Reflection for C++26
The P2996R4 document proposes a reduced set of static reflection features in C++26, using constant expressions, a reflection operator, metafunctions, and splicers. Implementation progress is ongoing by Lock3 and EDG.
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The document P2996R4 dated 2024-06-26 proposes a reduced set of features for static reflection in C++26. It suggests using constant expressions to produce reflection values of an opaque type std::meta::info, a reflection operator (^), consteval metafunctions, and splicers to generate grammatical elements from reflections. The proposal aims to facilitate code generation based on program structure observations, known as reflective metaprogramming. It emphasizes starting with a smaller feature set to expedite inclusion in the language, with plans for future enhancements. Notable additions include metafunctions for synthesizing simple struct and union types. The document defends the use of a single opaque reflection type to avoid constraining language evolution and implementation advantages. Implementation status indicates ongoing work by Lock3 and EDG, with available implementations on Compiler Explorer. The implementations cover significant features proposed, including namespace and template splicers, with examples showcased in both implementations. However, some language features like expansion statements are not yet included.
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21 commentsLooking at the examples (https://isocpp.org/files/papers/P2996R4.html#examples) what really stands out is the direct integration of type-syntax into the language. It fits in with a certain token-substitution way that connects back to templates. It also replaces some of the uglier operators (typeof?).
I hope it goes int! During the language's stagnation I left for a while, perhaps it'll be competitive again soon.
1. how immense the language has become, and how hard it got to learn and implement
2. how "modernising" C++ gives developers less incentives to convince management to switch to safer languages
While I like C++ and how crazy powerful it is, I also must admit decades of using it that teaching it to new developers has become immensely hard in the last few years, and the "easier" inevitably ends up being the unsafe one (what else can you do when the language itself tells you to refrain from using `new`?).
More specifically, with this I can iterate over struct members and get their names and types, but I cannot attach additional information to these members, like whether they should be serialized or under which name.
The referenced proposal P1887R1 covers this, but that's not included here, right?
P1887R1: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p18...
* What type of problems static reflection could solve, in general?
* Are there specific cases and / or situations where static reflection could resolve such case, even simplify an unnecessary complexity?But the `member_number` functions in § 3.2 look disturbing to me. It's not discernible how invalid arguments are handled. Normally I'd look at generated assembly to answer a question like that, but this probably doesn't make sense with compile-time-fu (`constexpr`)…
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