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Christof Meerwald
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 Posted: Thu Sep 28, 2006 7:38 pm Post subject: valid C++: template as base class |
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I am wondering if the following code is valid C++:
template<class T>
struct A
{
static void f()
{ }
};
struct B
: public A<int>
{
static void f()
{
A::f(); // is this valid?
}
};
int main()
{
B::f();
}
gcc, Visual C++ 2003 and Digital Mars don't like it,
but Intel C++ and HP aCC accept it.
Can anyone tell me if this is valid C++ and why or why not.
Christof
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....and what have you contributed to the Net?
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Alberto Ganesh Barbati
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| Posted: Thu Sep 28, 2006 10:51 pm Post subject: Re: valid C++: template as base class |
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Christof Meerwald ha scritto:
| Quote: | I am wondering if the following code is valid C++:
template<class T
struct A
{
static void f()
{ }
};
struct B
: public A<int
{
static void f()
{
A::f(); // is this valid?
}
};
|
No, it's not correct. Should be A<int>::f();
Ganesh
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Michael Tiomkin
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| Posted: Fri Sep 29, 2006 5:39 am Post subject: Re: valid C++: template as base class |
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Christof Meerwald wrote:
| Quote: | I am wondering if the following code is valid C++:
template<class T
struct A
{
static void f()
{ }
};
struct B
: public A<int
{
static void f()
{
A::f(); // is this valid?
|
The compiler doesn't know the 'T' type of A. You'd better use
A<int>::f();
| Quote: | }
};
int main()
{
B::f();
}
gcc, Visual C++ 2003 and Digital Mars don't like it,
but Intel C++ and HP aCC accept it.
Can anyone tell me if this is valid C++ and why or why not.
|
You used a template name without the type, and this is an error.
Inheriting from a template class is acceptable in C++.
Michael
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Gennaro Prota
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| Posted: Fri Sep 29, 2006 5:41 am Post subject: Re: valid C++: template as base class |
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On 28 Sep 2006 13:51:00 -0400, Alberto Ganesh Barbati
<AlbertoBarbati (AT) libero (DOT) it> wrote:
| Quote: | Christof Meerwald ha scritto:
I am wondering if the following code is valid C++:
template<class T
struct A
{
static void f()
{ }
};
struct B
: public A<int
{
static void f()
{
A::f(); // is this valid?
}
};
No, it's not correct. Should be A<int>::f();
|
I don't see why. Could you please quote the relevant sections from the
standard?
--
Gennaro Prota
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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Gennaro Prota
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| Posted: Fri Sep 29, 2006 8:24 pm Post subject: Re: valid C++: template as base class |
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On 29 Sep 2006 08:41:55 -0400, Alberto Ganesh Barbati
<AlbertoBarbati (AT) libero (DOT) it> wrote:
| Quote: | Gennaro Prota ha scritto:
On 28 Sep 2006 13:51:00 -0400, Alberto Ganesh Barbati
AlbertoBarbati (AT) libero (DOT) it> wrote:
Christof Meerwald ha scritto:
I am wondering if the following code is valid C++:
template<class T
struct A
{
static void f()
{ }
};
struct B
: public A<int
{
static void f()
{
A::f(); // is this valid?
}
};
No, it's not correct. Should be A<int>::f();
I don't see why. Could you please quote the relevant sections from the
standard?
The rules (§14.6.1) that allow the name A to be used without a parameter
list to refer to a particular specialization of the template A are
restricted to the scope defining A or a specialization of A.
|
Hmm... 14.6.1/2 seems to back up your interpretation. However the
immediately following paragraph says (emphasys mine):
The injected-class-name of a class template or class template
specialization can be used either with or without a
template-argument-list *wherever it is in scope*.
It seems to me the intent is for A to be found as an (accessible)
injected class name in the scope of the derived class. Frankly, I'd be
surprised of the contrary.
--
Gennaro Prota
--
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Alberto Ganesh Barbati
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| Posted: Sun Oct 01, 2006 2:58 pm Post subject: Re: valid C++: template as base class |
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Gennaro Prota ha scritto:
| Quote: |
Hmm... 14.6.1/2 seems to back up your interpretation. However the
immediately following paragraph says (emphasys mine):
The injected-class-name of a class template or class template
specialization can be used either with or without a
template-argument-list *wherever it is in scope*.
It seems to me the intent is for A to be found as an (accessible)
injected class name in the scope of the derived class. Frankly, I'd be
surprised of the contrary.
|
I read it again and again, and I couldn't find a flaw in your reasoning.
Frankly, I am very surprised to know that the name A (without <>) is
visible in the scope of B, but it now seems apparent that my belief is
only justified by the repeated use of non-conformant compilers.
BTW, Comeau Online accepts the code and it adds to the other three
compilers reported by other posters (Intel C++, HP aCC and Visual C++
2005). In front of a good argument and the agreement of four of the most
conformant compilers on the market, it's difficult to argue anymore ;-)
Ganesh
--
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Alberto Ganesh Barbati
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| Posted: Tue Oct 03, 2006 1:45 am Post subject: Re: valid C++: template as base class |
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Christof Meerwald ha scritto:
| Quote: |
My understanding is that the rules (14.6.1) have slightly changed and C++ 98
didn't allow it, but the latest working draft
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2009.pdf) does
allow it. Does anyone know when this change was made (was this included in
TC1)?
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Yes, they are already included in TC1.
Ganesh
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