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

From: Gabriel Dos Reis (gdr_at_[hidden])
Date: 2004-02-16 15:43:43

Brian McNamara <lorgon_at_[hidden]> writes:

| 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.

The base class "T" is dependent, therefore it is not examined during
parsing; therefore, it has nothing to say about "U".

| 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

Agreed.

|    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"),

Agreed.

|  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.

Again, agreed.

|  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.

Yes, because they're (alias) declarations of "U".

| Just my two cents here...

Thanks.  They're appreciated.

-- Gaby