Re: [boost] [introspection] Status

Subject: Re: [boost] [introspection] Status
From: Jeffrey Hellrung (jhellrung_at_[hidden])
Date: 2009-07-22 12:46:38

Richard Hazlewood wrote:
> I've actually been looking for a way to check for: does (or, perhaps can) 
> template function exist in type.
> 
> I've been experimenting with modifying your macro, so that the signature 
> becomes a template argument. E.g. so code such as:
> 
>   template <typename R, typename X>
>   void foo()
>   {
>     bool const has = has_non_const_member_function_bar<X, R (*)()>::value;
>     ...
>   }
> 
>   struct X
>   {
>     template <typename T>
>     T bar();
>   };
> 
>   foo<int, X>();
> 
> Could be made to work.
> 
> Trying:
> 
> 1] #define BOOST_HAS_NON_CONST_MEMBER_FUNCTION(Name,Sig)              \ 
> 2] namespace boost                                                    \ 
> 3] {                                                                  \ 
> 4]   namespace introspection                                          \ 
> 5]   {                                                                \ 
> 6]     template<class T, class F = Sig>                               \ 
> 7]     struct BOOST_PP_CAT(has_non_const_member_function_,Name)          \ 
> 8]     {                                                                 \ 
> 9]       typedef char NotFound;                                          \ 
> A]       struct Found { char x[2]; };                                    \ 
> B]                                                                       \ 
> C]       template< typename build_member_type<T,F>::type                 \ 
> D]               > struct member {};                                     \ 
> E]                                                                       \ 
> F]       template<class X> static Found test(member<&X::Name>*);         \ 
> G]       template<class X> static NotFound test( ... );                  \ 
> H]                                                                       \ 
> I]       static const bool value  = (sizeof(Found) == sizeof(test<T>(0))); \ 
> J]       typedef mpl::bool_<value> type;                                 \ 
> K]     };                                                                \ 
> L]   }                                                                   \ 
> M] }                                                                     \ 
> /**/ 
> 
> However, it doesn't appear to work (at least in VC71).  It looks like the 
> sizeof trick is not capable of stamping out the template function.
> 
> Do you know if this can be achieved?
> 
> Note: Lines [C-D, F] were modified from:
> 
>       template< class X                                                 \ 
>               , typename build_const_member_type<X,F>::type           \ 
>               > struct member {};                                       
> 
>       template<class X> static Found test(X, member<&X::Name>*);         \ 
> 
> Because VC71 would not compile it that way when Sig was a template arg (F). 
> [shrug].
> 
> This didn't seem to affect the original behaviour, was there a reason for it?
> 

I've been able to get a has_mem_fn-type trait to work on MSVC 8 and 9 by 
using the technique in the Adobe Move library's class_has_move_assign,

http://stlab.adobe.com/move_8hpp_source.html

Your lines [C-F] would look slightly different, roughly like

template< typename signature< T0, ..., TN >::type > struct sfinae { 
typedef yes_type type; };
template< class U > static typename sfinae< &U:: name >::type test(int);
template< class U > static no_type test(...);

I don't know if this is any better for MSVC 7, though, but you can try :/

- Jeff