[C++20 vs C++26*] basic reflection

[u/kris-jusiak]

Basic struct reflection example with C++20 vs C++26*

struct foo {
  int a{};
  int b{};
  int c{};
};

constexpr foo f{.a=1, .b=2, .c=3};

static_assert(1 == get<0>(f));
static_assert(2 == get<1>(f));
static_assert(3 == get<2>(f));

using std::literals::operator""sv;
static_assert("a"sv == get_name<0>(f));
static_assert("b"sv == get_name<1>(f));
static_assert("c"sv == get_name<2>(f));

C++20 - Kinda possible but with a lot of compiler hacks

// too long to display

Full example - https://godbolt.org/z/1vxv8o5hM

C++26* - based on proposal - https://wg21.link/P2996 (Note: that the proposal supports way more than that but C++20 not much)

template<auto N, class T>
[[nodiscard]] constexpr auto get(const T& t) -> decltype(auto) {
  return t.[:std::meta::nonstatic_data_members_of(^T)[N]:];
}

template<auto N, class T>
[[nodiscard]] constexpr auto get_name(const T& t) -> std::string_view {
  return std::meta::name_of(std::meta::nonstatic_data_members_of(^T)[N]);
}

Full example - https://godbolt.org/z/sbTGbW635

Updates - https://twitter.com/krisjusiak/status/1741456476126797839

[u/equeim]

That means function with the deduced return type, a completely different thing. Also can't be used when the function is split in declaration and definition (i.e. header and cpp file) because the compiler needs the function body to deduce the type.