error LNK2005, already defined?

Solution 1:

Why this error?

You broke the one definition rule and hence the linking error.

Suggested Solutions:

If you need the same named variable in the two cpp files then You need to use Nameless namespace(Anonymous Namespace) to avoid the error.

namespace 
{
    int k;
}

If you need to share the same variable across multiple files then you need to use extern.

A.h

extern int k;

A.cpp

#include "A.h"
int k = 0;

B.cpp

#include "A.h"

//Use `k` anywhere in the file 

Solution 2:

In the Project’s Settings, add /FORCE:MULTIPLE to the Linker’s Command Line options.

From MSDN: "Use /FORCE:MULTIPLE to create an output file whether or not LINK finds more than one definition for a symbol."

Solution 3:

If you want both to reference the same variable, one of them should have int k;, and the other should have extern int k;

For this situation, you typically put the definition (int k;) in one .cpp file, and put the declaration (extern int k;) in a header, to be included wherever you need access to that variable.

If you want each k to be a separate variable that just happen to have the same name, you can either mark them as static, like: static int k; (in all files, or at least all but one file). Alternatively, you can us an anonymous namespace:

namespace { 
   int k;
};

Again, in all but at most one of the files.

In C, the compiler generally isn't quite so picky about this. Specifically, C has a concept of a "tentative definition", so if you have something like int k; twice (in either the same or separate source files) each will be treated as a tentative definition, and there won't be a conflict between them. This can be a bit confusing, however, because you still can't have two definitions that both include initializers--a definition with an initializer is always a full definition, not a tentative definition. In other words, int k = 1; appearing twice would be an error, but int k; in one place and int k = 1; in another would not. In this case, the int k; would be treated as a tentative definition and the int k = 1; as a definition (and both refer to the same variable).

Solution 4:

Assuming you want 'k' to be a different value in different .cpp files (hence declaring it twice), try changing both files to

namespace {
    int k;
}

This guarantees that the name 'k' uniquely identifies 'k' across translation units. The old version static int k; is deprecated.

If you want them to point to the same value, change one to extern int k;.