Re: [Boost-users] Templates Question

From: Simon Buchan (simon_at_[hidden])
Date: 2005-10-30 19:00:57

David Abrahams wrote:
> "Suki" <suryakiran.gullapalli_at_[hidden]> writes:
> 
>> Hi,
>> I'm writing a templated class, and i dont want to use the class
>> otherthan for some predetermined types, say, int, double etc.
>>
>> This class has no meaning for typenames other than those few.
>>
>> =========================
>> template <class T>
>> myClass {....} ;
>>
>> myClass<int> intClass ;
>> myClass<float> floatClass ;
>> myClass<char> charClass ;
>> =========================
>>
>> The compiler should not complain for the first 2 instantiations, but
>> should bail out at the 3rd instantiation (charClass). Is there any way
>> i
>> can achieve this, using standard c++ or boost.
> 
> Declarative way:
> 
>   template <class T>
>   struct myClass_impl
>   {
>      ...
>   };
> 
>   template <class T>
>   struct myClass;
> 
>   template<>
>   myClass<int> : myClass_impl<int>
>   {};
> 
>   template<>
>   myClass<float> : myClass_impl<float>
>   {};
> 
>   template<>
>   myClass<char> : myClass_impl<char>
>   {};
> 
you mean 'struct myClass<foo> : public myClass_impl<foo>', I assume?

> Imperative way:
> 
>   #include <boost/mpl/or.hpp>
>   #include <boost/mpl/assert.hpp>
>   #include <boost/type_traits/is_same.hpp>
> 
>   template <class T>
>   struct myClass
>   {
>      BOOST_MPL_ASSERT((boost::mpl::or_<
>          boost::is_same<T,int>
>        , boost::is_same<T,float>
>        , boost::is_same<T,char>
>      >));
> 
>      ... // your class implementation here.
>   };
> 
This would slow down compile times a bit, though, would it not? (but 
give better error messages!) Not that it matters much.

Where's the functional way? :-D (Don't answer that)

Perhaps abstracting the check to a traits class would be helpful? 
Depends on what it's for, I guess.

-- 
don't quote this