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