Re: [boost] Re: [OT] Hiding template parameters [was: FC++ Formal review is...]

From: Brian McNamara (lorgon_at_[hidden])
Date: 2004-02-16 14:15:17

On Mon, Feb 16, 2004 at 06:46:26PM +0100, Gabriel Dos Reis wrote:
> David Abrahams <dave_at_[hidden]> writes:
> | Gabriel Dos Reis <gdr_at_[hidden]> writes:
> | > I understand the reasoning of letting the user says what the name
> | > should refer to.  Which makes me inclined to make the construct
> | > ill-formed: That is not different from the rule that says you cannot 
> | > reclare a template-parameter in its scope.
> | 
> | That would be horrible for generic code.
> 
> Care to explain why?
> 
> |  What are the requirements
> | on the T parameter of this class template?
> | 
> |    template <class T>
> |    struct Der : T
> |    {
> |       template <class U>
> |       void f(U x)
> |       {}
> |    };
> 
> Should those requirements be different from those for
> 
>      template<class S>
>      struct Der : S
>      {
>         template<class U>
>         void f(U x)
>         { } 
>      };

Apologies ahead of time if I am putting words in anyone's mouth.

I think Dave wants to imply that the class T in his example is not
allowed to have a member named U according to Gaby's proposed rule.

But this isn't how the rule would work; the way I see it, _these_:

   struct Base { typedef int U; };

   template <class U>
   struct Derived : Base { };   // illegal

   struct Derived2 : Base { 
      template <class U>
      void f(U u) {}            // illegal
   };

would be illegal ("the name U already has another meaning; choose
another name for your template parameter"), whereas _these_:

   struct Base { typedef int U; };

   template <class B, class U>
   struct Derived : B { };
   /* ... Derived<Base> ... */

   template <class B>
   struct Derived2 : B { 
      template <class U>
      void f(U u) {}
   };
   /* ... Derived2<Base> ... */

are both fine, since Base's "U" is hidden by virtue of being a dependent
name.  There is still an issue of what happens if one of these last two
classes issues a "using B::U" in its body; presumably then "U" ought to
become outlawed as a valid template parameter name.

Just my two cents here...

-- 
-Brian McNamara (lorgon_at_[hidden])