Q117794: PRB: Compiler Errors when Implementing CObject-Derived Class

Article: Q117794
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbnokeyword kbLangCPP kbMFC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbG
Last Modified: 07-MAY-2001

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

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft C/C++ for MS-DOS, version 7.0 
   - Microsoft Visual C++, versions 1.0, 1.5, 1.51, 1.52 
   - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.10, 4.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

When you implement a class derived from CObject and your code is written so that
the copy constructor or assignment operator for the class needs to be called,
the compiler may report errors similar to the following:

  error C2660: 'CSample::CSample' : function does not take 1 parameters
  error C2582: 'CSample' : 'operator =' function is unavailable

You can reproduce the problem by compiling the sample code in the "RESOLUTION"
section, below.

NOTE: Using Visual C++, 32-bit Edition, versions 2.x and 4.0, the sample code
shown in this article generates the following error messages:

  error C2558: 'CSample::CSample' : no copy constructor available
  error C2582: 'CSample' : 'operator =' function is unavailable

CAUSE
=====

The reason for the compiler errors is that CObject declares a private copy
constructor and assignment operator in the AFX.H file. The compiler does not
generate a default copy constructor and assignment operator for the
CObject-derived class because of this. Because the compiler does not find these
functions declared in the class, it reports the errors.

RESOLUTION
==========

To avoid the compiler errors, you need to implement a copy constructor and
assignment operator for the CObject-derived class. This is illustrated in the
sample code below. You can avoid the errors by uncommenting the lines indicated
in the sample code.

Sample Code
-----------

  /* Compile options needed: /c
  */ 

     // replace with #define _CONSOLE when compiling for Windows NT
     #define _DOS

     #include <afx.h>

     class CSample : public CObject
     {
     private:
         short m_nValue;
     public:
     // uncomment the lines below to avoid the compiler errors
     //    CSample() {}
     //    CSample( const CSample &s )  // copy ctor
     //        { m_nValue = s.m_nValue; }
     //    CSample& operator=( const CSample &s )  // assignment operator
     //        { m_nValue = s.m_nValue; return *this; }
     };

     void main()
     {
         CSample a;
         CSample b = a;
         a = b;
     }

REFERENCES
==========

"Visual C++ and C/C++, version 7.0 Language Reference" manuals, Section 11.7,
"Copying Class Objects."

Visual C++ 4.0 Books Online: Visual C++ Books, C/C++, C++ Language Reference,
Special Memeber Functions, Copying Class Objects.

Additional query words: 7.00 1.00 1.50 2.00 2.10 2.50 1.51 1.52 2.51 3.00 3.10 4.00 arguments kbNoUpdate

======================================================================
Keywords          : kbnokeyword kbLangCPP kbMFC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:
Issue type        : kbprb

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

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.