Bring a typename into scope from a templated base class

Bring a typename into scope from a templated base class

[John Fine]

I am porting some code into gcc 4.1.2 from a much more permissive 
compiler.  I need to bring many names into the scopes of a templated 
classes from templated base classes.  (In the more permissive compiler 
they are in scope without extra effort).

For function names, a "using" declaration seems to work (though I've 
only tried a few examples so far).

For typenames, I've seen other code that uses "using typename".  But I 
can't get that to work and can't understand the C++ standard well enough 
to know why it doesn't work.

So here is a minimal example.  Put the following in a .cpp file and 
compile with -c to see the error.  (The compiler can't parse "iterator 
it" because it doesn't know that "iterator" is a typename.

template<class T>
struct A {
typedef T* iterator; };

template<class T>
struct B : A<T> {
using typename A<T>::iterator;
// typedef typename A<T>::iterator iterator;
iterator it; };

B<int> x;

Is there a correct way to do what I want "using typename" to do?
If I use the "typedef typename" instead of the "using typename", it 
makes this example work, but it doesn't quite mean the same thing.  I'll 
use that if there isn't a way to make "using typename" work.  But I'd 
like to first understand the problem.

Re: Bring a typename into scope from a templated base class

[Brian Dessent]

John Fine wrote:

> Is there a correct way to do what I want "using typename" to do?
> If I use the "typedef typename" instead of the "using typename", it
> makes this example work, but it doesn't quite mean the same thing.  I'll
> use that if there isn't a way to make "using typename" work.  But I'd
> like to first understand the problem.

I think what you want is:

template<class T>
struct B : A<T> {
typename A<T>::iterator it; };

See also:
<http://gcc.gnu.org/gcc-3.4/changes.html>
<http://kegel.com/gcc/gcc4.html>

Brian

Re: Bring a typename into scope from a templated base class

[John Fine]

Brian Dessent wrote:
> I think what you want is:
>
> template<class T>
> struct B : A<T> {
> typename A<T>::iterator it; };
>   

Thankyou, but that misses the point.  I'm not trying to fix the trivial 
example.  I'm trying to fix real code where the change you suggest would 
be massive and would make the code unreadable.

Even the typedef method is much better than changing every use of the 
type throughout the class.

I wrote the minimal example in the hopes that someone could explain how 
to do what I want to do:  Bring a typename into scope from a templated 
base class
> See also:
> <http://gcc.gnu.org/gcc-3.4/changes.html>
> <http://kegel.com/gcc/gcc4.html>
>
>   
I expect somewhere in all the pages those two link to there is some 
discussion of this issues.  But starting from the top of those two, I 
have no idea how to dig down to it.  Near misses are easy to find, for 
example:

http://womble.decadentplace.org.uk/c++/template-faq.html#base-lookup

My question is sort of a combination of questions 1 and 2 of that faq.  
But the answers don't combine.