PRB: C2250 Error Generated with Ambiguous Multiple Inheritance

• The Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft C/C++ for MS-DOS, version 7.0 - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52 - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SYMPTOMS

With the Microsoft C/C++ compiler, the error

   Error C2250: 'identifier': Ambiguous Inheritance of 'class::member'

CAUSE

This is expected C++ compiler behavior.

In the Sample Code below, the error is generated because D::f is ambiguous. That is, both class B and class C redefine their member f(), inherited from the virtual base class A, and class D inherits from B and C. As class D has only one vtable slot for the virtual function f(), and both B::f and C::f cannot be put there, the compiler generates error C2550.

RESOLUTION

There are two ways to avoid the error:

• Remove or change the name of the member function causing the ambiguity. In the Sample Code below, the error can be fixed by removing (or commenting) the declaration of either B::f(), C::f(), or both.
• Override the virtual function in the most-derived class. In the Sample Code below, this means adding a declaration of "void f();" to class D. Then, if so desired, D::f can be defined to call f() from any of the base classes. For example,

     void D::f()
     {
        B::f();
        C::f();
     }
  
  

MORE INFORMATION

Any expression that refers to a class member must make an unambiguous reference. Unfortunately, multiple inheritance introduces the possibility for names to be inherited along more than one path. The class-member names along these paths are not necessarily unique. These name conflicts are called "ambiguities." When the compiler detects an ambiguity, it generates an error message.

Sample Code

   /* Compiler options needed: /c
   */

   class A
   {
   public:
       int a;
       virtual void e();
       virtual void f();
   };

   class B : public virtual A
   {
   public:
      void f();
      int a;
   };

   class C : public virtual A
   {
   public:
      void f();
   };

   class D : public B, public C
   {
   public:
      void d();
   };  // VC++ 4.0 - 2 C2250 errors
       //   'D' : ambiguous inheritance of 'B::f'
       //   'D' : ambiguous inheritance of 'C::f'

   void D::d()
   {
     a++; // OK: only one function 'a' in class 'D'
     e(); // OK: only one function 'e' in class 'D'
     f(); // error C2250 or error C2385 :
          //   two function 'f's in class 'D'
   }