etl/docs/types/meta.md

title
meta

{{< callout type="info">}} Header: meta.h
From: 20.50.0
Similar to: std::meta {{< /callout >}}

C++26 static reflection (P2996) in namespace etl::meta.

etl::meta mirrors std::meta, but is usable in both STL and non-STL builds. In an STL build it is a thin layer of using-declarations over std::meta. In a non-STL build ETL supplies the minimal declarations that the compiler's reflection builtins require, so no standard library is needed.

#include "etl/meta.h"

Compiler support

Static reflection is a very new language feature, and at the time of writing GCC 16 is the only compiler that implements enough of P2996 to be usable. Clang (including the Bloomberg clang-p2996 fork) and MSVC are not supported.

etl/meta.h may always be included. It defines a single macro, ETL_HAS_REFLECTION, as 1 or 0; when it is 0 the header defines nothing else. This is the only reflection feature switch in the ETL, and user code should test it before using anything in etl::meta.

#include "etl/meta.h"

#if ETL_HAS_REFLECTION
  static_assert(etl::meta::identifier_of(^^my_type) == "my_type");
#endif

GCC requires the -freflection option in addition to -std=c++26. The reference toolchain is defined in .devcontainer/gcc16.

Usage

Names are used exactly as their std::meta equivalents.

struct point
{
  int x;
  int y;
};

constexpr auto members =
  etl::meta::nonstatic_data_members_of(^^point,
                                       etl::meta::access_context::unchecked());

static_assert(etl::meta::identifier_of(members[0]) == "x");
static_assert(etl::meta::type_of(members[0])       == ^^int);
static_assert(etl::meta::size_of(^^point)          == 2 * sizeof(int));

identifier_of, display_string_of and symbol_of return std::string_view in an STL build and etl::string_view in a non-STL build.

info_array

GCC cannot keep the heap allocation of a std::vector<info> alive across a constant evaluation, so a template for over the result of members_of does not compile. etl::meta::info_array is a fixed-size, allocation-free array of info that can be iterated instead.

constexpr auto ctx = etl::meta::access_context::unchecked();
constexpr auto count = etl::meta::nonstatic_data_members_of(^^point, ctx).size();
constexpr auto members =
  etl::meta::to_info_array<count>(etl::meta::nonstatic_data_members_of(^^point, ctx));

template for (constexpr auto member : members)
{
  // ...
}

Member	Description
begin()	Pointer to the first element.
end()	Pointer past the last element.
size()	The number of elements.
operator[](size_t)	The element at the index.

All members are consteval.

Tests

test_meta.cpp