (P2996R5<\/a>).<\/p>\n\n\n\n

“We not only want to observe the structure of the program: We also want to ease generating code that depends on those observations. That combination is sometimes referred to as \u201creflective metaprogramming<\/strong>\u201d, but within WG21 discussion the term \u201creflection\u201d has often been used informally to refer to the same general idea.<\/em>“<\/p>\n\n\n\n

“This proposal is not intended to be the end-game as far as reflection and compile-time metaprogramming are concerned. Instead, we expect it will be a useful core around which more powerful features will be added incrementally over time. In particular, we believe that most or all the remaining features explored in P1240R2 and that code injection (along the lines described in [P2237R0<\/a>]) are desirable directions to pursue.<\/em>“<\/p>\n\n\n\n

History<\/h2>\n\n\n\n

The history of reflection in C++ is based on template metaprogramming. Template metaprogramming started around 1994, and so does reflection. C++98 got runtime reflection (RTTI<\/a>) and function template type deduction<\/a>. The type traits library in C++11 improved C++ capabilities. In C++26, we will probably get general reflection support.<\/p>\n\n\n\n

My Strategy<\/h2>\n\n\n\n

I will base my reflection presentation in C++26 on proposal P2996R and use examples from it.<\/p>\n\n\n\n

First, let me jump forward and back from the reflection value to the grammatical elements.<\/p>\n\n\n\n

Grammatical Elements <=> Reflection Value <\/h2>\n\n\n\n

The following program starts in the grammatical domain, jumps to the reflection domain, and back to the grammatical domain.<\/p>\n\n\n\n

\/\/ forthAndBack.cpp (P2996R5)<\/span>\n\n#include <iostream><\/span>\n#include <cassert><\/span>\n#include <concepts><\/span>\n\nint<\/span> main<\/span>() {\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\n    static_assert(std::<\/span>same_as<<\/span>decltype(x), int<\/span>><\/span>);\n    static_assert(std::<\/span>same_as<<\/span>decltype(c), char<\/span>><\/span>);\n    assert(x ==<\/span> 42<\/span>);\n    assert(c ==<\/span> '*'<\/span>);\n}\n<\/pre><\/div>\n\n\n\n