Re: [Boost-users] [multi-index] member arguments [Was: how to use in heritmember as index]

From: Filip Konvička (filip.konvicka_at_[hidden])
Date: 2007-06-28 02:56:58

>> I'm just curious - is there a reason why "member" takes 3 template
>> parameters, and not just one, like in
>>
>>       member<&Peer::userid>
>>
>> ? The first two types can be extracted from the member pointer, so I
>> find this a bit redundant...
>>     
>
> It certainly looks redundant, but it turns out you really cannot do what
> you propose:
>
>   template<...>
>   struct member;
>
> What should "..." look like? It's not a type parameter, but it cannot
> be made a non-type parameter either since we don't know the type
> of the pointer-to-member being passed...
>
> In decltype- (or typeof-) enabled compilers a terser syntax could be
> achieved by using some macro hackery:
>
> template<typename PtrToMemberType,PtrToMemberType PtrToMember>
> struct member;
>
> #define MEMBER(PtrToMember) \
> member<decltype(PtrToMember),PtrToMember>
>
> So maybe when C++0x arrives we can take advantage of this.
>   

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...

Cheers,
Filip