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

From: David Abrahams (dave_at_[hidden])
Date: 2004-02-16 16:04:27

Brian McNamara <lorgon_at_[hidden]> writes:

> 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.

That's what I thought he meant.

> 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...

OK, I think I understand now.  I think it's a bizarre and inconsistent
rule.  Shall we outlaw:

  struct Base { static int x; };

  struct Derived : Base { 
     void f(int x) {}     // Horrors! we're masking a base class member!
  };

Sorry for the sarcasm; it'll wear off by tomorrow.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com