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The implicit-argument mechanism makes use of what is called the
this
pointer. Every member function has an implied parameter, named
this, whose value is a pointer to the member function's class-object
argument.
When you want to refer to the class argument of a member function, you
could write down this along with a dereferencing asterisk. Thus,
you could define the display_capacity function inside the
box_car class definition as follows:
Argument is the same as the
argument handed to display_capacity
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virtual void display_capacity ( ) {cout << (*this).volume ( );}
Alternatively, you can use the class-pointer operator to go through the pointer:
virtual void display_capacity ( ) {cout << this -> volume ( );}
Fortunately, C++ allows you to leave out the first argument entirely:
virtual void display_capacity ( ) {cout << volume ( );}
Thus, the real result of leaving out an explicit class-object argument is
that the dereferenced this pointer is used to supply a default
class-object argument.