Microsoft KB Archive/128805

= FIX: Explicit Call to Virtual Destructor Corrupts Stack =

Article ID: 128805

Article Last Modified on 7/5/2005

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APPLIES TO


 * Microsoft Visual C++ 1.0 Professional Edition
 * Microsoft Visual C++ 2.0 Professional Edition
 * Microsoft Visual C++ 2.1

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This article was previously published under Q128805



SYMPTOMS
Making an explicit call to a destructor that has been declared virtual can corrupt the stack. This problem may show up as:


 * Local variables change unexpectedly. -or-


 * Access violations occur on return from functions.



CAUSE
An explicit call to a virtual destructor generates a call to a helper function called the scalar deleting destructor. In addition to calling the destructor, the scalar deleting destructor also calls operator delete, depending on the value of a flag that is pushed onto the stack before the call.

Because the compiler is not correctly pushing the flags before calling the scalar deleting destructor, four extra bytes are popped off the stack each time an explicit call to a destructor is made. Also, if the last value stored on the stack before the call happens to be 1, operator delete is also called.



RESOLUTION
To work around this problem, use the scope resolution operator to specify which destructor should be called. This will avoid the call through the virtual table, so that the destructor is called without calling the scalar deleting destructor.



STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was fixed in Microsoft Visual C++, 32-bit Edition, version 4.0.



MORE INFORMATION
The following program demonstrates both the problem and the solution. When compiled without the /D "GOOD_CODE" compiler option, the ptr->~MyClass call corrupts the stack, so the loop is only executed only six times, and operator delete is accidentally called.

The following is the output when you compile the sample code without using the /D "GOOD_CODE" compiler option: ~MyClass(2D0340) i=0, after ptr->~MyClass ~MyClass(2D134C) i=1, after ptr->~MyClass ~MyClass(2D1354) i=2, after ptr->~MyClass ~MyClass(2D135C) i=3, after ptr->~MyClass ~MyClass(2D1364) MyClass::operator delete(2D1364) i=4, after ptr->~MyClass ~MyClass(2D136C) MyClass::operator delete(2D136C) i=5, after ptr->~MyClass out of the loop, i=4227241 To see the correct output, compile the code with the /D "GOOD_CODE" compiler option.

Sample Code
/* Compile options needed: none class MyClass { public: void operator delete(void *ptr) {   printf("\nMyClass::operator delete(%lX)", (long)ptr); }  virtual ~MyClass {   printf("\n~MyClass(%lX)", (long)this); } }; void main(void) {  int i;   MyClass *ptr; int j;
 * 1) include 
 * 1) include 

for (i=0; i < 100; i++) {   ptr = new MyClass; j = 1; // Used to put '1' onto the stack

#ifndef GOOD_CODE // This call corrupts the stack, and "accidentally" // calls delete ptr->~MyClass; #else // This call works fine ptr->MyClass::~MyClass; #endif

printf("\ni=%d, after ptr->~MyClass", i); fflush(stdout); }

printf("\nout of the loop, i=%d", i); }

Additional query words: 1.00 2.00 2.10 8.0 8.00 9.0 9.00 9.1 9.10

Keywords: kbbug kbfix kbcpponly kbcompiler KB128805

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