Re: [Boost-users] enable_if question

From: jajarvi (jajarvi_at_[hidden])
Date: 2004-06-08 07:52:20

On Jun 7, 2004, at 7:11 PM, Hurd, Matthew wrote:

>
SFINAE does not kick in, because the condition 
(is_arithmetic<std::string>::value) is not dependent on any template
arguments.

   Cheers, Jaakko

>     template< bool >
>     typename boost::enable_if_c<
> boost::is_arithmetic<std::string>::value, void >::type
>     exchange_code( boost::call_traits<std::string>::param_type
> new_exchange_code )
>     {
>         lock lk(guard_ );
>         exchange_code<false>( new_exchange_code);
>     }
>
>     template< bool >
>     typename boost::disable_if_c<
> boost::is_arithmetic<std::string>::value  , void >::type
>     exchange_code( boost::call_traits<std::string>::param_type
> new_exchange_code )
>     {
>         exchange_code<false>( new_exchange_code);
>     }
>
> Forget the relevance of the is_arithmetic bit, I use my own needs_lock
> thingo.
>
> For a start:
>
> The compiler correctly identifies that ::type is not a member, but
> SFINAE does not seem to kick in if I'm reading it correctly.
> ____________________
> c:\source\matt\trading_engine\controller\instrument.hpp(66) : error
> C2039: 'type' : is not a member of 'boost::enable_if_c<B,T>'
>         with
>         [
>             B=false,
>             T=void
>         ]
>         c:\source\matt\trading_engine\controller\instrument.hpp(143) :
> see reference to class template instantiation 'te::instrument<S>' being
> compiled
> ____________________
>
>     template< bool >
>     typename boost::disable_if_c<
> boost::is_arithmetic<std::string>::value  , void >::type
>     exchange_code( boost::call_traits<std::string>::param_type
> new_exchange_code )
>     {
>         exchange_code<false>( new_exchange_code);
>     }
>
> Is fine...
>
> Flipping the conditions round by changing to is_arithmetic< int > flips
> the failures around.
>
> Same issue with the extra parameter approach:
>
>     template< bool >
>     void
>     exchange_code( boost::call_traits<std::string>::param_type
> new_exchange_code,
>     typename boost::enable_if_c< boost::is_arithmetic<int>::value, void
>> ::type* dummy = 0  )
>     {
>         lock lk(guard_ );
>         exchange_code<false>( new_exchange_code);
>     }
>
>     template< bool >
>     void
>     exchange_code( boost::call_traits<std::string>::param_type
> new_exchange_code,
>     typename boost::disable_if_c< boost::is_arithmetic<int>::value  ,
> void >::type* dummy = 0 )
>     {
>         exchange_code<false>( new_exchange_code);
>     }
>
> Is SFINAE meant to work in this way with member templates with
> enable_if, vc7.1 and boost 1.31?
>
> Something simple is probably escaping me...
>
> Any thoughts?
>
>
> Regards,
>
> Matt Hurd.
> _______________
>
> Matt Hurd
> +61.2.8226.5029
> hurdm_at_[hidden]
> Susquehanna
> _______________
>
>
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