Re: [Boost-users] [multi-index] member arguments [Was: how to useinheritmember as index]

From: Joaquín Mª López Muñoz (joaquin_at_[hidden])
Date: 2007-06-28 04:27:29

Filip Konvi?ka ha escrito:

> Joaquín Mª López Muñoz (28.6.2007 9:49):
> > Filip Konvi?ka ha escrito:
> > [...]
> >
> >
> >> I use this template in my code:
> >>
> >>   template<typename T>
> >>   struct get_pointer_to_member_details;
> >>
> >>   template<typename Class, typename Member>
> >>   struct get_pointer_to_member_details<Member Class::*> {
> >>     typedef Class   class_type;
> >>     typedef Member  member_type;
> >>   };
> >>
> >> Using this, you can write (and I do) something like
> >>
> >>   template<typename MemberPointerType>
> >>   struct member {
> >>     MemberPointerType pMember;
> >>     typedef typename
> >> get_pointer_to_member_details<MemberPointerType>::class_type   class_type;
> >>     typedef typename
> >> get_pointer_to_member_details<MemberPointerType>::member_type  result_type;
> >>
> >>     member(MemberPointerType pMember) : pMember(pMember) {}
> >>    // ...
> >> }
> >>
> >> I wonder then, is it MSVC 8 that allows something non-standard, or is it
> >> OK? I think I tried with recent gcc as well and it worked...
> >>
> >
> > Your code is fine, but it does not do what boost::multi_index::member is
> > meant to do, namely accepting the pointer-to-member as a template
> > parameter --rather, you're taking it as a runtime variable in construction
> > time.
> Ah, I see...but duplicating the argument using a macro might work, like
> this:
>
> #define MEMBER(MemPtr) member<get_details<MemPtr>::class_type,
> get_details<MemPtr>::member_type, MemPtr>

Umm... no, not quite so. MemPtr is a non-type parameter, so you'd have to
write:

#define MEMBER(MemPtr) \
member< \
  get_details<decltype(MemPtr)>::class_type, \
  get_details<decltype(MemPtr)>::member_type, \
  MemPtr>

So we have the need for a typeof-like facility agian :(

> The reason why I think this is better is that when you change the member
> type in class declaration, you have to go through all member<> usages
> and change the type there too.

I agree with you having a macro like this makes usage simpler --but I haven't
provided the macro because typeof or decltype is not standard and/or widely
supported, and I don't know how to implement the macro without some of
those.

If you use GCC, which provides typeof(), then indeed you can write your
private MEMBER utility in the manner described.

Joaquín M López Muñoz
Telefónica, Investigación y Desarrollo