I have a static class member variable as follows:
struct A
{
static void Set (int i) { v = i; }
static int& Get () { return v; }
static int v;
};
int A::v; // define A::v in the cpp file
A::v will have external linkage and there will only be one instance of this
variable in the executable:
However, let's say I don't want to define the variable in the cpp file (eg.
it's a template class and I don't want users to have to define the
variable).
Is there anything wrong with defining it in a static member function which
returns a reference to the variable? Will there also always only be one copy
of the local static variable? Any other unforseen problems? Thanks in
advance for any comments.
struct A
{
static void Set (int i) { v() = i; }
static int& Get () { return v(); }
static int& v()
{
static v;
return v;
}
};
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[ comp.lang.c++.m oderated. First time posters: Do this! ]
Jul 22 '05
16  2985 
Renjith Mohan wrote: Another solution is to use an unnamed namespace which is semantically
equivalent to internal linkage and is a mcuh more simple and elegant
solution.
Thus in a header file you can do this.
namespace
{
int v;
}
struct A
{
static void Set (int i) { v = i; }
static int& Get () { return v; }
};
I strongly disagree. I am not sure, but it seems to me, that you've got
ODR violation here. The exact meaning of 'return v' is different in every
TU because of the fact that an unnamed namespace does have a name, though
hidden and distinct for every TU.
Aside from (possible) ODR violation, there are other problems. This places
a distinct instance of an object from the unnamed namespace in every TU
that includes the header with the unnamed namespace, what results in quite
different semantics because now every TU operates on an own instance of v.
--
Maxim Yegorushkin
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ]
"Maxim Yegorushkin" <e-*****@yandex.ru > schrieb im Newsbeitrag
news:opshq9w3y9 ti5cme@wkcg6rir wp... Renjith Mohan wrote:
Another solution is to use an unnamed namespace which is semantically
equivalent to internal linkage and is a mcuh more simple and elegant
solution.
Thus in a header file you can do this.
namespace
{
int v;
}
struct A
{
static void Set (int i) { v = i; }
static int& Get () { return v; }
};
I strongly disagree. I am not sure, but it seems to me, that you've got
ODR violation here.
Don't think so. Since v has internal linkage....
Aside from (possible) ODR violation, there are other problems. This places
a distinct instance of an object from the unnamed namespace in every TU
that includes the header with the unnamed namespace, what results in quite
different semantics because now every TU operates on an own instance of v.
That IS a problem.
Thomas
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[ comp.lang.c++.m oderated. First time posters: Do this! ]
"Thomas Mang" <no****@nospam. invalid> wrote... "Maxim Yegorushkin" <e-*****@yandex.ru > schrieb im Newsbeitrag
news:opshq9w3y9 ti5cme@wkcg6rir wp... Renjith Mohan wrote:
> Another solution is to use an unnamed namespace which is semantically
> equivalent to internal linkage and is a mcuh more simple and elegant
> solution.
> Thus in a header file you can do this.
> namespace
> {
> int v;
> }
> struct A
> {
> static void Set (int i) { v = i; }
> static int& Get () { return v; }
>
> };
I strongly disagree. I am not sure, but it seems to me, that you've got
ODR violation here.
Don't think so. Since v has internal linkage....
A big misconception. 'v' has _external_ linkage, but it doesn't matter.
Every file that includes that header has its own 'v', since its name has
the name of the anonymous namespace added to it.
[..the other problem is correctly identified..]
V
In article <41************ ***********@use net.univie.ac.a t>, Thomas Mang
<no****@nospam. invalid> writes "Maxim Yegorushkin" <e-*****@yandex.ru > schrieb im Newsbeitrag
news:opshq9w3y 9ti5cme@wkcg6ri rwp... Renjith Mohan wrote:
> Another solution is to use an unnamed namespace which is semantically
> equivalent to internal linkage and is a mcuh more simple and elegant
> solution.
> Thus in a header file you can do this.
> namespace
> {
> int v;
> }
> struct A
> {
> static void Set (int i) { v = i; }
> static int& Get () { return v; }
>
> };
I strongly disagree. I am not sure, but it seems to me, that you've got
ODR violation here.
Don't think so. Since v has internal linkage....
It doesn't, it has external linkage but with an unutterable name.
However that is not really relevant because there is a potential ODR
violation which ever way you take it. If the above header file is
included in more than one TU the definition of struct A will be an ODR
issue.
--
Francis Glassborow ACCU
Author of 'You Can Do It!' see http://www.spellen.org/youcandoit
For project ideas and contributions: http://www.spellen.org/youcandoit/projects
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[ comp.lang.c++.m oderated. First time posters: Do this! ]
"Maxim Yegorushkin" <e-*****@yandex.ru > wrote in message
news:<opshq9w3y 9ti5cme@wkcg6ri rwp>... Renjith Mohan wrote:
Another solution is to use an unnamed namespace which is semantically
equivalent to internal linkage and is a mcuh more simple and elegant
solution.
Thus in a header file you can do this.
namespace
{
int v;
}
struct A
{
static void Set (int i) { v = i; }
static int& Get () { return v; }
};
I strongly disagree. I am not sure, but it seems to me, that you've got
ODR violation here. The exact meaning of 'return v' is different in every
TU because of the fact that an unnamed namespace does have a name, though
hidden and distinct for every TU.
Aside from (possible) ODR violation, there are other problems. This places
a distinct instance of an object from the unnamed namespace in every TU
that includes the header with the unnamed namespace, what results in quite
different semantics because now every TU operates on an own instance of v.
I understand my misconception. Thanks for pointing this out. I seem to
have been over enthusiastic about the clause 7.3.1.1 - Unnamed
namespaces para 2 which says something like this
"The use of the static keyword is deprecated when declaring objects in
a namespace scope (see annex depr); the unnamed-namespace provides a
superior alternative."
As has been pointed out this applies to "namespace scope" members only
and even with such variables this must be used with caution as the
generated namespace name(for the unnamed namespace) in each TU is
implementation dependent which may create binary differences of the
executable which is beyond programmers control.
But a doubt remains; since the standard does recommend it for
"namespace scope" variables, does it violate ODR for such variables?.
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[ comp.lang.c++.m oderated. First time posters: Do this! ]
Thomas Mang wrote: "Maxim Yegorushkin" <e-*****@yandex.ru > schrieb im Newsbeitrag news:opshq9w3y9 ti5cme@wkcg6rir wp...Renjith Mohan wrote:Another solution is to use an unnamed namespace which is semantically
equivalent to internal linkage and is a mcuh more simple and elegant
solution.
Thus in a header file you can do this.
namespace
{
int v;
}
struct A
{
static void Set (int i) { v = i; }
static int& Get () { return v; }
};
I strongly disagree. I am not sure, but it seems to me, that you've got
ODR violation here.
Don't think so. Since v has internal linkage....
No, v does not have internal linkage. Things inside the unnamed
namespace have external linkage. They just have a name that is known
only to the current translation unit, making them inaccessible from
other translation units. The ultimate effect is very similar to making
v static, but there are some subtle differences that must be understood.
The one-definition rule (ODR) violation is not with v, it is with A::Set
and A::Get. Within each translation unit, A::Set and A::Get refer to
the v that is unique to that translation unit. Therefore, A::Set and
A::Get are not the same in all translation units, which is not allowed.
--
David Olsen qg********@yaho o.com
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.m oderated. First time posters: Do this! ] re**********@ho tmail.com (Renjith Mohan) wrote in message
news:<cd******* *************** ****@posting.go ogle.com>...
But a doubt remains; since the standard does recommend it for
"namespace scope" variables, does it violate ODR for such variables?.
In this context, you get ODR violations using either (deprecated)
namespace scope statics or a nameless namespace. Using a nameless
namespace doesn't make it any more wrong :-)
Point of interest: boost::bind uses a nameless namespace in a header
to define _1, _2 etc. (the parameter placeholders). I've yet to get a
grip on the pros and cons of this - not that it's something you have
to understand to use bind itself.
James
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