In C++, the convention is to declare the classes and functions in a header file (.h file) and implement them in the source file (.cpp file).
Your Subtract.h file (declarations) should have only this:
class Subtract {
public:
int minus(int a, int b);
};
Your Subtract.cpp file (implementation) should have this:
#include "Subtract.h"
int Subtract::minus(int a, int b)
{
return a-b;
}
Then you #include "Subtract.h" in your newtestclass.cpp file.