Unusual link errors in Borland C++

static data members must be defined in a separate .cpp file, in addition to being declared in a header file. When only the header file is compiled and the linker can't find that symbol in one of the compiled binaries, you get this kind of error.
Make sure that the static data member is defined somewhere in the project. Also ensure that the definition and the declaration match, i.e., the name, the type etc are the same in both of them.