Knowledge Base

You receive a link error when you build an application that contains a base class in Visual C++

Article ID: 130486

Article Last Modified on 12/9/2005

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 Enterprise Edition
Microsoft Visual C++ 5.0 Professional Edition
Microsoft Visual C++ .NET 2003 Standard Edition
Microsoft Visual C++ .NET 2002 Standard Edition
Microsoft Visual C++ 2005 Express Edition

This article was previously published under Q130486

SYMPTOMS

When building an application that contains a base class with a pure virtual operator= function and a class derived from the base class with a defined operator= function, a following link error is generated for the base class's operator=:

error LNK2001: unresolved external symbol

This also occurs if the classes reside in a DLL and are declared with the __declspec( dllexport ) storage-class attribute.

In Visual C++ .NET you will receive the following error message:

LNK2019: unresolved external symbol

CAUSE

This behavior is by design. The assignment operator is not inherited. Normal inheritance rules do not apply, and declaring it pure virtual does not have the usual affect.

RESOLUTION

Since operator= is not inherited, any declaration of operator= in the base class is unused and unnecessary. Do not declare the operator= in the base class.

MORE INFORMATION

When built, the following code sample will generate the discussed LNK2001 error. Comment the declaration of the operator=() member in the base class to resolve.

Sample Code

/* Compile options needed: none
*/ 

class A
{
public :
   // To workaround LNK2001, comment the following line.
   virtual const A& operator=( const A& f ) = 0;
};

class B : public A
{
public :
   const A& operator=( const A& g ) {return g;}
};

B aB1, aB2;

void main( void )
{
   aB2 = aB1;
}

Additional query words: 9.00 9.10

Keywords: kberrmsg kbtshoot kbprb kbcpponly kbcompiler KB130486