Microsoft KB Archive/246550

= BUG: Local Static Variables Not Reinitialized After an Exception =

Article ID: 246550

Article Last Modified on 7/5/2005

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APPLIES TO


 * Microsoft Visual C++ 6.0 Enterprise Edition
 * Microsoft Visual C++ 6.0 Professional Edition
 * Microsoft Visual C++ 6.0 Standard Edition

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This article was previously published under Q246550



SYMPTOMS
After the constructor for a local static variable throws an exception, the constructor is not called the next time there is a call to the function in which the variable is declared.



CAUSE
The Visual C++ Compiler has not correctly implemented the behavior dictated by the ANSI C++ specification. The ANSI specification, section 6.7 states in part:

"The zero-initialization [construction] of all local objects with static storage duration is performed before any other initialization takes place. ... such an object is initialized the first time control passes through its declaration; such an object is considered initialized upon the completion of its initialization [upon completion of its constructor] . If the initializations exits by throwing an exception, the initialization is not complete, so it will be tried again [constructor called again] the next time control enters the declaration. ..."



RESOLUTION
There are several potential ways to workaround the behavior, including removing the static keyword from the variable declaration, having the class constructor catch and handle the exception, not using exceptions, using a separate initialization function to emulate proper behavior, and so forth. The actual approach you choose will be greatly dependent upon your individual project. See the "More Information" section for an example of the separate initialization approach.



STATUS
Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.



Steps to Reproduce Behavior
The following code sample illustrates the problem. Define the "workaround to see the separate initialization function approach.

// compile options: /GX
 * 1) include

class MyClass { public: MyClass { throw(int); } MyClass(const MyClass& cClass) {} MyClass& operator=(const MyClass &co) {return *this;} virtual ~MyClass {} };

class MyClassWrapper : public MyClass { public: // Here, m_InitFlag is set to false in the initialize list // (Before MyClass is called), and then set to true // in the body of the constructor (after MyClass is called) MyClassWrapper : m_InitFlag(false) {m_InitFlag = true;} void init {         if(!m_InitFlag) {              MyClass tmp; *((MyClass *)this) = tmp; m_InitFlag = true; }    } private: bool m_InitFlag; };
 * 1) ifdef WORKAROUND
 * 1) endif //WORKAROUND

typedef MyClassWrapper StaticClass; typedef MyClass StaticClass;
 * 1) ifdef WORKAROUND
 * 1) else
 * 1) endif //WORKAROUND

void MyFunction {    static StaticClass MyObject; MyObject.init; }
 * 1) ifdef WORKAROUND
 * 1) endif //WORKAROUND

int main {    using std::cout;

for(int i = 0; i < 2; i++) {         try {              MyFunction; cout << "No Exceptions Thrown / All Exceptions Handled\n"; }         catch(int) {              cout << "An int Exception Was Thrown and Not Handled\n"; }    }

return 0; } // end program Output: An int Exception Was Thrown and Not Handled No Exceptions Thrown / All Exceptions Handled Output if WORKAROUND defined: An int Exception Was Thrown and Not Handled An int Exception Was Thrown and Not Handled

Additional query words: Exception Constructor ANSI

Keywords: kbbug kbcpponly kbpending KB246550

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