Q152211: FIX: Constructor Gets Called Multiple Times on the Same Object

Article: Q152211
Product(s): Microsoft C Compiler
Version(s): 4.1,4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbCompiler kbCPPonly kbVC kbVC410bug kbVC500bug kbVC600bug kbNoUpdate
Last Modified: 11-FEB-2002

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), used with:
   - Microsoft Visual C++, 32-bit Editions, version 4.1 
   - Microsoft Visual C++, 32-bit Enterprise Edition, versions 4.2, 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, versions 4.2, 5.0, 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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SYMPTOMS
========

The constructor gets called more than once on the same global object.

CAUSE
=====

This error occurs when a file has both a global object and a function that
references the global object as extern. In this case, the compiler incorrectly
generates code that calls the constructor on the "extern myClass object;"
statement.

RESOLUTION
==========

Remove the line that contains extern reference to the global. You can safely
remove this line since the global is in file scope.

-or-

Create two separate files, one that contains the global object, and one that
contains the function that references the global object as extern.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article.

This problem was corrected in Microsoft Visual C++ .NET.

MORE INFORMATION
================

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

  // This sample demonstrates the problem of the constructor getting
  // called twice.
  /* Compile options needed: cl test.cpp
  */ 
  // TEST.CPP
  // ========
  #include "stdio.h"
  class myClass {
  public:
    myClass() {printf("myClass Constructor called\n");};
    void test() {printf("test() called\n");};
  } object;

  void main()
  {
  // Remove the next line for the first workaround:
    extern myClass object;
    object.test();
  }

  // sample output: (without extern line removed)
  myClass Constructor called
  myClass Constructor called
  test() called

  // This sample demonstrates the workaround that causes the program
  // to only call the constructor once.
  /* Compile options needed: cl Test2A.cpp Test2B.cpp
  */ 
  // Test2.h
  // ========
  #include "stdio.h"
  class myClass {
  public:
    myClass() {printf("myClass Constructor called\n");};
    void test() {printf("test() called\n");};
  };
  // Test2A.cpp
  // ==========
  #include "Test2.h"
  myClass object;

  // Test2B.cpp
  // ==========
  #include "Test2.h"
  void main()
  {
    extern myClass object;
    object.test();
  }

  // sample output:
  myClass Constructor called
  test() called

Additional query words: 10.10 10.20

======================================================================
Keywords          : kbCompiler kbCPPonly kbVC kbVC410bug kbVC500bug kbVC600bug kbNoUpdate 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :4.1,4.2,5.0,6.0
Issue type        : kbbug
Solution Type     : kbfix

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

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