std::meta::substitute, c++26 reflection example

c++, c++26 reflection: std::meta::substitute.

Requires gcc 16.1 to compile.

Enable it:

-freflection

gcc 16.1 is ahead of visual c++ and clang for supporting and compiling c++ reflection code.

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std::meta::substitute

std::meta::substitute returns a reflection,
it accepts a reflection template and a range of reflections
which denote the template parameter list of the reflection.

namespace std::meta
{
	template <
		std::meta::reflection_range range_type
			= std::initializer_list<std::meta::info>
	>
	consteval std::meta::info substitute(
		std::meta::info,
		range_type &&
	);
}

For example,

constexpr auto ref_val = std::meta::substitute(
	^^std::tuple,
	std::vector<std::meta::info>{^^int, ^^float, ^^std::string}
);

Accept a reflection template:
^^std::tuple
Accept a range of reflections which denote the template parameter list of the reflection:
std::vector<std::meta::info>{^^int, ^^float, ^^std::string}
Return a reflection:
the reflection of std::tuple<int, float, std::string>

c++ example

This c++ example code is compiled on gcc 16.1.

#include <meta>
#include <iostream>
#include <tuple>
#include <stdfloat>

namespace nss
{
	template <typename ... type_list>
	class some_door
	{
	public:
		void print() const
		{
			((std::cout << sizeof (type_list) << " "), ...);
			std::cout << std::endl;
		}
	};
}

int main()
{
	constexpr auto ref_type_1 = std::meta::substitute(
		^^std::tuple,
		std::vector<std::meta::info>{
			^^int,
			^^float,
			^^std::string,
			^^char
		}
	);
	constexpr auto ref_type_2 = std::meta::substitute(
		^^std::array,
		std::vector<std::meta::info>{
			^^std::string,
			std::meta::reflect_constant(3)
		}
	);
	constexpr auto ref_type_3 = std::meta::substitute(
		^^nss::some_door,
		std::vector<std::meta::info>{
			^^int,
			^^float,
			^^nss::some_door<int, float>
		}
	);

	static_assert(std::same_as<typename [:ref_type_1:], std::tuple<int, float, std::string, char>>);
	static_assert(std::same_as<typename [:ref_type_2:], std::array<std::string, 3>>);
	static_assert(
		std::same_as<
			typename [:ref_type_3:],
			nss::some_door<
				int,
				float,
				nss::some_door<
					int,
					float
				>
			>
		>
	);
	typename [:ref_type_3:] door;
	door.print();	// 4 4 1
}

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