PRB: offsetof() Macro Fails with Virtual Base Class

• Microsoft C/C++ Compiler (CL.EXE) included with: - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 4.0, 4.1, 4.2

SYMPTOMS

Use of the offsetof() macro may cause the compiler to generate incorrect code when used with classes that have virtual base classes. This incorrect code results in a null pointer access, which causes an access violation when the application is executed.

This problem occurs only for members declared in the virtual base class.

Note that according to the ANSI C++ Draft Working Papers, clause 18.1.5, the offsetof macro is restricted to POD union and POD structure types. POD types may not have base classes. Using the macro contrary to this restriction results in undefined behavior.

RESOLUTION

The offsetof() macro returns the offset in bytes of a member from the beginning of a class by casing NULL to a pointer of class/struct type. To work around this problem, cast the address of the object to a char *, cast the address of the member to a char *, and subtract the address of the object from the address of the member. Please see the Sample Code in the MORE INFORMATION section for an example.

While this workaround generates code that is as efficient as the offsetof() macro, it does require that the class/struct be instanced, while offsetof() does not.

STATUS

This is by design.

MORE INFORMATION

The following code demonstrates the problem and the workaround.

Sample Code

   /* Compile options needed: none
   */

   #include <stdio.h>
   #include <iostream.h>
   #include <stddef.h>

   class A { public: int a; };
   class B : virtual public A { public: int v_a; };
   class C : virtual public A { public: int v_b; };
   class D : virtual public B, virtual public C { public: int b; };

   int main()
   {

      D xD;

      xD.b   = 16;
      xD.v_a = 8;
      xD.v_b = 4;
      xD.a   = 2;

      cout << "sizeof(D) == " << sizeof(D)   << endl;
      cout << "sizeof(C) == " << sizeof(C)   << endl;
      cout << "sizeof(B) == " << sizeof(B)   << endl;
      cout << "sizeof(A) == " << sizeof(A)   << endl << endl;

   // The following code section works around the offsetof() problem
      cout << "offsetof(D.b)   == " << (char *)&xD.b - (char *)&xD << endl;
      cout << "offsetof(D.v_a) == " << (char *)&xD.v_a - (char *)&xD <<
      endl;
      cout << "offsetof(D.v_b) == " << (char *)&xD.v_b - (char *)&xD <<
      endl;
      cout << "offsetof(D.a)   == " << (char *)&xD.a - (char *)&xD << endl;
      cout << endl;

   // The following statement is generated correctly
      cout << "offsetof(D.b)   == " << offsetof(D, b)   << endl;

   // The following three statements cause access violations when executed
      cout << "offsetof(D.v_a) == " << offsetof(D, v_a) << endl;
      cout << "offsetof(D.v_b) == " << offsetof(D, v_b) << endl;
      cout << "offsetof(D.a)   == " << offsetof(D, a)   << endl << endl;

      cout << "Press Enter...";
      cin.get();

      return 0;
   }