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 _membersof 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_locationof to emulate this feature, wich is nice :).