FIX: extern Declaration Generates Extra Constructor Call

View products that this article applies to.

This article was previously published under Q167359

SYMPTOMS

An extern declaration incorrectly causes an extra constructor call when the extern declaration and the global declaration appear in the same translation unit.

NOTE: Here, translation unit means the source file and any included files considered as though they were all one file. Please see the example code in the "More Information" section.

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RESOLUTION

There are many potential workarounds. The basic condition is that the extern declaration cannot appear after the global variable declaration in the same translation unit. One of the easiest ways to assure this condition is to place the offending global variable declaration(s) in a source file of their own that is included in the project, but not included in any other source files (please see the example code in the "More Information" section).

A supported fix for Visual C++ 6.0 that corrects this problem is now available from Microsoft, but it has not been fully regression tested and should be applied only to systems experiencing this specific problem.

To resolve this problem immediately, contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information on support costs, please go to the following address on the World Wide Web:

http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS (http://support.microsoft.com/default.aspx?scid=fh;en-us;cntactms)

The English version of this fix should have the following file attributes or later:

Collapse this tableExpand this table

Name	Size	Date	Time	Version	Platform
c1xx.dll	1,196,083	2/8/2000	5:39 AM	12.00.8769.0	x86

NOTE: If this product was already installed on your computer when you purchased it from the Original Equipment Manufacturer (OEM) and you need this fix, please call the Pay Per Incident number listed on the preceding Web site. If you contact Microsoft to obtain this fix, a fee may be charged. This fee is refundable if it is determined that you only require the fix you requested. However, this fee is non-refundable if you request additional technical support, if your no-charge technical support period has expired, or if you are not eligible for standard no-charge technical support.

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STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article. This bug was corrected in the latest service pack for Visual Studio 6.0.

For additional information about Visual Studio service packs, click the following article numbers to view the articles in the Microsoft Knowledge Base:

194022 (http://support.microsoft.com/kb/194022/EN-US/ ) INFO: Visual Studio 6.0 Service Packs, What, Where, Why

194295 (http://support.microsoft.com/kb/194295/EN-US/ ) HOWTO: Tell That a Visual Studio Service Pack Is Installed

To download the latest Visual Studio service pack, visit the following Microsoft Web site:

http://msdn2.microsoft.com/en-us/vstudio/Aa718353.aspx (http://msdn2.microsoft.com/en-us/vstudio/Aa718353.aspx)

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MORE INFORMATION

Sample Code

/* Compile Options Needed: /GX */ 
// File MyClass.h

#include <iostream>

#pragma once

struct MyClass {
   MyClass()
            {std::cout << "MyClass ctor Exectuing" << std::endl;}
};

MyClass X;
// End File MyClass.h

// File main.cpp
#include "myclass.h"

extern MyClass X;

int main()
{
   return 0;
}
// End File main.cpp

When you build and run the preceding code, you will see that MyClass::MyClass() is called twice. One of the calls is generated by the global variable declaration in the file MyClass.h and the other by the extern declaration in the file main.cpp. One possible workaround is to remove the declaration of the global variable X from MyClass.h and create a third file, globals.cpp that contains the declaration of the global variable X. By doing this, the declaration of the global variable resides in a different translation unit than the extern declaration(s).

/* Compile Options Needed: /GX */ 
// File MyClass.h

#include <iostream>

#pragma once

struct MyClass {
   MyClass()
            {std::cout << "MyClass ctor Exectuing" << std::endl;}
};

// End File MyClass.h

// File Globals.cpp
#include "MyClass.h"

MyClass X;
// End File Globals.cpp

// File main.cpp
#include "myclass.h"

extern MyClass X;

int main()
{
   return 0;
}
// End File main.cpp

When you build and run the program, it now shows that MyClass::MyClass() is only called once.

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