memory allocation for a struct

[c94wjpn]

If I define

struct A
{
     float x,y,z;
     A(xx,yy,zz) { x=xx;y=yy;z=zz;};
     virtual ~A(void) {};
}

then

   A a(0.0,1.0,2.0);
   float* p2;

   p2 = reinterpret_cast<float*>(&a);

   float f;
   f = p2 [2];

   assert(f==a.z);

Can I be certain that the member variables of A will appear in memory in the right order like an array, in spite of the fact that A has a virtual function table? Is this part of the C++ spec? Is it true of all platforms and implementations?