PRB: C2250 Error Generated with Ambiguous Multiple Inheritance

• The Microsoft C/C++ Compiler (CL.EXE)
  • Microsoft C/C++ for MS-DOS, version 7.0
  • Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
  • Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0

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'
} 

Additional query words: 8.00 8.00c 9.00 9.10

Keywords : kbCompiler kbCPPonly kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Version : 7.0 1.0 1.5 1.51 1.52 2.0 2.1 4.
Platform : MS-DOS NT WINDOWS
Issue type : kbprb