RE: [boost] Re: Policy or Trait?

From: Rozental, Gennadiy (gennadiy.rozental_at_[hidden])
Date: 2004-04-02 16:46:26

> I thought I'd try an example to see what happens.
>  whether this is best example I dont know....whatever
> Many more variants of course possible.
> operator()() could be moved into multiply_traits etc
> which would give user yet more control.
> Perhaps that is the difference. Move all the implementation 
> out of the class gives the user more flexibility. But is it 
> the  flexibility that se wants. (Sorry boost pros uses she 
> above, but men users, women developers, viceversa...just a 
> personal thing)
> 
> First observation from below ....Obviously the application 
> has a great deal to do with it.! :-) Many more experiments to 
> do ... but ...  Interesting debate.
> 
> regards
> Andy Little
> 
> -----------
> 
>     #include <iostream>
>     template< typename A , typename B >
>     struct multiply_traits;
> 
> // use traits
>     template< typename A , typename B >
>     struct multiply1{
> 
>         typedef multiply_traits<A,B> traits;
>         typename traits::result_type operator ()
>             (typename traits::first_argument_type a,
>             typename traits::second_argument_type b)
>         {
>             return a * b;
>         }
> 
>     };
> //use policy
>     template< typename A , typename B, typename Policy>
>     struct multiply2{
> 
>         typename Policy::result_type operator ()
>             (typename Policy::first_argument_type a,
>             typename Policy::second_argument_type b)
>         {
>             return a * b;
>         }
> 
>      //   or Policy ()(a,b);  etc if ---> policy
> 
>     };

And give me single example where second design is preferable to first?

Actually I think the best design would look like this:

namespace {

struct multiply_t
{
     template< typename A , typename B >
     struct sig : multiply_traits<A,B> {};

    template< typename A , typename B >
    typename sig<A,B>::result_type
    operator ()( typename sig<A,B>::first_argument_type a,
                        typename sig<A,B>::second_argument_type b )
    {
       return a * b;
    }
 } multiply;

}

int main()
{
   std::cout << multiply( 10, 1.5 ) <<'\n';

   MyInt i( 10 )
   std::cout << multiply( i, 10 ) <<'\n';
}

Gennadiy.