Virtual Base Bug

[root]

/*
 * bug19991104.cc
 *
 * By: Mike Bird mgb@yosemite.net
 *
 * Class heirarchy:              VB    ... a virtual base class
 *                            D1    D2
 *                               X
 *                               Y
 *
 * At least one piece of virtual base class information is being
 * initialised too late so that a virtual call on an object of type Y
 * currently constructed only as far as being an object of type X
 * via a pointer of type D1 is not being adjusted correctly.  I have a lot
 * of similar code which works fine; it seems to be just this one case.
 *
 * Linux 2.2.12 on an i586
 * First found in Redhat's 2.91.66 and reproduced in 2.95.2 built from scratch
 *
 * Compilation command:
 *   g++ -o bug19991104 bug19991104.cc
 *
 * Should produce:      D1-1 D2-2 D1-1 D2-2 D1-1 D2-2
 * Typically produces:  D1-1 D2-2 D1-$ D2- D1-1 D2-2
 *
 * Once you've seen the problem, changing the cerr stuff to just ++ will
 * allow you to remove the include file and make the problem much smaller,
 * which is the reason I'm sending original C++ instead of pre-processed.
 */

#include <iostream>

struct VB {
  virtual void F() = 0;
};

struct D1: virtual public VB {
  char name1;
  D1() : name1('1') {}
  virtual void F() { cerr << " D1-" << name1; }
};

struct D2: virtual public VB {
  char name2;
  D2() : name2('2') {}
  virtual void F() { cerr << " D2-" << name2; }
};

void G(D1* g)
{
  g->F();
}

struct X: public D1, public D2 {
  X() {
    F();      // (1) This works
    G(this);  // (2) This produces garbage because something is not ready
  }
  void F() { D1::F(); D2::F(); }
};

struct Y: public X {
  char something_is_needed_here_else_bug_wont_show;
  Y() : X() {}
};

int main() {
  Y y;
  G(&y);  // (3) This works because virtuals are ready, a little late
}