RE: [Boost-Users] A strange question-)

From: Aleksey Gurtovoy (alexy_at_[hidden])
Date: 2002-11-13 20:55:24

Sam Gentile wrote:
> I'm sorry, no I still don't understand. Could you please say more?

Well, it would help me if you've said what exact points in my answer you are
having troubles with :).

Aleksey

 
> 
> -----Original Message-----
> From: Aleksey Gurtovoy [mailto:alexy_at_[hidden]] 
> Sent: Wednesday, November 13, 2002 3:19 AM
> To: 'Boost-Users_at_[hidden]'
> Subject: RE: [Boost-Users] A strange question-)
> 
>  
> 
> Sam Gentile wrote:
> > I went there and indeed it can't get any simpler - at least 
> > in theory-)
> > Read the document
> > C:\Boost\boost_1_29_0\libs\type_traits\c++_type_traits.htm 
> > and its quite
> > simple:
> > 
> > template <typename T> 
> > struct is_void
> > { static const bool value = false; };
> >  
> > template <> 
> > struct is_void<void>
> > { static const bool value = true; };
> > 
> > That's exactly what I want to show. So I go to the header files and
> > instead:
> > 
> > //* is a type T void - is_void<T>
> > 
> > BOOST_TT_AUX_BOOL_TRAIT_DEF1(is_void,T,false)
> > 
> > BOOST_TT_AUX_BOOL_TRAIT_SPEC1(is_void,void,true)
> > 
> >  
> > 
> > Huh? So I look in bool_trait_def.hpp:
> > 
> > #define BOOST_TT_AUX_BOOL_TRAIT_DEF1(trait,T,C) \
> > 
> > template< typename T > struct trait \
> > 
> >     : mpl::bool_c< C > \
> > 
> > { \
> > 
> >     BOOST_TT_AUX_BOOL_TRAIT_VALUE_DECL(C) \
> > 
> >     BOOST_MPL_AUX_LAMBDA_SUPPORT(1,trait,(T)) \
> > 
> > }; \
> 
> 
> Well, the original code doesn't have all these excessive newlines ;),
> but
> yes, basically that's how it's implemented.
> 
> > Argh! Why is it done like this? This makes it much more 
> > complicated and unreadable than it has to be. Why are macros 
> > being used everywhere?
> 
> Practical engineering matters. If you consider that the fact 
> that there
> are
> ~60 trait templates in the library and every time when a new 
> version of
> a
> compiler comes out, or when somebody starts porting the 
> library to a new
> platform, there is a chance that you need to change half of those
> definitions, from, for instance,
> 
>     template <typename T> 
>     struct is_void
>         : mpl::bool_c<false>
>     {
>     };
> 
> to
> 
>     template <typename T> 
>     struct is_void
>     {
>         BOOST_STATIC_CONSTANT(bool, value = false);
>         typedef mpl::bool_c<value> type;
>     };
> 
> it might all make sense, after all :). Something along the 
> lines of both
> of
> the above scenarios has actually happened, a couple of times. It's not
> the
> whole picture yet, but IMO it's enough of the motivation already, so I
> won't
> go too deep into details :). 
> 
> Hope this answers your question,
> Aleksey
> 
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