Pointer conversions

Conversions (either implicit or explicit) from a derived class pointer or reference to a base class pointer or reference must refer unambiguously to the same accessible base class object. (An accessible base class is a publicly derived base class that is neither hidden nor ambiguous in the inheritance hierarchy.) For example:

class W { /* ... */ };
class X : public W { /* ... */ };
class Y : public W { /* ... */ };
class Z : public X, public Y { /* ... */ };
int main ()
{
      Z z;
      X* xptr = &z;       // valid
      Y* yptr = &z;       // valid
      W* wptr = &z;       // error, ambiguous reference to class W
                          // X's W or Y's W ?
}

You can use virtual base classes to avoid ambiguous reference. For example:

class W { /* ... */ };
class X : public virtual W { /* ... */ };
class Y : public virtual W { /* ... */ };
class Z : public X, public Y { /* ... */ };
int main ()
{
      Z z;
      X* xptr = &z;      // valid
      Y* yptr = &z;      // valid
      W* wptr = &z;      // valid, W is virtual therefore only one
                         // W subobject exists
}

A pointer to a member of a base class can be converted to a pointer to a member of a derived class if the following conditions are true:

• The conversion is not ambiguous. The conversion is ambiguous if multiple instances of the base class are in the derived class.
• A pointer to the derived class can be converted to a pointer to the base class. If this is the case, the base class is said to be accessible.
• Member types must match. For example suppose class A is a base class of class B. You cannot convert a pointer to member of A of type int to a pointer to member of type B of type float.
• The base class cannot be virtual.