Multiply defined symbol

[Christopher Thomas McGinlay]

The following generates a multiply defined symbol error (LNK2005) in my
linker (MSVC6), leaving me mystified!

There are 3 files:

header.h
--------
#ifndef INCLUDE_TEST
#define INCLUDE_TEST

namespace A
{
int myinteger; //problem
enum token
{
NAME, NUMBER, END
};
int f();
}

#endif

imp.cpp
-------
#include "header.h"

A::f()
{
return 1;
}

driver.cpp
----------
#include "header.h"

int main()
{
return 1;
}

Apparently A::myinteger is already defined in driver.obj. Any ideas, as
I think I've got the wrong end of this particular stick.

Thanks, Chris

--
Ascent Software Limited Registered in Scotland: SC201671
www.ascent.zetnet.co.uk Bank Of Scotland Buildings
Lerwick, Shetland, ZE1 0EB

[ralph]

Christopher Thomas McGinlay <ascent@zetnet.co.uk> wrote:
>The following generates a multiply defined symbol error (LNK2005) in my

>linker (MSVC6), leaving me mystified!
>
>There are 3 files:
>
>header.h
>--------
>#ifndef INCLUDE_TEST
>#define INCLUDE_TEST
>
>namespace A
>{
> int myinteger; //problem
> enum token
> {
> NAME, NUMBER, END
> };
> int f();
>}
>
>#endif
>
>imp.cpp
>-------
>#include "header.h"
>
>A::f()
>{
> return 1;
>}
>
>driver.cpp
>----------
>#include "header.h"
>
>int main()
>{
> return 1;
>}
>
>Apparently A::myinteger is already defined in driver.obj. Any ideas, as

>I think I've got the wrong end of this particular stick.
>
>Thanks, Chris
>
>--
>Ascent Software Limited Registered in Scotland: SC201671
>www.ascent.zetnet.co.uk Bank Of Scotland Buildings
> Lerwick, Shetland, ZE1 0EB
>

Somebody must have told you that 'namespaces' would resolve your 'extern'
declarations in multiple files. They do not.

They help to resolve ambiguities with-in a translation unit. Princple reason
for the existence of namespaces is to allow you to create multiple libraries
and worry less about name-collision. Not to KEEP it from happening but to
provide a way to resolve it when it does.

However, when the smoke clears and it is time to put all the units together
- you can have only "one" globally defined object/type/etc. with the same
name. Think of it as having to resolve to "one and only one" address where
the thing is kept in the final compiled code.

In the example above each .cpp file (translation unit) has the following:

// imp.cpp
int myinteger; // courtesy of header.h
// declared and defined global - there be storage
...

// driver.cpp
int myinteger; // courtesy of header.h
// declared and defined global - there be storage
...
// program.exe
// Error! Both translation units declare and define an
// address for a global int by the same name.

Use either 'extern', const, #define, or enum, as other respondents have suggested.

[Christopher Thomas McGinlay]

>
> Somebody must have told you that 'namespaces' would resolve your 'extern'
> declarations in multiple files. They do not.

Aahh, well I have to be honest, I made up this misconception all by
myself! Now I do feel 'a bit of a wally' as it were. Never mind.

Thanks though!

Chris




--
Ascent Software Limited Registered in Scotland: SC201671
www.ascent.zetnet.co.uk Bank Of Scotland Buildings
Lerwick, Shetland, ZE1 0EB