BUG: Constructor Gets Called Multiple Times on the Same Object

• The C/C++ Compiler (CL.EXE), used with:
  • Microsoft Visual C++, 32-bit Editions, versions 4.1, 4.2, 5.0, 6.0

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.

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 
Version           : winnt:4.1,4.2,5.0,6.0
Platform          : winnt 
Issue type        : kbbug 

Last Reviewed: July 27, 1999