BUG: extern Declaration Generates Extra Constructor Call

• Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2, 5.0

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 below.

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 below).

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. We are researching this bug and will post new information here in the Microsoft Knowledge Base as it becomes available.

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

    /* 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