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@meta-comm.com] Sent: Wednesday, November 13, 2002 3:19 AM To: 'Boost-Users@yahoogroups.com' 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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