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

From: Aleksey Gurtovoy (alexy_at_[hidden])
Date: 2002-11-13 03:18:48

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