header files revisited: extern vs. static

[John Goche]

Hello,

The extern keyword can be used in C and C++
to share global variables between files by
declaring the variable in header file and
defining it in only one of the two files.
For example, if x were not declared
extern in the header file below then
the linker would complain about x
being multiply defined.

--------------------
// main.cpp

#include "hello.h"

int x;

int main() {

x = 0;

hello();

}
--------------------
// hello.h

#ifndef HELLO_H
#define HELLO_H

extern int x;

void hello();

#endif // HELLO_H
--------------------
// hello.cpp

#include <iostream>
#include "hello.h"

void hello() {

std::cout << x << std::endl;

}
----------------------------------
output: 0
----------------------------------

However, I don't quite understand how
using the keyword static in the header
file allows us to define the variable
in the header file and include the
header file in multiple files.
After all, both global variables
and static variables are placed
in the static data segment of a
computer program when loaded
into memory so what is the
difference? Also, why is
1 still being printed in
the second case instead
of 0?

--------------------
// main.cpp

#include "hello.h"

int main() {

x = 0;

hello();

}
--------------------
// hello.h

#ifndef HELLO_H
#define HELLO_H

static int x = 1;

void hello();

#endif // HELLO_H
--------------------
// hello.cpp

#include <iostream>
#include "hello.h"

void hello() {

std::cout << x << std::endl;

}
----------------------------------
output: 1
----------------------------------

Thanks,

JG

[Bart]

John Goche wrote:
<snip>

However, I don't quite understand how
using the keyword static in the header
file allows us to define the variable
in the header file and include the
header file in multiple files.
After all, both global variables
and static variables are placed
in the static data segment of a
computer program when loaded
into memory so what is the
difference? Also, why is
1 still being printed in
the second case instead
of 0?

The keyword 'static' declares an object with internal linkage. The
#include just copies and pastes the contents of the header into the
source code. This means you've just defined TWO different objects with
internal linkage - one in each of your translation units. Probably not
what you intended, is it?

Regards,
Bart.

[Julián Albo]

John Goche wrote:

However, I don't quite understand how
using the keyword static in the header
file allows us to define the variable
in the header file and include the
header file in multiple files.
After all, both global variables
and static variables are placed
in the static data segment of a
computer program when loaded
into memory so what is the
difference?

Non static global variables have external linkage, the linker can identify
the variables by his name and resolve his usage from different object
files. The static ones are internal, his names are not exported and can be
used only from the same object file. If you have two static variables with
the same name in different translations units, they are two different
variables. The result is the same if you put it in a header file or in each
of the cpp that include that header, header files are just like some way of
automated copy&paste.

A compiler can use a different build model that the supposed in this
explanation, but the result must be equivalent.

--
Salu2

[Victor Bazarov]

Julián Albo wrote:

[..]
Non static global variables have external linkage, [..]

Nit-pick: .. unless they are declared 'const'.

[Steve Pope]

Bart <ba***********@ gmail.comwrote:

>The keyword 'static' declares an object with internal linkage. The
#include just copies and pastes the contents of the header into the
source code. This means you've just defined TWO different objects with
internal linkage - one in each of your translation units. Probably not
what you intended, is it?

What does the phrase "internal linkage" mean here? It almost
sounds like an oxymoron. What is being linked to what? Does
the linker need to manipulate such a static variable?

Steve

[Jack Klein]

On Mon, 30 Oct 2006 21:33:40 +0000 (UTC), sp*****@speedym ail.org
(Steve Pope) wrote in comp.lang.c++:

Bart <ba***********@ gmail.comwrote:

The keyword 'static' declares an object with internal linkage. The
#include just copies and pastes the contents of the header into the
source code. This means you've just defined TWO different objects with
internal linkage - one in each of your translation units. Probably not
what you intended, is it?

What does the phrase "internal linkage" mean here? It almost
sounds like an oxymoron. What is being linked to what? Does
the linker need to manipulate such a static variable?

Steve

Every identifier in a C++ program has several attributes, and linkage
is one of them. There are exactly three types of linkage, external,
internal, and no linkage.

Internal linkage is defined in the C++ standard like this: "When a
name has internal linkage, the entity it denotes can be referred to by
name from other scopes in the same translation unit."

This differs from identifiers defined at block scope, which cannot be
referred to by name outside of the scope in which it was defined.

--
Jack Klein
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[Steve Pope]

Jack Klein <ja*******@spam cop.netwrote:

>(Steve Pope) wrote in comp.lang.c++:

>What does the phrase "internal linkage" mean here? It almost
sounds like an oxymoron. What is being linked to what? Does
the linker need to manipulate such a static variable?

>Every identifier in a C++ program has several attributes, and linkage
is one of them. There are exactly three types of linkage, external,
internal, and no linkage.

>Internal linkage is defined in the C++ standard like this: "When a
name has internal linkage, the entity it denotes can be referred to by
name from other scopes in the same translation unit."

>This differs from identifiers defined at block scope, which cannot be
referred to by name outside of the scope in which it was defined.

Thanks. I find the choice of words -- "internal linkage" --
un-intuitive, relative to say "local to one source file", but
that's of no consequence here. ;)

Steve
of little

[Victor Bazarov]

Steve Pope wrote:

[..] I find the choice of words -- "internal linkage" --
un-intuitive, relative to say "local to one source file", but
that's of no consequence here. ;)

It's unintuitive for a newcomer who never dealt with the table
of *external* symbols for a module (no offence intended). What
is the opposite of 'external'? See?

'External' has unfortunately two meanings. And I don't think
there is a way to overcome that. OOH, it's a symbol that cannot
be resolved locally, and as such is deemed to come from outside.
OTOH it's a symbol visible from outside, a symbol to which other
modules can refer (thus for other modules it also is 'external')
by name. 'Internal' is basically the opposite of both of those
meanings, i.e. defined inside and referred inside [the module].

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask

[Geo]

Victor Bazarov wrote:

Julián Albo wrote:

[..]
Non static global variables have external linkage, [..]

Nit-pick: .. unless they are declared 'const'.

If they are 'const', are they 'variables' !

[Bart]

Geo wrote:

Victor Bazarov wrote:

Julián Albo wrote:

[..]
Non static global variables have external linkage, [..]

Nit-pick: .. unless they are declared 'const'.

If they are 'const', are they 'variables' !

Yes. Technically, the C++ standard defines a 'variable' as being
"introduced by the declaration of an object". That's different than the
mathematical meaning of a 'variable'.

Regards,
Bart.