New subject: [Boost-Users] A strange question-)

13 Nov 2002

      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

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

Aleksey Gurtovoy

Sam Gentile

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