Ambiguous base classes (C++ only)

When you derive classes, ambiguities can result if base and derived classes have members with the same names. Access to a base class member is ambiguous if you use a name or qualified name that does not refer to a unique function or object. The declaration of a member with an ambiguous name in a derived class is not an error. The ambiguity is only flagged as an error if you use the ambiguous member name.

For example, suppose that two classes named A and B both have a member named x, and a class named C inherits from both A and B. An attempt to access x from class C would be ambiguous. You can resolve ambiguity by qualifying a member with its class name using the scope resolution (::) operator.

class B1 {
public:
  int i;
  int j;
  void g(int) { }
};

class B2 {
public:
  int j;
  void g() { }
};

class D : public B1, public B2 {
public:
  int i;
};

int main() {
  D dobj;
  D *dptr = &dobj;
  dptr->i = 5;
//  dptr->j = 10;
  dptr->B1::j = 10;
//  dobj.g();
  dobj.B2::g();
}

The statement dptr->j = 10 is ambiguous because the name j appears both in B1 and B2. The statement dobj.g() is ambiguous because the name g appears both in B1 and B2, even though B1::g(int) and B2::g() have different parameters.

The compiler checks for ambiguities at compile time. Because ambiguity checking occurs before access control or type checking, ambiguities may result even if only one of several members with the same name is accessible from the derived class.