Multiple Inclusion Guards Work in MSVC++ .NET...right?

[Fritz Foetzl]

I'm flummoxed. I'm a veteran C++ programmer from the Unix/Linux camp,
trying to learn Visual C++. I'm trying to build a project in which I
need to include one header in a couple of different files, but the
classic multiple inclusion problem is biting me hard. The
#ifndef..#defin e..#endif method doesn't seem to be working, although
all the documentation I've read indicates that it should.

As a small example, I have an empty console project with three files:
globals.h, functions.cpp and driver.cpp. They look like this:

// ----------------------------------------------------
// globals.h
#ifndef _GLOBALS_
#define _GLOBALS_

const char *msg = "Wiggety wack";

#endif
// EOF

// ----------------------------------------------------
// functions.cpp
#include <iostream>
using namespace std;

#include "globals.h"

void getWiggety ()
{
cout << msg << endl;
}
// EOF

// ----------------------------------------------------
// driver.cpp
#include <iostream>
using namespace std;

#include "globals.h"

extern void getWiggety (void);

int main (void)
{
cout << msg << endl;
getWiggety ();
return 0;
}
// EOF

This won't link, because msg is declared twice, in spite my
#ifndef..#defin e..#endif in globals.h. I've gone and looked at
<iostream>, and it's protected against multiple inclusion the same way
as I'm doing it. I'm also including it in two places but...the linker
doesn't complain about std::cout et. al.

WTFO?

ff

[Jonathan Turkanis]

"John Carson" <do***********@ datafast.net.au > wrote in message
news:40******** @usenet.per.par adox.net.au...

"Jonathan Turkanis" <te******@kanga roologic.com> wrote in message

Observe that there is no explicit mention of pointers under the
first dashed point dealing with the use of the static keyword, yet we know that declaring a pointer static will give it internal
linkage. Accordingly, I infer that pointers are included as
"objects" under both dashed points.

I don't follow your reasoning here. The passage is talking about
const, not static. I believe objects and pointers are usually treated separately by the standard.

Look again. The first dashed point says:

"an object, reference, function or function template that is

explicitly declared static"

I see what your argument was now:

1. We've all heard that delaring a pointer static has gives it
internal linkage.
2. This must be the passage which states that rule.
3. Therefore, this passage must be refering to pointers (among
other things)

Right? :-)

This get's the interpretive porcess exactly backwards. I'd like to
think we should start with the text, read it carefully, and figure out
what it means. Of course, in this case, I did a horrible job.

Jonathan

[John Carson]

"Jonathan Turkanis" <te******@kanga roologic.com> wrote in message
news:bu******** ****@ID-216073.news.uni-berlin.de

I see what your argument was now:

1. We've all heard that delaring a pointer static has gives it
internal linkage.
2. This must be the passage which states that rule.
3. Therefore, this passage must be refering to pointers (among
other things)

Right? :-)
Exactly.
This get's the interpretive porcess exactly backwards. I'd like to
think we should start with the text, read it carefully, and figure out
what it means.

Yes, that would be ideal. But the standard often isn't that easy to
interpret. To be a really good interpreter of the standard, I think you
would have to read the whole thing from the beginning --- which I am
disinclined to do.
--
John Carson
1. To reply to email address, remove donald
2. Don't reply to email address (post here instead)

[E. Robert Tisdale]

Fritz Foetzl wrote:

[snip]

This won't link.
Try this:
cat globals.h #ifndef GUARD_GLOBALS_H
#define GUARD_GLOBALS_H 1

const
char *const msg = "Wiggety wack";
extern void getWiggety(void );

#endif//GUARD_GLOBALS_H
cat functions.cpp #include <iostream>
#include "globals.h"

void getWiggety(void ) {
std::cout << msg << std::endl;
}
cat driver.cpp #include <iostream>
#include "globals.h"

int main(int argc, char* argv[]) {
std::cout << msg << std::endl;
getWiggety();
return 0;
}
g++ -Wall -ansi -pedantic -o driver driver.cpp functions.cpp
./driver

Wiggety wack
Wiggety wack

[Fritz Foetzl]

"Jonathan Turkanis" <te******@kanga roologic.com> wrote in message news:<bu******* *****@ID-216073.news.uni-berlin.de>...

I think your problem has nothing to do with include guards. The header
"globals.h" is properly included twice, once when each of the files
"driver.cpp " and "functions. cpp" is compiled. The problem is that msg
is being defined twice in the same program, violating the ODR.

You should probably decalre it extern and define it in "global.cpp ",
or use an inline function instead

Maybe it works differently with gcc, or maybe I've just never
adequately understood the problem. Declaring my globals extern in the
header and defining them in a seperate .cpp file works beautifully.

I'm over that hurdle. Thanks for your help!

ff

[Jonathan Turkanis]

"Fritz Foetzl" <fr**********@h otmail.com> wrote in message
news:d2******** *************** ***@posting.goo gle.com...

"Jonathan Turkanis" <te******@kanga roologic.com> wrote in message news:<bu******* *****@ID-216073.news.uni-berlin.de>...

I think your problem has nothing to do with include guards. The header "globals.h" is properly included twice, once when each of the files "driver.cpp " and "functions. cpp" is compiled. The problem is that msg is being defined twice in the same program, violating the ODR.

You should probably decalre it extern and define it in "global.cpp ", or use an inline function instead
Maybe it works differently with gcc, or maybe I've just never
adequately understood the problem. Declaring my globals extern in

the header and defining them in a seperate .cpp file works beautifully.

I'm over that hurdle. Thanks for your help!

ff

Glad to help. Now if I could just erase my others posts in this
thread, in which I misread the standard half a dozen times, I'd be
happy forever!

Jonathan