• if something varies, it changes in nuances
• co-prefix expresses relation: when two things vary, they vary the same way

regarding A..

• B' is covariant from B
• C' is contravariant from C

A = factory     B = product

A' = special factory     B' = special product

class IHardware { };
class MaschineController : public IHardware { };
class TraktorKontrol : public IHardware { };
  
class IFactory {
public:
  virtual IHardware *createHardware() const = 0;
};

class MaschineFactory : public IFactory {
public:
  MaschineController *createHardware() const override;
};

class TraktorFactory : public IFactory {
public:
  TraktorKontrol *createHardware() const override;
};

§ 10.3.7 Virtual functions The return type of an overriding function shall be either identical to the return type of the overridden function or covariant with the classes of the functions. If a function D::f overrides a function B::f, the return types of the functions are covariant if they satisfy the following criteria:

• both are pointers to classes, both are lvalue references to classes, or both are rvalue references to classes
• the class in the return type of B::f is the same class as the class in the return type of D::f, or is an unambiguous and accessible direct or indirect base class of the class in the return type of D::f
• both pointers or references have the same cv-qualification and the class type in the return type of D::f has the same cv-qualification as or less cv-qualification than the class type in the return type of B::f

class IFactory {
public:
  virtual const IHardware *createHardware() const = 0;
};

class MaschineFactory : public IFactory {
public:
  MaschineController *createHardware() const override;
};

class TraktorFactory : public IFactory {
public:
  TraktorKontrol *createHardware() const override;
};

class IHardware {
public:
  virtual const char *getBrand() const = 0;
};

class MaschineController : public IHardware { 
public:
  char *getBrand() const override;
};

class TraktorKontrol : public IHardware { 
public:
  char *getBrand() const override;
};