Re: [Describe] Review - Part 2: Class types

From: Alexander Grund (alexander.grund_at_[hidden])
Date: 2021-03-15 07:47:12

Am 14.03.21 um 18:33 schrieb Peter Dimov via Boost:
> mod_virtual is necessary for the implementation of mod_inherited for base
> classes (virtual bases of the same type must only be returned once.)
>
> Since the library has to compute it anyway, I might as well return it to the user.
>
> My original design (which was about a compiler implementation of these
> reflection facilities) had more such output-only "modifiers", such as mod_final
> and mod_overridden. These, along with mod_virtual, were supposed to
> be returned in member function descriptors - the compiler knows all this and
> we can't get it in any other way, so why not return it. In the library
> implementation, only mod_virtual for bases remained. :-)

This reminds me of a question I had earlier but forgot: Given C++17 
constexpr if I think it would be useful to return ALL members and then 
constexpr if on the type.

E.g. currently it is not possible to do:

boost::mp11::mp_for_each<describe_members<T, mod_any_access>>([&](auto D){
 Â  if constexpr(is_function(D)) handle_function
 Â  if constexpr(is_variable(D)) handle_variable
}

I think the situation where this would be most useful is:

boost::mp11::mp_for_each<describe_members<T, mod_any_access | 
mod_function>>([&](auto D){
 Â  some_computation
 Â  // use the result of the computation to call functions which may be 
static
 Â  if constexpr(is_static(D)) handle_static_function
 Â  else handle_member_function
}

I.e. the problem with the toggles is, that they are toggles only. It is 
impossible to return both static and not-static members, which 
especially for functions is useful
In general it is impossible to return ALL members, e.g. 
`boost::mp11::mp_for_each<describe_members<T>>` could be useful, 
especially for a compiler implementation

Also a reminder that those toggles should stand out. E.g. I was 
wondering whether mod_hidden meant, that only hidden members are 
returned or in addition. :)
Just thinking out loud: `describe_members<T, mod_any_access, 
mod_function, mod_static>` and `describe_members<T, mod_any_access, 
mod_data, mod_nonstatic>` could work too and make that clear if those 
are kept as toggles.