3 ms·

Most of what you sent under this link falls under the general idea of reflection, e.g. the quoted macros, and is fairly similar to the kind of things that are b

Most of what you sent under this link falls under the general idea of reflection, e.g. the quoted macros, and is fairly similar to the kind of things that are being proposed for c++. Of course all the reflection systems proposed for c++ are purely compile-time, no one even proposed runtime reflection I think

still_grokking 4y ago
For most people "reflection" is a synonym to "runtime reflection". In contrast quotes and splices aren't reflection at all! The whole point about them is that they're "black box". You (usually) can't "extract information" form a quote in a safe way (so it's the exact contrary to reflection). That's still something academia is trying to solve (even there are some proposals out)¹. Where can I learn more about the C++ proposals to compare? --- ¹ https://dl.acm.org/doi/10.1145/3136000.3136005 https://dl.acm.org/doi/10.1145/3136000.3136005 ¹ https://dl.acm.org/doi/10.1145/3158101 https://dl.acm.org/doi/10.1145/3158101 And the recent work that underlines the solution included in Scala 3: ¹ https://se.informatik.uni-tuebingen.de/publications/stucki21multistage.pdf https://se.informatik.uni-tuebingen.de/publications/stucki21...
- jcelerier 4y ago
  > For most people "reflection" is a synonym to "runtime reflection". I mean, that's just because many languages don't even have a compile-time so of course reflection can only be runtime. Every compiled language has at least some amount of extremely basic concept of compile-time reflection, e.g. sizeof in C for instance which yields a constant expression ; likewise, `if constexpr(std::is_same_v<T, some_other_type>) { ... }` which we can do since C++17 is very much in the domain of compile-time reflection. Here's the most recent paper: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1240r2.pdf https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p12... If you want this is a very good read: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0707r3.pdf https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p07... Finally, there is an implementation of the reflection TS: https://clementpirelli.wordpress.com/2021/12/08/cpp-reflection-ts-first-look/ https://clementpirelli.wordpress.com/2021/12/08/cpp-reflecti...
  - still_grokking 4y ago
    Oh, cool, direct links to the relevant papers! It's not always easy to find the relevant stuff in C++ land. Thanks a lot! I have to read this stuff first, but my gut feeling would still be that `constexpr` and such isn't reflection at all. The `sizeof` example seems more in this direction (because some expression gets actually inspected).
    - jcelerier 4y ago
      i'm talking about "if constexpr", not "constexpr" ; "if constexpr" allows to make choices at compile time depending on properties of a variable and that is definitely what reflection is about. e.g. void f(auto x) { // common code path if constexpr(x is floating-point) { // code path 1 } else if constexpr(x is a string) { // code path 2 } // common code path }
      - still_grokking 4y ago
        Oh, I see. You're right. Thanks for the pointer.

still_grokking 4y ago

For most people "reflection" is a synonym to "runtime reflection". In contrast quotes and splices aren't reflection at all! The whole point about them is that they're "black box". You (usually) can't "extract information" form a quote in a safe way (so it's the exact contrary to reflection). That's still something academia is trying to solve (even there are some proposals out)¹. Where can I learn more about the C++ proposals to compare? --- ¹ https://dl.acm.org/doi/10.1145/3136000.3136005 https://dl.acm.org/doi/10.1145/3136000.3136005 ¹ https://dl.acm.org/doi/10.1145/3158101 https://dl.acm.org/doi/10.1145/3158101 And the recent work that underlines the solution included in Scala 3: ¹ https://se.informatik.uni-tuebingen.de/publications/stucki21multistage.pdf https://se.informatik.uni-tuebingen.de/publications/stucki21...

jcelerier 4y ago
> For most people "reflection" is a synonym to "runtime reflection". I mean, that's just because many languages don't even have a compile-time so of course reflection can only be runtime. Every compiled language has at least some amount of extremely basic concept of compile-time reflection, e.g. sizeof in C for instance which yields a constant expression ; likewise, `if constexpr(std::is_same_v<T, some_other_type>) { ... }` which we can do since C++17 is very much in the domain of compile-time reflection. Here's the most recent paper: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p1240r2.pdf https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p12... If you want this is a very good read: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0707r3.pdf https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p07... Finally, there is an implementation of the reflection TS: https://clementpirelli.wordpress.com/2021/12/08/cpp-reflection-ts-first-look/ https://clementpirelli.wordpress.com/2021/12/08/cpp-reflecti...
- still_grokking 4y ago
  Oh, cool, direct links to the relevant papers! It's not always easy to find the relevant stuff in C++ land. Thanks a lot! I have to read this stuff first, but my gut feeling would still be that `constexpr` and such isn't reflection at all. The `sizeof` example seems more in this direction (because some expression gets actually inspected).
  - jcelerier 4y ago
    i'm talking about "if constexpr", not "constexpr" ; "if constexpr" allows to make choices at compile time depending on properties of a variable and that is definitely what reflection is about. e.g. void f(auto x) { // common code path if constexpr(x is floating-point) { // code path 1 } else if constexpr(x is a string) { // code path 2 } // common code path }
    - still_grokking 4y ago
      Oh, I see. You're right. Thanks for the pointer.