FIX: /Og Causes vfptr Initialization Problems

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

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:

   ARTICLE-ID: Q170365
   TITLE     : 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;
   }