Use forward declarations in C++ in all the following cases:

Master.h

class Master {
private:
    // Declare a member to be a pointer or a reference to the incomplete type
    Forward *ptr1;     
    Forward &ptr2;

public:

    // Declare functions or methods which accepts/return incomplete types:
    void ByValue(Forward by_value);
    Forward OrReturnValue();

    // Define functions or methods which accepts/return pointers/references 
    // to the incomplete type (but without using its members):
    void OrByPointer(Forward* by_pointer);
    void OrByReference(const Forward& by_reference);
    Forward& ReturnByRef();
    Forward* ReturnByPointer();

};