Question

I have to implement a factory method pattern in C++. The class (C) that is to be “assembled” by the factory is abstract and is inherited by some other class (D). So I don’t want C to have a constructor. But to create an instance of D, C has to have at least a protected constructor. But then someone might derive from C and create an instance of C in this way. I don’t want this to happen. So I created a protected constructor that takes a pointer to a C object for derived classes to use, code is below. The question is whether that is the correct way of dealing with this issue.

class C {
private:
 C() {}
protected:
 C(const C* c) {}
friend class CFactory
};

class D: public C
{
private:
 D(const C* c): C(c) {}
friend class CFactory;
};

class CFactory
{
public:
 static C* createC() {
  C* ptr = new C();
  ptr = new D(ptr); // There is garbage collection on the project, so no memory leak.
 }
};

Answer 1

It’s probably a better option to give C a private constructor and make D a friend class of C.