My understanding is that, generally (because (again in my understanding) this is all compiler-specific, not mandated by the standard) is that in C++, there's a single pointer that points to the combo of the vtable and then the data, after it. See http://www.drdobbs.com/cpp/storage-layout-of-polymorphic-obj... for example. While a Shape is a position, outline, and fill, if you have virtual functions, the pointer points to a vtable pointer, and then all of the data.

In other words, in Rust, a pointer to a shape is

  (pointer to vtable, pointer to data)

whereas, in C++, a pointer to a shape is

   (pointer to shape)

where shape is

   (pointer to vtable, data)

If that's incorrect, I'm quite happy to be corrected! This isn't an area I'm an expert in.