FIX: Constructor Gets Called Multiple Times on the Same Object

View products that this article applies to.

This article was previously published under Q152211

SYMPTOMS

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

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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.

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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.

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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.

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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

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