Because the VPTR determines the virtual
function behavior of the object, you can see how it’s critical that the
VPTR always be pointing to the proper VTABLE. You don’t ever want to be
able to make a call to a virtual function before the VPTR is properly
initialized. Of course, the place where initialization can be guaranteed is in
the constructor, but none of the Instrument examples has a
constructor.
This is where creation of the default
constructor is essential. In the Instrument examples, the compiler
creates a default constructor that does nothing except initialize the VPTR. This
constructor, of course, is automatically called for all Instrument
objects before you can do anything with them, so you know that it’s always
safe to call virtual functions.
