error LNK2005, already defined?

by

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

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