Microsoft KB Archive/151500

= FIX: /Og Causes vfptr Initialization Problems =

Article ID: 151500

Article Last Modified on 12/10/2003

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APPLIES TO


 * Microsoft Visual C++ 4.2 Enterprise Edition
 * Microsoft Visual C++ 5.0 Enterprise Edition
 * Microsoft Visual C++ 4.2 Professional Edition
 * Microsoft Visual C++ 5.0 Professional Edition

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This article was previously published under Q151500



SYMPTOMS
Under some circumstances, virtually calling a class member function from within the class's constructor may cause the wrong version of the member function to be called when global optimizations are enabled (/Og or #pragma optimize("g",on) ). Virtually calling a class member function means assigning the value of this to a pointer variable and attempting to call virtual member functions via that pointer. Please see the sample code in the More Information Section below.



CAUSE
The vfptr is not being properly initialized. In the sample code below, p in effect becomes an A* instead of a B*.



RESOLUTION
One workaround is to disable global optimizations for the class constructor. Please note the sections of code that compile conditionally for the constant 'WORKAROUND' in the sample code below.

Another workaround is not to call the member function(s) virtually. That is call them directly via the this pointer, ( this->f ), or to simply call the function using the implied this pointer, ( f ).



STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug has been corrected in Visual Studio 97 Service Pack 1.

For additional information about the Visual Studio 97 Service Pack 1, please see the following article in the Microsoft Knowledge Base:

170365 INFO: Visual Studio 97 Service Packs - What, Where, and Why



MORE INFORMATION
/* Compile Options: /Og // uncomment the following to illustrate the workaround which // disables global optimizations for the constructor //#define WORKAROUND

class A  { public: virtual void f = 0; virtual void g; };

void A::g {  }

class B : public A  { public: B; virtual void f; virtual void g; };

void B::f {  }

void B::g {  }

#ifdef WORKAROUND #pragma optimize("g",off) #endif  // WORKAROUND

B::B {  B* p = this; // If p->g is changed to this->g, or simply g, the // correct member function (B::g) is called. p->g; // Actually calls A::g // if p->f is changed to this->f, or simply f, the // correct member function (B::f) is called. p->f; // Actually calls A::f, which causes a runtime error }

#ifdef WORKAROUND #pragma optimize("g",on) #endif  //WORKAROUND

int main {  B b;   return 0; }

Additional query words: /Og Global Optimization

Keywords: kbbug kbfix kbvs97sp1fix kbcode kbcompiler KB151500

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