Re: Returning nested class within template class from member
function.
On Dec 2, 3:33 pm, Hendrik Schober <spamt...@gmx.de> wrote:
James Kanze wrote:
On Dec 2, 9:18 am, Hendrik Schober <spamt...@gmx.de> wrote:
jason.cipri...@gmail.com wrote:
[...]
All right, but now that I've fixed *my* stupid mistake,
Comeau compiles it fine, except MS's VC++ 2008 compiler
choked on it:
1>warning C4346: 'MyClass<T>::Nested' : dependent name is not a type
1> prefix with 'typename' to indicate a type
1>error C2143: syntax error : missing ';' before 'MyClass<T>::f'
But adding a "typename" in the middle there made it happy
for some reason:
template <typename T> typename MyClass<T>::Nested MyClass<T>::f () {
return Nested();
}
I'd say the 'typename' is necessary ('Nested' is a nested
identifier depending on a template parameter, after all),
but this seems to counter all my experience regarding
whether Comeau or VC is right, whenever they disagree.
Am I wrong here, or would this really be a bug in Comeau??
Check the options it was invoked with. Comeau is a very
professional product, which means that it doesn't go around
breaking existing code if it can help it. So it almost
certainly has modes which accept this code, and others which
don't.
Good thought, however, it does accept this
template <typename T> class MyClass {
public:
class Nested { };
Nested f ();
};
template <typename T> MyClass<T>::Nested MyClass<T>::f()
{
return Nested();
}
as
Comeau C/C++ 4.3.10.1 (Oct 6 2008 11:28:09) for ONLINE_EVALUATION_BE=
TA2
Copyright 1988-2008 Comeau Computing. All rights reserved.
MODE:strict errors C++ noC++0x_extensions
so it seems indeed this is a bug in Comeau.
I don't think so. The typename here is optional, since Nested
is not a dependent name, but a locally declared name. If you
used MyClass<T>::Nested in the definition of another template,
the typename would be necessary, but not here. But IMHO, the
standard is far from clear about this. It goes into detail with
regards to when an expression is dependant, but it really
doesn't say much about names in declarations. About the only
really relevant text I can find is right at the start of =A714.6,
where it says that "Three kinds of names can be used[...]", and
places names declared within the template itself in the same
category as the name of the template, i.e. locally declared. So
while Nested in MyClass<T>::Nested obviously depends on T, it
doesn't depend on T in the sense of the standard. (This makes
sense, of course, in that the compiler can easily determine that
Nested is a type; in the definition of template members, it
doesn't have to worry about specializations.)
FWIW: both g++ and Sun CC require the typename as well.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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