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From "Travis Vitek" <Travis.Vi...@roguewave.com>
Subject RE: implementation of Unary Traits
Date Mon, 30 Jun 2008 22:47:52 GMT
 

Martin Sebor wrote:
>Travis Vitek wrote:
>>  
>> 

[...]

>
>I can think of a couple of options that satisfy these. I'm not sure
>how palatable I find them yet, but I might get over their (initial)
>lack of appeal if the payoff is worth it. YMMV.
>
>One is to drop the cv specializations of the __rw_is_xxx traits and
>define the standard traits in terms of both remove_cv and the __rw
>traits, like so:
>
>   template <class _TypeT>
>   struct is_void:
>       integral_constant<bool,
>           _RW::__rw_is_void<typename
>               _RW::__rw_remove_cv<_TypeT>::type>::value>
>   { };
>
>or more concisely:
>
>   template <class _TypeT>
>   struct is_void:
>       integral_constant<bool, _RWSTD_IS_VOID (_TypeT)::value>
>   { };
>
>with _RWSTD_IS_VOID() being #defined like so:
>
>   #define _RWSTD_IS_VOID(T) \
>       _RW::__rw_is_void<typename _RW::__rw_remove_cv<T>::type>
>
>All internal code would always need to use _RWSTD_IS_VOID() and
>never _RW::__rw_is_void directly.
>

I think there is a problem with _RWSTD_IS_VOID() being defined like this
because it can't be used outside a template (because of the typename
keyword being used in the macro definition).

  $ cat u.cpp && g++ u.cpp
  namespace __rw {


  template <class T>
  struct __rw_remove_cv { typedef T type; };

  template <class T>
  struct __rw_is_void { enum { value = 0 }; };

  template <>
  struct __rw_is_void<void> { enum { value = 1 }; };


  }

  #define _RW __rw

  #define _RWSTD_IS_VOID(T) \
      _RW::__rw_is_void<typename _RW::__rw_remove_cv<T>::type>

  int main ()
  {
      const bool b = _RWSTD_IS_VOID(void)::value;
      return b == 0;
  }
  u.cpp: In function 'int main()':
  u.cpp:23: error: using 'typename' outside of template


Travis

>

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