C++20 Idioms for Parameter Packs

Re:C++20 Idioms for Parameter Packs by mazieres on Tuesday February 27, 2024 @08:34PM (#39532782)

Actually not. Try to implement a compile-time initialized const char* that represents some compile-time value like sizeof(X) for some data structure X. You'll see that you need recursion.

A place where this kind of pattern is useful is in having a generic accessor type for fields, and wanting to extend it to tuples. So for example accessor(x) might return x.field, and accessor.name might be "field." To make it work for tuples, you need a string for every possible tuple index. E.g.:

  template<typename T, size_t N> struct tuple_accessor;
  template<typename...T, size_t N>
  struct tuple_accessor<std::tuple<T...>, N> {
    constexpr decltype(auto) operator(auto &&t) const {
      return get<N>(std::forward<decltype(t)>(t));
    }
    static constexpr const char *name = index_string<N>();
  };

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Re:C++20 Idioms for Parameter Packs by mgaunard on Wednesday February 28, 2024 @08:31AM (#39537611)

for demonstration purposes only.

  #include <array>
  #include <tuple>

  constexpr std::array<char, 11> itos(unsigned n) {
      std::array<char, 11> s;
      unsigned i = 0;
      while (n) {
          s[i++] = '0' + (n % 10);
          n /= 10;
      }
      s[i] = '\0';
      return s;
  }
  template<auto V>
  struct static_value {
      static constexpr auto value = V;
  };

  template<class T, unsigned N>
  struct tuple_accessor;
  
  template<class...T, unsigned N>
  struct tuple_accessor<std::tuple<T...>, N> {
      static constexpr const char *name = static_value<itos(N)>::value.data();
  };

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Re:C++20 Idioms for Parameter Packs by mazieres on Wednesday February 28, 2024 @11:05PM (#39546301)

Your constexpr function is not legal because it doesn't initialize all the array elements, even though the compiler might let you get away with it. That's easily fixed. More importantly, though, your static_value::value is not a const char *.

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Re:C++20 Idioms for Parameter Packs by mgaunard on Thursday February 29, 2024 @03:14AM (#39547377)

That restriction was lifted in 2020.

tuple_accessor::name is a const char* which is what you asked for. static_value here is an implementation detail and the type of its members is irrelevant.

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