Q130486: PRB: LNK2001 on Pure Virtual Operator=

Article: Q130486
Product(s): Microsoft C Compiler
Version(s): 1.0,2.0,2.1,4.0,5.0
Operating System(s): 
Keyword(s): kbCompiler kbCPPonly kbVC100 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 13-JUN-2002

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
- Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
- Microsoft Visual C++.NET (2002) 
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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          : kbCompiler kbCPPonly kbVC100 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbvc100 kbVC500 kbVC200 kbVC210 kbVC32bitSearch kbVCNET kbVC500Search
Version           : :1.0,2.0,2.1,4.0,5.0
Issue type        : kbprb

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