Microsoft KB Archive/104186

= PRB: C2250 Error Generated with Ambiguous Multiple Inheritance =

Article ID: 104186

Article Last Modified on 7/5/2005

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APPLIES TO


 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 1.5 Professional Edition
 * Microsoft Visual C++ 1.51
 * Microsoft Visual C++ 1.52 Professional Edition
 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 2.1
 * Microsoft Visual C++ 4.0 Standard Edition
 * Microsoft Visual C++ 5.0 Standard Edition
 * Microsoft Visual C++ 6.0 Service Pack 5

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This article was previously published under Q104186





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: kbprb kbcpponly kbcompiler KB104186

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