[c++] error LNK2005, already defined?

The linker tells you that you have the variable k defined multiple times. Indeed, you have a definition in A.cpp and another in B.cpp. Both compilation units produce a corresponding object file that the linker uses to create your program. The problem is that in your case the linker does not know whic definition of k to use. In C++ you can have only one defintion of the same construct (variable, type, function).

To fix it, you will have to decide what your goal is

• If you want to have two variables, both named k, you can use an anonymous namespace in both .cpp files, then refer to k as you are doing now:

.

namespace {
  int k;
}

• You can rename one of the ks to something else, thus avoiding the duplicate defintion.
• If you want to have only once definition of k and use that in both .cpp files, you need to declare in one as extern int k;, and leave it as it is in the other. This will tell the linker to use the one definition (the unchanged version) in both cases -- extern implies that the variable is defined in another compilation unit.