Nested class template as friend

[Ike Naar]

Given the following C++ snippet:

template < typename T > class A { // 1
public : // 2
class AA { } ; // 3
} ; // 4
// 5
template < typename T > class B { // 6
friend class A < T > :: AA ; // 7
} ; // 8

It compiles cleanly with Sun C++, HP C++, Intel ecc 7.1,
GNU g++ 2.95.3 and GNU g++ 2.96 .
However, GNU g++ 3.2 gives the following warning:

a.C:7: warning: `typename A<T>::AA' is implicitly a typename
a.C:7: warning: implicit typename is deprecated, please see the
documentation for details

Is there really anything wrong with the friend declaration, or is it a bug in
GNU g++ 3.2 ?
Replacing the keyword 'class' in line 7 by 'typename' makes
it compile cleanly with GNU g++, but gives compilation
errors with all non-GNU compilers.
Replacing 'class' by 'class typename' is not an option either.

Any thoughts?

Kind regards,
Ike

--
mail to ike at iae dot nl

[Rolf Magnus]

Re: Nested class template as friend

Ike Naar wrote:
[color=blue]
> Given the following C++ snippet:
>
> template < typename T > class A { // 1
> public : // 2
> class AA { } ; // 3
> } ; // 4
> // 5
> template < typename T > class B { // 6
> friend class A < T > :: AA ; // 7
> } ; // 8
>
> It compiles cleanly with Sun C++, HP C++, Intel ecc 7.1,
> GNU g++ 2.95.3 and GNU g++ 2.96 .
> However, GNU g++ 3.2 gives the following warning:
>
> a.C:7: warning: `typename A<T>::AA' is implicitly a typename
> a.C:7: warning: implicit typename is deprecated, please see the
> documentation for details
>
> Is there really anything wrong with the friend declaration, or is it a
> bug in GNU g++ 3.2 ?[/color]

When you need to use a type that depends on a template parameter, you
need to tell the compiler that it's a type by adding the typename
keyword.
[color=blue]
> Replacing the keyword 'class' in line 7 by 'typename' makes
> it compile cleanly with GNU g++, but gives compilation
> errors with all non-GNU compilers.
> Replacing 'class' by 'class typename' is not an option either.[/color]

Try the other way round:

friend typename class A < T > :: AA ; // 7

[Ike Naar]

Re: Nested class template as friend

: Ike Naar wrote:
:> template < typename T > class A { // 1
:> public : // 2
:> class AA { } ; // 3
:> } ; // 4
:> // 5
:> template < typename T > class B { // 6
:> friend class A < T > :: AA ; // 7
:> } ; // 8
:>
:> It compiles cleanly with Sun C++, HP C++, Intel ecc 7.1,
:> GNU g++ 2.95.3 and GNU g++ 2.96 but not with GNU g++ 3.2

Rolf Magnus <ramagnus@t-online.de> wrote:
: When you need to use a type that depends on a template parameter, you
: need to tell the compiler that it's a type by adding the typename
: keyword.

:Ike Naar:
:> Replacing the keyword 'class' in line 7 by 'typename' makes
:> it compile cleanly with GNU g++, but gives compilation
:> errors with all non-GNU compilers.
:> Replacing 'class' by 'class typename' is not an option either.

Rolf Magnus:
: Try the other way round:
: friend typename class A < T > :: AA ; // 7

That's also problematic.

GNU: a.C:7: parse error before `<' token

HP:
Error 19: "a.C", line 7 # Unexpected 'class'.
friend typename class A < T > :: AA ;

Intel: a.C(7): error: expected an identifier
friend typename class A < T > :: AA ;

--
mail to ike at iae dot nl

[Jonathan Turkanis]

Re: Nested class template as friend


"Ike Naar" <nospam@nospam. invalid> wrote in message
news:xBa%b.5621 $ks5.19339@typh oon.bart.nl...[color=blue]
> Given the following C++ snippet:
>[/color]
[color=blue]
>
> a.C:7: warning: `typename A<T>::AA' is implicitly a typename
> a.C:7: warning: implicit typename is deprecated, please see the
> documentation for details
>
> Is there really anything wrong with the friend declaration, or is it[/color]
a bug in[color=blue]
> GNU g++ 3.2 ?[/color]

I haven't checked the standard on this point, but I think GCC is wrong
to warn here. Using 'class' tells the compiler to expect a class.
Using typename after class is defintely wrong.

GCC 3.3.1 accepts with no warnings, even using -pedantic. Comeau also
accepts it, so I'd just ignore the warning.

Jonathan