[Boost-users] enable_if question

From: Hurd, Matthew (hurdm_at_[hidden])
Date: 2004-06-07 19:11:11

I'm just trying to play with enable_if and learn something.

I just mean to replace this template method's code:

    template< bool >
    void exchange_code( boost::call_traits<std::string>::param_type
new_exchange_code )
    {
        if ( boost::is_arithmetic<std::string>::value ) {
            lock lk(guard_ );
            exchange_code<false>( new_exchange_code);
        } else {
            exchange_code<false>( new_exchange_code);
        }
    }

With this:

    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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