members_of declaration order guarantee

[Pespon]

Hello, I'm really enjoying trying out your project, and I'd like to thank everyone who is making it possible.

Feature Proposition
I think it would be interesting to guarantee that the order of the members infos returned by members_of matches their declaration order.

Reasoning
If this guarantee is provided, it enables the use of tags delimiting scopes in the code.
I think this guarantee will become even more interesting as support for methods and functions expands.

I hope this example will speak for itself:

struct magic_attribute_1
{
	int n;
	constexpr magic_attribute_1(int in_n)
		: n (in_n)
	{}
};

template<std::size_t n>
struct magic_attribute_2
{
	std::array<char, n> magic{};
	constexpr magic_attribute_2(char const (&in_magic)[n])
	{ std::ranges::copy(in_magic, this->magic.data()); }
};

template<auto ... magics>
struct members_group_begin
{};

struct members_group_end
{};

struct baz
{
	float im_not_in_a_group;

	using properties_group = members_group_begin<magic_attribute_1(12)>;

	std::string m1;
	int			m2;

	using group_1 = members_group_begin<magic_attribute_2("more magic")>;

	void f1();
	void f2();

	using group_2 = members_group_begin<magic_attribute_2("even more magic")>;

	void f3();
	using some_int_type = int;

	using there_is_no_more_group = members_group_end;

	char im_not_in_a_group_neither;
};

[katzdm]

Hey @Pespon ! Thanks for these bug reports and ideas, and thanks for checking out the project!

The guarantees that we're offering for P2996 are as follows:

Non-static data members are indexed in the order in which they are declared, but the order of other kinds of members is unspecified. [ Note 1: Base classes are not members. — end note ]

We guarantee this order because the language specification already requires that compilers keep track of the order of non-static data members - after all, they're initialized in declaration order. On the other hand, there is no such requirement for compilers to track e.g., the relative ordering of a non-static data member and a type alias (as in your example above). We aren't confident that implementing such a "global ordering" on all member declarations would be feasible for all implementations, so we're restricting the ordering requirement to those members whose order must already be known.

[Pespon]

Thanks for your answer. I just noticed that you may use source_location_of to emulate this feature, wich is nice :).