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Keith H Duggar
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 Posted: Tue Sep 19, 2006 4:45 am Post subject: qualified names in namespace member declarations |
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There is some previous discussion of this; but, I could not
find definitive and unanimous agreement and interpretation
of the standard.
namespace A {
class A::SomeClass { } ;
A::SomeClass A::some_func() ;
}
namespace A {
A::SomeClass A::some_func() { return A::SomeClass() ; }
}
Which of the redundant A:: qualifications above are legal?
Is section 8.3 of the standard relevant? What other sections
bear on this question?
On our system, g++ and the Sun compiler allow the above while
the Intel compiler gives the "qualified name not allowed in
namespace member declaration" error discussed here a few
years back.
Finally, is it legal to define some_func without wrapping in
namespace A { } as one does for class member functions?
A::SomeClass A::some_func() { return A::SomeClass() ; }
Thanks in advance!
-- Keith -- Fraud 6
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Greg Herlihy
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| Posted: Tue Sep 19, 2006 3:29 pm Post subject: Re: qualified names in namespace member declarations |
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Keith H Duggar wrote:
| Quote: | There is some previous discussion of this; but, I could not
find definitive and unanimous agreement and interpretation
of the standard.
namespace A {
class A::SomeClass { } ;
A::SomeClass A::some_func() ;
}
namespace A {
A::SomeClass A::some_func() { return A::SomeClass() ; }
}
Which of the redundant A:: qualifications above are legal?
Is section 8.3 of the standard relevant? What other sections
bear on this question?
|
Neither of the "A::" qualifications are redundant; and both - being
unmatched - are illegal. §8.3 is relevant because it states that the
qualified name of a namespace can be used to refer that that namespace
only from outside of the named namespace. So whichever namespace (or
class) "A" is meant to refer to - it cannot be the enclosing A
namespace. And since no other namespace or class named "A" are anywhere
in evidence, the program is ill-formed.
| Quote: | On our system, g++ and the Sun compiler allow the above while
the Intel compiler gives the "qualified name not allowed in
namespace member declaration" error discussed here a few
years back.
|
I am happy to report that the gcc 4.01 compiler rejects these
declarations with the same error as the Intel compiler. So it appears
that the steady march of C++ compilers toward full compliance,
continues.
| Quote: | Finally, is it legal to define some_func without wrapping in
namespace A { } as one does for class member functions?
A::SomeClass A::some_func() { return A::SomeClass() ; }
|
Consider this "corrected" version of the program above:
namespace A
{
namespace A
{
class SomeClass;
SomeClass some_func();
}
class A::SomeClass{ };
A::SomeClass A::some_func()
{
return A::SomeClass();
}
}
This program is well-formed because the namespace "A" (along with its
"SomeClass" class and "some_func" function declarations) is present.
So the answer is "yes" - it is legal to define a function, class or
variable outside of its namespace (by providing the appropriate
qualifications.) But, as shown in the above example, there must always
be a matching, unqualified declaration within the namespace in order
for the external definition to be legal.
Greg
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Aleksey Loginov
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| Posted: Thu Sep 21, 2006 3:38 pm Post subject: Re: qualified names in namespace member declarations |
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|
Greg Herlihy wrote:
| Quote: |
I am happy to report that the gcc 4.01 compiler rejects these
declarations with the same error as the Intel compiler. So it appears
that the steady march of C++ compilers toward full compliance,
continues.
|
what about
namespace A {
class SomeClass { } ;
A::SomeClass some_func() ;
}
namespace A {
A::SomeClass some_func() { return A::SomeClass(); }
}
Comeau compile it.
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Greg Herlihy
Guest
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| Posted: Sun Sep 24, 2006 6:55 pm Post subject: Re: qualified names in namespace member declarations |
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Aleksey Loginov wrote:
| Quote: | what about
namespace A {
class SomeClass { } ;
A::SomeClass some_func() ;
}
namespace A {
A::SomeClass some_func() { return A::SomeClass(); }
}
Comeau compile it.
|
Good point. I should have limited my example to declarations and
function names - since other names, such as type names and variable
names can be namespace qualified anywhere. In fact, my answer should
have been to wait for C++0x. It appears that the Committee has decided
to drop 8.3's restriction and allow qualified names within their own
namespace.
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#482
So from what I can tell, the original code will then be legal in the
future.
And as an editorial aside: I don't really see much benefit from this
decision. It seems to grant names in C++ an even greater potential for
ambiguity - and does so just for the convenience of the programmer
being able to type a longer name.
Greg
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Manfred von Willich
Guest
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| Posted: Thu Sep 28, 2006 8:48 pm Post subject: Re: qualified names in namespace member declarations |
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Greg Herlihy wrote:
| Quote: | And as an editorial aside: I don't really see much benefit from this
decision. It seems to grant names in C++ an even greater potential for
ambiguity - and does so just for the convenience of the programmer
being able to type a longer name.
|
Despite being a relative outsider, it is difficult for me to refrain
from adding my two cents' worth to this one.
Con:
* It is not clear to me whether any existing code will break - in
particular, code in which class or namespace is declared inside a
namespace of the same name. Name lookup from the innermost namespace
to the outermost may resolve this ambiguity correctly in all cases -
something the experts have probably looked into.
Pro:
* The rule for disambiguating any name qualification name should become
uniform for all contexts. Most importantly, the rule will be intuitive
and easily remembered.
* The change would bring the treatment of qualified names for
namespaces and classes closer together. Ideally, I would like to see
class declarations being defined as generating a namespace with the
same name, and that all name qualification refers to namespace names
only.
I remember a discussion in this group relating to injected class names
that may make the latter point less applicable, though I am unsure of
this. I would prefer "injected" to mean that a class (or namespace)
name used as the first component in a qualified name is visible by
virtue of searching outwards through nested namespaces/classes, rather
than that it is in effect redeclared as nested inside its own scope.
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