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]

Topic titled "Difference between a global int and a global static int"
seems to be up your alley:
http://groups-beta.google.com/group...n+global+static&rnum=1&hl=en#28589a26025956fe

Topic titled "global static variable help" has some useful info:
http://groups-beta.google.com/group..."global+static"&rnum=2&hl=en#fff744d2fa6b2ded

Topic titled "Static needed on global variables" is anothe good one I
think:
http://groups-beta.google.com/group...+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]

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.

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]

[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,

 

[Phlip]

I know it's bad design to use global variables.

It's bad luck to make primitive things globally mutatible.

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.

'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.

 

[Victor Bazarov]

[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.

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]

* Tony Johansson:

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.

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.

 

[kannansekar]

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]

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).

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,

 

[Alf P. Steinbach]

* Donovan Rebbechi:

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 ?

One usage is as a template argument.