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Boris Kolpackov
Guest
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 Posted: Tue Jul 13, 2004 2:50 am Post subject: Can a using-declaration name a namespace? |
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|
Good day,
There is a defect report #406
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#460
that questions whether the following is legal or not (which is
legal according to the standard):
namespace n
{
namespace m
{
}
}
using n::m;
While the defect report does not say what exactly the defect is,
it has the following note:
"Notes from the March 2004 meeting:
We agree that it should be an error."
Can somebody explain to the rest of us why exactly it should be
an error?
thanks,
-boris
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Sandeep
Guest
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| Posted: Tue Jul 13, 2004 2:04 pm Post subject: Re: Can a using-declaration name a namespace? |
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[email]boris (AT) kolpackov (DOT) net[/email] (Boris Kolpackov) wrote in message news:<2lgaksFcl1qcU1 (AT) uni-berlin (DOT) de>...
| Quote: | Good day,
There is a defect report #406
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#460
that questions whether the following is legal or not (which is
legal according to the standard):
namespace n
{
namespace m
{
}
}
using n::m;
While the defect report does not say what exactly the defect is,
it has the following note:
"Notes from the March 2004 meeting:
We agree that it should be an error."
Can somebody explain to the rest of us why exactly it should be
an error?
thanks,
-boris
---
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|
Hi,
I am novice to the C++,so if u think i am wrong then plz ignore my
comment.
The question was whether following is legal or not...
| Quote: | namespace n
{
namespace m
{
}
}
using n::m;
|
It is not legal .The problem might be the "namespace" word that we
need to use after "using" .
The legal syntax is
using namespace n::m;
It will work.
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Steve Clamage
Guest
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| Posted: Fri Jul 16, 2004 1:04 am Post subject: Re: Can a using-declaration name a namespace? |
|
|
Boris Kolpackov wrote:
| Quote: | Good day,
There is a defect report #406
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#460
that questions whether the following is legal or not (which is
legal according to the standard):
namespace n
{
namespace m
{
}
}
using n::m;
While the defect report does not say what exactly the defect is,
it has the following note:
"Notes from the March 2004 meeting:
We agree that it should be an error."
Can somebody explain to the rest of us why exactly it should be
an error?
thanks,
-boris
|
Here's my take on the issue.
To get access to names in namespace foo, you write a using-directive:
using namespace foo;
not a using-declaration:
using foo;
A using-declaration introduces the *name* into the current namespace.
It is not the same as a using-directive.
So in your example, the name "m" from namespace "n" would be
introduced into the global namespace. But "m" is the name of a
namespace, so introducing the name doesn't have any useful effect.
I wasn't present for the discussion of this issue, but I suppose the
idea was to make
using <namespace-name> ;
an error, since it probably is a programmer mistake anyway.
---
Steve Clamage, [email]stephen.clamage (AT) sun (DOT) com[/email]
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Gabriel Dos Reis
Guest
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| Posted: Fri Jul 16, 2004 3:14 am Post subject: Re: Can a using-declaration name a namespace? |
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|
[email]stephen.clamage (AT) sun (DOT) com[/email] (Steve Clamage) writes:
| Quote: | Boris Kolpackov wrote:
Good day,
There is a defect report #406
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#460
that questions whether the following is legal or not (which is
legal according to the standard):
namespace n
{
namespace m
{
}
}
using n::m;
While the defect report does not say what exactly the defect is,
it has the following note:
"Notes from the March 2004 meeting:
We agree that it should be an error."
Can somebody explain to the rest of us why exactly it should be an
error?
thanks,
-boris
Here's my take on the issue.
To get access to names in namespace foo, you write a using-directive:
using namespace foo;
not a using-declaration:
using foo;
|
Yes but that is not what Boris presented.
As you recalled in your message, a using-directive does not have the
same effect as a using-declaration. A using-declaration selectively
brings into scope a specific set of declarations. Using-directives on
the other hand profondly modifies the way look-up is done. They are
quite different things and I believe that reducing the issue to that
of using-directives is not right.
The inconsistency, as I understand it from Boris's message, is:
namespace N
{
struct S { };
namespace M { }
}
using N::S; // OK
using N::M; // would be ERROR according to the #406
I must confess I'm confused as to why it should be an error. As far
as I can tell, the effect is to bring the name "M" into scope, not to
dump the declarations in N::M:: into the current scope.
I believe that the resolution of #406 is defective.
| Quote: | A using-declaration introduces the *name* into the current namespace.
It is not the same as a using-directive.
|
Yes, agreed. But Boris does not want using-directive. He wants to
introduces the *name* "m" into the current scope. I see no technical
reason to forbid that.
| Quote: | So in your example, the name "m" from namespace "n" would be
introduced into the global namespace. But "m" is the name of a
namespace, so introducing the name doesn't have any useful effect.
|
Why?
It has the effect that he would just say "m" witthout:
(1) the qualification
(2) without dumping the whole content into the current scope.
| Quote: | I wasn't present for the discussion of this issue, but I suppose the
idea was to make
using <namespace-name> ;
an error, since it probably is a programmer mistake anyway.
|
That might be the idea but I don't find it particularly compelling.
Why would it be likely an error to say
using <namespace-name> ;
and
using <class-name> ;
would not?
I would suggest we trust the programmer and have, simple, general rules.
--
Gabriel Dos Reis
[email]gdr (AT) cs (DOT) tamu.edu[/email]
Texas A&M University -- Computer Science Department
301, Bright Building -- College Station, TX 77843-3112
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Victor Bazarov
Guest
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| Posted: Fri Jul 16, 2004 11:25 pm Post subject: Re: Can a using-declaration name a namespace? |
|
|
Gabriel Dos Reis wrote:
| Quote: | stephen.clamage (AT) sun (DOT) com (Steve Clamage) writes:
| Boris Kolpackov wrote:
| > Good day,
| > There is a defect report #406
| > http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#460
| > that questions whether the following is legal or not (which is
| > legal according to the standard):
| > namespace n
| > {
| > namespace m
| > {
| > }
| > }
| > using n::m;
| > While the defect report does not say what exactly the defect is,
| > it has the following note:
| > "Notes from the March 2004 meeting:
| > We agree that it should be an error."
| > Can somebody explain to the rest of us why exactly it should be an
| > error?
| > thanks,
| > -boris
|
| Here's my take on the issue.
|
| To get access to names in namespace foo, you write a using-directive:
| using namespace foo;
| not a using-declaration:
| using foo;
Yes but that is not what Boris presented.
As you recalled in your message, a using-directive does not have the
same effect as a using-declaration. A using-declaration selectively
brings into scope a specific set of declarations. Using-directives on
the other hand profondly modifies the way look-up is done. They are
quite different things and I believe that reducing the issue to that
of using-directives is not right.
The inconsistency, as I understand it from Boris's message, is:
namespace N
{
struct S { };
namespace M { }
}
using N::S; // OK
using N::M; // would be ERROR according to the #406
I must confess I'm confused as to why it should be an error. As far
as I can tell, the effect is to bring the name "M" into scope, not to
dump the declarations in N::M:: into the current scope.
I believe that the resolution of #406 is defective.
| A using-declaration introduces the *name* into the current namespace.
| It is not the same as a using-directive.
Yes, agreed. But Boris does not want using-directive. He wants to
introduces the *name* "m" into the current scope. I see no technical
reason to forbid that.
|
I believe the name "m" needs classification when it's injected. And
that classification is "either it's a namespace or it isn't". Just
"adding a name" won't work because names of namespaces are treated
differently from names of types and names of objects.
| Quote: |
| So in your example, the name "m" from namespace "n" would be
| introduced into the global namespace. But "m" is the name of a
| namespace, so introducing the name doesn't have any useful effect.
Why?
|
Because it won't.
| Quote: |
It has the effect that he would just say "m" witthout:
(1) the qualification
(2) without dumping the whole content into the current scope.
|
There is already a mechanism in the language to do that. It's called
"namespace aliasing".
| Quote: |
| I wasn't present for the discussion of this issue, but I suppose the
| idea was to make
| using <namespace-name> ;
| an error, since it probably is a programmer mistake anyway.
That might be the idea but I don't find it particularly compelling.
Why would it be likely an error to say
using <namespace-name> ;
and
using <class-name> ;
would not?
I would suggest we trust the programmer and have, simple, general rules.
|
A simple, general rule exists. It basically makes any namespace name
_special_. The name, when declared, is not added to the list of "all
known names", but gets put in a "all known namespaces". So, declaring
a namespace (to introduce its particular name into the set of namespaces
for, say, searching for a name when resolving a qualified name) is
already there:
namespace nm = n::m;
..
Correct me if I am wrong.
Victor
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Boris Kolpackov
Guest
|
| Posted: Sat Jul 17, 2004 2:42 am Post subject: Re: Can a using-declaration name a namespace? |
|
|
[email]stephen.clamage (AT) sun (DOT) com[/email] (Steve Clamage) writes:
| Quote: | So in your example, the name "m" from namespace "n" would be
introduced into the global namespace. But "m" is the name of a
namespace, so introducing the name doesn't have any useful effect.
|
It is as useful as any other using-declaration -- it brings mentioned
name into a scope which allows me to refer to such a name without any
further qualification:
namespace n
{
namespace m
{
void f ();
}
}
void g ()
{
n::m::f ();
using n::m;
// Now `m' is in the scope so I can use it unqualified.
m::f ();
}
Hope it is clear now.
-boris
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Max Polk
Guest
|
| Posted: Sat Jul 17, 2004 5:05 am Post subject: Re: Can a using-declaration name a namespace? |
|
|
Gabriel Dos Reis wrote:
| Quote: | namespace N
{
struct S { };
namespace M { }
}
using N::S; // OK
using N::M; // would be ERROR according to the #406
I must confess I'm confused as to why it should be an error. As far
as I can tell, the effect is to bring the name "M" into scope, not to
dump the declarations in N::M:: into the current scope.
....
It has the effect that he would just say "m" witthout:
(1) the qualification
(2) without dumping the whole content into the current scope.
|
Essentially you want to have the effect of saying M::Z instead of N::M::Z
[where Z is in the M namespace], but without bringing S and everything else in
N into the current scope.
This actually presents a problem if there is another namespace M adjacent to
N, as in:
namespace N
{
struct S { };
namespace M
{
struct Z { };
}
}
namespace M
{
struct Z { };
}
M::Z a; // this is not N::M::Z
using N::M; // your suggestion
M::Z b; // ambiguous
If you *were* able to bring the N::M namespace into current scope, M::Z would
then be ambiguous, and it would be *impossible* to refer to the Z inside M
which is in turn not inside N.
You might wish to do this:
::M::Z b; // solved ambiguity?
But you would be preventing somebody from wrapping your header file with all
these definitions inside a namespace to keep it separate from their code:
namespace VENDOR
{
#include "filename.h"
}
You haven't solved the ambiguity because ::M::Z isn't right, it's really
prefixed by VENDOR by whoever is including your code.
Basically you raise an ambiguity impossible to solve.
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Gabriel Dos Reis
Guest
|
| Posted: Sat Jul 17, 2004 7:16 am Post subject: Re: Can a using-declaration name a namespace? |
|
|
[email]ux9i003 (AT) yahoo (DOT) com[/email] (Max Polk) writes:
[...]
| Quote: | This actually presents a problem if there is another namespace M
adjacent to N, as in:
|
That is a non-argument. See below.
| Quote: | namespace N
{
struct S { };
namespace M
{
struct Z { };
}
}
namespace M
{
struct Z { };
}
M::Z a; // this is not N::M::Z
using N::M; // your suggestion
|
That particular configuration would be invalid in similar ways as
namespace N {
struct S { };
};
struct S { };
using N::S;
by general rules.
[...]
| Quote: | Basically you raise an ambiguity impossible to solve.
|
No. See above.
--
Gabriel Dos Reis
[email]gdr (AT) cs (DOT) tamu.edu[/email]
Texas A&M University -- Computer Science Department
301, Bright Building -- College Station, TX 77843-3112
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Seungbeom Kim
Guest
|
| Posted: Sun Jul 18, 2004 12:10 am Post subject: Re: Can a using-declaration name a namespace? |
|
|
Max Polk wrote:
| Quote: |
This actually presents a problem if there is another namespace M
adjacent to N, as in:
namespace N
{
struct S { };
namespace M
{
struct Z { };
}
}
namespace M
{
struct Z { };
}
M::Z a; // this is not N::M::Z
using N::M; // your suggestion
M::Z b; // ambiguous
[...]
Basically you raise an ambiguity impossible to solve.
|
Is the situation different in any way with M being a name of a type or
an object instead of a namespace? I don't think so. If the possibility
of ambiguity were a problem that prevented us from using-declarations,
we wouldn't be able to use any using-declaration at all, would we?
--
Seungbeom Kim
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Bo Persson
Guest
|
| Posted: Sun Jul 18, 2004 12:12 am Post subject: Re: Can a using-declaration name a namespace? |
|
|
"Boris Kolpackov" <boris (AT) kolpackov (DOT) net> skrev i meddelandet
news:2lqa97FfrhqbU1 (AT) uni-berlin (DOT) de...
| Quote: | stephen.clamage (AT) sun (DOT) com (Steve Clamage) writes:
So in your example, the name "m" from namespace "n" would be
introduced into the global namespace. But "m" is the name of a
namespace, so introducing the name doesn't have any useful effect.
It is as useful as any other using-declaration -- it brings mentioned
name into a scope which allows me to refer to such a name without any
further qualification:
namespace n
{
namespace m
{
void f ();
}
}
void g ()
{
n::m::f ();
using n::m;
|
But this feature already exists, you just write it:
namespace m = n::m;
| Quote: |
// Now `m' is in the scope so I can use it unqualified.
|
Right!
Bo Persson
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Gabriel Dos Reis
Guest
|
| Posted: Sun Jul 18, 2004 3:50 am Post subject: Re: Can a using-declaration name a namespace? |
|
|
[email]bop (AT) gmb (DOT) dk[/email] ("Bo Persson") writes:
[...]
| Quote: | namespace n
{
namespace m
{
void f ();
}
}
void g ()
{
n::m::f ();
using n::m;
But this feature already exists,
|
No, it does not.
| Quote: | you just write it:
namespace m = n::m;
|
Then the same reasoning implies that using-declaration that brings in
type-names should be abolished because we have typedef.
namespace N {
struct S { };
};
using N::S; // should be rejected, because it could be
// written typedef N::S S;
--
Gabriel Dos Reis
[email]gdr (AT) cs (DOT) tamu.edu[/email]
Texas A&M University -- Computer Science Department
301, Bright Building -- College Station, TX 77843-3112
---
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Gabriel Dos Reis
Guest
|
| Posted: Tue Jul 20, 2004 6:14 pm Post subject: Re: Can a using-declaration name a namespace? |
|
|
Victor Bazarov <v.Abazarov (AT) comAcast (DOT) net> writes:
[...]
| Quote: | The inconsistency, as I understand it from Boris's message, is:
namespace N
{
struct S { };
namespace M { }
}
using N::S; // OK
using N::M; // would be ERROR according to the #406
I must confess I'm confused as to why it should be an error. As far
as I can tell, the effect is to bring the name "M" into scope, not to
dump the declarations in N::M:: into the current scope.
I believe that the resolution of #406 is defective.
| A using-declaration introduces the *name* into the current
namespace.
| It is not the same as a using-directive.
Yes, agreed. But Boris does not want using-directive. He wants to
introduces the *name* "m" into the current scope. I see no technical
reason to forbid that.
I believe the name "m" needs classification when it's injected. And
|
When the name is brought into scope through a using-declaration, it is
not forgetfully done so. That is, by the time the using-declaration
was processed, the compiler looked into the scope named by the
nested-qualifier and determined that effectively "m" is a namespace.
When it bring that name into scope, it annotates it (someway of the
other) as to what it names. That, irrespectively of whether that name
designate a type-name or a fubhar. So the objection you present is not
a valid one.
| Quote: | that classification is "either it's a namespace or it isn't". Just
"adding a name" won't work because names of namespaces are treated
differently from names of types and names of objects.
|
Yes, that but a using-declatation (whether it brings a namespace-name
or a fubhar-name into scope) does not forget about what that name
happens to designate in reality. Nobody is asking that "m" to be
introduced as a symbol in void.
| Quote: | | So in your example, the name "m" from namespace "n" would be
| introduced into the global namespace. But "m" is the name of a
| namespace, so introducing the name doesn't have any useful effect.
Why?
Because it won't.
|
That is untrue, for at least the two useful effects I listed below
| Quote: | It has the effect that he would just say "m" witthout:
(1) the qualification
(2) without dumping the whole content into the current scope.
There is already a mechanism in the language to do that. It's called
"namespace aliasing".
|
So what? Following your logic a using-declaration that names a type
should be banned because we already have a mechanism in the language
to do that, namely typedef. I believe that the coherence of that
reasoning does not resist any serious analyzis.
| Quote: | | I wasn't present for the discussion of this issue, but I suppose
the
| idea was to make
| using <namespace-name> ;
| an error, since it probably is a programmer mistake anyway.
That might be the idea but I don't find it particularly compelling.
Why would it be likely an error to say
using <namespace-name> ;
and
using <class-name> ;
would not?
I would suggest we trust the programmer and have, simple, general
rules.
A simple, general rule exists. It basically makes any namespace name
_special_.
|
No, it does not.
| Quote: | The name, when declared, is not added to the list of "all
known names", but gets put in a "all known namespaces".
|
just as the same is done for class-names, typedef-names or non-type names.
| Quote: | So, declaring
a namespace (to introduce its particular name into the set of namespaces
for, say, searching for a name when resolving a qualified name) is
already there:
namespace nm = n::m;
.
Correct me if I am wrong.
|
Repeat the same for type-names.
--
Gabriel Dos Reis
[email]gdr (AT) cs (DOT) tamu.edu[/email]
Texas A&M University -- Computer Science Department
301, Bright Building -- College Station, TX 77843-3112
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