PRB: LNK2001 on Pure Virtual Operator=

ID: Q130486

The information in this article applies to:

The Microsoft C/C++ Compiler (CL.EXE)
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0

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.

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
Version :
Platform : NT WINDOWS
Issue type : kbprb