Prev: Programatically rearranging icons
Next: utime() and GMT
From: Triple-DES on 11 Nov 2009 03:21
On 10 Nov, 13:51, "Igor Tandetnik" <itandet...(a)mvps.org> wrote:
> Triple-DES wrote:
> > In some instances, Comeau (online) is more lenient with typename than
> > the Standard. Example:
>
> > template <typename T>
> > struct A
> > {
> >    struct B { };
> >    B b1_;    // ok
> >    A::B b2_; // ill-formed, 'typename' required
>
> > };
>
> > Comeau online compiles this without a diagnostic.
>
> typename keyword is not required here. There's an example in 14.6 that shows this exact situation, and no typename keyword:
>
> 14.6p2 A name used in a template declaration or definition and that is dependent on a template-parameter is assumed not to name a type *unless the applicable name lookup finds a type name* [emphasis mine] or the name is qualified by the keyword typename. [Example:
> // no B declared here
> class X;
> template<class T> class Y {
>   class Z; // forward declaration of member class
>   void f() {
>     X* a1; // declare pointer to X
>     T* a2; // declare pointer to T
>     Y* a3; // declare pointer to Y<T>
>
>     // ****** This is the part similar to your example.
>     Z* a4; // declare pointer to Z
>
>     typedef typename T::A TA;
>     TA* a5; // declare pointer to T's A
>     typename T::A* a6; // declare pointer to T's A
>     T::A* a7; // T::A is not a type name:
>               // multiply T::A by a7; ill-formed,
>                                           // no visible declaration of a7
>     B* a8; // B is not a type name:
>                                  // multiply B by a8; ill-formed,
>                                   // no visible declarations of B and a8
>   }};
>
> -end example]

This is not the same situation. In my example, a previously declared
member, B, is referred to _using a qualified name_, in which case the
keyword typename is always required. While

Z* a4; // is well-formed,
Y::Z * a4 // is ill-formed. 'typename' is required here.
From: Igor Tandetnik on 11 Nov 2009 08:07
Triple-DES wrote:
> This is not the same situation. In my example, a previously declared
> member, B, is referred to _using a qualified name_, in which case the
> keyword typename is always required.

I suppose you refer to 14.6p6: "...The keyword typename shall always be specified when the member is referred to using a qualified name..." Apparently, Comeau follows DRs 409 and 224, which remove this sentence as well as the rationale for the requirement:

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#409
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#224

--
With best wishes,
Igor Tandetnik

With sufficient thrust, pigs fly just fine. However, this is not necessarily a good idea. It is hard to be sure where they are going to land, and it could be dangerous sitting under them as they fly overhead. -- RFC 1925
First  |  Prev  | 
Pages: 1 2 3
Prev: Programatically rearranging icons
Next: utime() and GMT