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#include <experimental/meta>

#include <iostream>

enum MyEnum { x, y, e = -1, f, z = 99 };

struct S {

int key:24;

int flags:8;

double value;

} s = { 100, 0x0d, 42.0 };

int main() {

constexpr std::meta::info refl = ^MyEnum;� std::cout << name_of(refl) << '\n';

std::cout << name_of(enumerators_of(refl)[1]) << '\n';

std::cout << [:enumerators_of(refl)[4]:] << '\n';

std::cout << name_of(nonstatic_data_members_of(^S)[1]) << '\n';

std::cout << s.[:nonstatic_data_members_of(^S)[1]:] << '\n';

}

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std::meta::info

[: … :]

Constant evaluation guarantees

Metafunctions (lots!)

Core language details (ODR, …)

Source identifiers as string-like arguments and return values.

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API elements that produce text

namespace std::meta {

consteval str_type name_of(info);

consteval str_type qualified_name_of(info);

consteval str_type display_name_of(info);

}

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API elements that consume text (as of P2996R2)

namespace std::meta {

struct data_member_options_t {

optional<string_view> name;

bool is_static = false;

optional<int> alignment;

optional<int> width;

};

consteval info data_member_spec(info type,

data_member_options_t options = {});

consteval info define_class(info class_type, span<info const>);

}

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Constraints

Round-tripping must work
Output to std::cout must work reasonably
Some text may not be source-like text (std::meta::display_name_of)

Currently only expected to be “output text” with no need for round-tripping

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Tension

After phase 1 of translation, source is effectively Unicode

That’s good

There are solid Unicode types in standard C++

UTF-8 types in particular

Support in the standard library is inadequate

In particular, no std::cout << u8”Hello, World\n”;

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Proposal sketch #1 (Daveed’s preferred)

Traffic in both std::string_view and std::u8string_view (and maybe std::string and std::u8string)
Require round-tripping

Either using an implementation-defined identifier-exchange representation, or by standardizing such a representation (e.g., using UCNs)

�namespace std::meta {� template<char_or_char8_t CharT = char> consteval� std::basic_string_view<CharT> name_of(info);�}

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Proposal sketch #2

Traffic only in std::u8string_view (and maybe std::u8string)
(Naturally round-trips)
Ensure UTF-8 output works for std::cout and std::format in C++26

Via a separate paper
Danger of standardization race condition

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Proposal sketch #3 (Victor’s idea)

Introduce a new type (std::meta::identifier?) that encapsulates the internal encoding

Could have a specific formatter in the <format> library

Can be consumed directly by the define_class interface

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