DOCUMENT:Q104186  29-NOV-2001  [visualc]
TITLE   :PRB: C2250 Error Generated with Ambiguous Multiple Inheritance
PRODUCT :Microsoft C Compiler
PROD/VER::1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
OPER/SYS:
KEYWORDS:kbCompiler kbCPPonly kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kb

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-------------------------------------------------------------------------------
The information in this article applies to:

 - Microsoft C/C++ for MS-DOS 
 - 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'

may be generated when a class inherits from two or more classes, at least two of
which derive from the same virtual base class and redefine the same base class
virtual 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 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbZNotKeyword8 kbvc150 kbvc100 kbZNotKeyword3 kbVC500 kbVC600 kbVC151 kbVC200 kbVC210 kbVC32bitSearch kbVC16bitSearch kbVC152 kbVC500Search
Version           : :1.0,1.5,1.51,1.52,2.0,2.1,4.0,5.0,6.0
Issue type        : kbprb

=============================================================================

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