global variables

[Tony Johansson]

Hello!

I know it's bad design to use global variables. I just want to ask a
question about them.

Is global variables and global static variables the same. These are define
outside any function at the top of the a file where you have them.

//Tony

[shumaker@cs.fsu.edu]

Re: global variables

Topic titled "Difference between a global int and a global static int"
seems to be up your alley:

http://groups-beta.google.com/group/comp.lang.c++/browse_thread/thread/596eecb0df5383a4/28589a26025956fe?q=difference+between+global+static&rnum=1&hl=en#28589a26025956fe

Topic titled "global static variable help" has some useful info:

http://groups-beta.google.com/group/comp.lang.c++/browse_thread/thread/893fb120c94747f0/fff744d2fa6b2ded?q=global+versus+%22global+static%22&rnum=2&hl=en#fff744d2fa6b2ded

Topic titled "Static needed on global variables" is anothe good one I
think:

http://groups-beta.google.com/group/comp.lang.c/browse_thread/thread/3220b60144be7de4/6deb276814d2da0b?q=difference+between+global+static&rnum=19&hl=en#6deb276814d2da0b

You can find lots of info by hitting the "Groups" link at google.com
and searching there, which is how I found the above posts and many more.

[Donovan Rebbechi]

Re: global variables

On 2005-05-20, Tony Johansson <johansson.ande rsson@telia.com > wrote:[color=blue]
> Hello!
>
> I know it's bad design to use global variables. I just want to ask a
> question about them.
>
> Is global variables and global static variables the same. These are define
> outside any function at the top of the a file where you have them.[/color]

If it's declared as static, it has "internal linkage". This means it is not
visible outside the translation unit in which it is defined. In C++, there is
not so much need for file globals, but in C they can be useful. For example,
if your C program consists of several modules, each which need to store some
state information, then each module can be implemented as a .c file, and then
the state information can be stored in internal-linkage global variables.
Basically, it's one of many ways to do encapsulation in C.

In C++, I'd be less inclined to create global variables of any kind. I'd be more
likely to implement a module system by usually putting state information in
classes and using inheritance to allow different modules to have a common
interface but differing state information.

Cheers,
--
Donovan Rebbechi

http://pegasus.rutgers.edu/~elflord/

[Donovan Rebbechi]

Re: global variables

On 2005-05-20, Donovan Rebbechi <abuse@aol.co m> wrote:[color=blue]
> On 2005-05-20, Tony Johansson <johansson.ande rsson@telia.com > wrote:[color=green]
>> Hello!
>>
>> I know it's bad design to use global variables. I just want to ask a
>> question about them.
>>
>> Is global variables and global static variables the same. These are define
>> outside any function at the top of the a file where you have them.[/color][/color]

[snip]

Just an add-on -- one can also have internal linkage for functions, which is
very useful. In C++, you usually do this via unnamed namespaces.

Cheers,
--
Donovan Rebbechi

http://pegasus.rutgers.edu/~elflord/

[Phlip]

Re: global variables

Tony Johansson wrote:
[color=blue]
> I know it's bad design to use global variables.[/color]

It's bad luck to make primitive things globally mutatible.
[color=blue]
> I just want to ask a
> question about them.
>
> Is global variables and global static variables the same. These are define
> outside any function at the top of the a file where you have them.[/color]

'static' data variables at file scope are an excellent encapsulation
technique. They are not the same because if two translation units both see

static int foo;

then each gets its own copy of foo. Changes to one foo won't affect the
other file, and changes are the biggest problem with global variables.

--
Phlip

http://www.c2.com/cgi/wiki?ZeekLand

[Victor Bazarov]

Re: global variables

Donovan Rebbechi wrote:
[color=blue]
> On 2005-05-20, Donovan Rebbechi <abuse@aol.co m> wrote:
>[color=green]
>>On 2005-05-20, Tony Johansson <johansson.ande rsson@telia.com > wrote:
>>[color=darkred]
>>>Hello!
>>>
>>>I know it's bad design to use global variables. I just want to ask a
>>>question about them.
>>>
>>>Is global variables and global static variables the same. These are define
>>>outside any function at the top of the a file where you have them.[/color][/color]
>
>
> [snip]
>
> Just an add-on -- one can also have internal linkage for functions, which is
> very useful. In C++, you usually do this via unnamed namespaces.[/color]

Wrong. In C++ any function in an unnamed namespace still has external
linkage by default. You give internal linkage to an object or a function
if you declare it 'static' or 'const' without 'extern' (for objects).

V

[Alf P. Steinbach]

Re: global variables

* Tony Johansson:[color=blue]
>
> I know it's bad design to use global variables. I just want to ask a
> question about them.
>
> Is global variables and global static variables the same. These are define
> outside any function at the top of the a file where you have them.[/color]

This is not primarily a language question but a terminology question.

With respect to a some piece of C++ source code S a variable is "global"
if it's available in all of S plus possibly in code outside S, and it's
"local" if it's only available within S, possibly not within all of S.

I.e.,

"local" => inside only
"global" => all of inside + possibly also outside

Used without qualification the meaning is somewhat language dependent; in
C++ unqualified "local" usually refers to S = a function, and unqualified
"global" usually refers to S = a translation unit, then meaning a variable
that's not only available throughout S but also outside S, in all units.

With that default in place, a "global variable" (unqualified) means a
variable available throughout the whole statically linked program, S =
compilation unit, whereas a "global static variable" (qualified) means
a variable available throughout a single compilation unit, S = the code
from the variable declaration to the end of the compilation unit text.

In standard C++ available throughout the complete statically linked program
is the most global a variable can be.

In in-practice C++ it's possible for a variable to be even more global,
exported to dynamically linked libraries.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[kannansekar@gmail.com]

Re: global variables

actually when you say,
int variable;
outside of all function , it means
extern int variable; automatically which means, in another translation
unit, I can declare
extern int variable , and will be able to access the value,
but
static int variable; will not allow you to do that.
But how that is done ?, every translation unit has something called
"exported" symbols, in that
'variable' will be included if you write ( int variable;) at the global
level.

Kannan Somasekar

[Donovan Rebbechi]

Re: global variables

On 2005-05-20, Victor Bazarov <v.Abazarov@com Acast.net> wrote:
[color=blue]
> Wrong. In C++ any function in an unnamed namespace still has external
> linkage by default. You give internal linkage to an object or a function
> if you declare it 'static' or 'const' without 'extern' (for objects).[/color]

Doh! So it does. I suppose there's a reason for this. When would one want/need
a function in an unnamed namespace to have external linkage ?

Cheers,
--
Donovan Rebbechi

http://pegasus.rutgers.edu/~elflord/

[Alf P. Steinbach]

Re: global variables

* Donovan Rebbechi:[color=blue]
> On 2005-05-20, Victor Bazarov <v.Abazarov@com Acast.net> wrote:
>[color=green]
> > Wrong. In C++ any function in an unnamed namespace still has external
> > linkage by default. You give internal linkage to an object or a function
> > if you declare it 'static' or 'const' without 'extern' (for objects).[/color]
>
> Doh! So it does. I suppose there's a reason for this. When would one want/need
> a function in an unnamed namespace to have external linkage ?[/color]

One usage is as a template argument.

--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?