CPP13 — Is public _ptr member mandatory?

Summary: In a number of places in the sample code for the C++ binding, the symbol
_ptr is used as a member variable in _var types. In all of these the
actual member is shown as private, so it is clear that actual
implementations could use something else.

The one exception to this is in section 20.10 on the Mapping for Struct
Types. This discusses (without sample code) the mapping for string and
object reference members of structs.