Re: [boost] About member extraction

From: John Torjo (john.lists_at_[hidden])
Date: 2003-07-14 09:17:49

>
>Subject: Re: [boost] About member extraction
>   From: Joaquín Mª López Muñoz <joaquin_at_[hidden]>
>   Date: Mon, 14 Jul 2003 14:24:37 +0200
>     To: Boost mailing list <boost_at_[hidden]>
>
>
>
>Daryle Walker ha escrito:
>
>> But doesn't the "PtrToMember" template parameter already imply the
>> "Type" and "Class" parameters?  So specifying all three would be
>> redundant.  Could we reduce it by:
>>
>> //================================================================
>> template < typename PtrToMember >
>> struct member_extractor
>> {
>>    // Don't know if this is a real type-traits class
>>    BOOST_STATIC_ASSERT(is_pointer_data_member<PtrToMember>::value);
>>
>>    // The extractor traits classes aren't real (yet, maybe)
>>    typedef get_class_type<PtrToMember>  argument_type;
>>    typedef get_member_type<PtrToMember>   return_type;
>>
>>    return_type const &  operator ()( argument_type const &c ) const
>>      { return c.*PtrToMember; }
>>
>>    return_type &  operator ()( argument_type &c ) const
>>      { return c.*PtrToMember; }
>> };
>
>Of the approaches you propose, this is the one that I like best, but
>I'm afraid it cannot be implemented: Note that PtrToMember is a
>*type* (something like int A::*), not a pointer to member.
>member_extractor would have to defined as
>
>template < typename PtrToMember, PtrToMember ptr >
>struct member_extractor
>
>and used as
>
>member_extractor<int A::*,&A::x>; // x is an int member of A

Actually, I think it's worse (I don't have a C++ compiler with me right now:-( )

Something in the lines of:

template < typename type, typename result, result type::* ptr >
struct member_extractor

>
>which takes us again to the redundancy we were trying to avoid.
>Something in the way of eliminating this redundancy, however, would be
>a boon. Maybe some metaprogramming gurus here can think of
>something.
>

I would really love to see a solution to this ;-)

Best,
John

>Joaquín M López Muñoz
>Telefónica, Investigación y Desarrollo
>
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