<s>
In	O
object-oriented	B-Language
programming	I-Language
,	O
a	O
virtual	B-Application
base	I-Application
class	I-Application
is	O
a	O
nested	O
inner	B-Language
class	I-Language
whose	O
functions	O
and	O
member	O
variables	O
can	O
be	O
overridden	O
and	O
redefined	O
by	O
subclasses	O
of	O
an	O
outer	O
class	O
.	O
</s>
<s>
Virtual	O
classes	O
are	O
analogous	O
to	O
virtual	B-Application
functions	I-Application
.	O
</s>
<s>
The	O
run	O
time	O
type	O
of	O
a	O
virtual	B-Application
class	I-Application
depends	O
on	O
the	O
run	O
time	O
type	O
of	O
an	O
object	O
of	O
the	O
outer	O
class	O
.	O
</s>
<s>
(	O
Just	O
like	O
the	O
run	O
time	O
type	O
of	O
an	O
object	O
decides	O
which	O
virtual	B-Application
function	I-Application
should	O
be	O
used	O
.	O
)	O
</s>
<s>
A	O
run	O
time	O
instance	O
type	O
of	O
the	O
outer	O
class	O
object	O
not	O
only	O
decides	O
on	O
the	O
polymorphic	O
type	O
of	O
its	O
own	O
type	O
object	O
,	O
but	O
also	O
on	O
a	O
whole	O
family	O
tree	O
of	O
virtual	B-Application
class	I-Application
members	O
.	O
</s>
<s>
Virtual	O
classes	O
solve	O
the	O
extensibility	B-Architecture
problem	O
of	O
extending	O
data	O
abstraction	O
with	O
new	O
functions	O
and	O
representations	O
.	O
</s>
<s>
Like	O
virtual	B-Application
functions	I-Application
,	O
virtual	O
classes	O
follow	O
the	O
same	O
rules	O
of	O
definition	O
,	O
overriding	B-Language
,	O
and	O
reference	O
.	O
</s>
<s>
When	O
a	O
derived	O
class	O
inherits	O
from	O
a	O
base	O
class	O
,	O
it	O
must	O
define	O
or	O
override	B-Language
the	O
virtual	O
inner	O
classes	O
it	O
inherited	O
from	O
the	O
base	O
class	O
.	O
</s>
<s>
The	O
type	O
of	O
the	O
outer	O
class	O
determines	O
the	O
run	O
time	O
of	O
the	O
inner	O
virtual	B-Application
class	I-Application
.	O
</s>
<s>
For	O
example	O
,	O
a	O
base	O
class	O
Machine	O
could	O
have	O
a	O
virtual	B-Application
class	I-Application
Parts	O
.	O
</s>
<s>
Subclass	O
Car	O
would	O
implement	O
Parts	O
differently	O
than	O
the	O
subclass	O
Bicycle	O
,	O
but	O
the	O
programmer	O
can	O
call	O
any	O
methods	O
in	O
the	O
virtual	O
inner	B-Language
class	I-Language
Parts	O
on	O
any	O
class	O
Machine	O
object	O
,	O
and	O
get	O
the	O
Parts	O
implementation	O
of	O
that	O
specific	O
derived	O
class	O
.	O
</s>
<s>
Functions	O
like	O
get_fuel_type( )	O
can	O
be	O
added	O
to	O
the	O
virtual	B-Application
class	I-Application
Parts	O
by	O
the	O
derived	O
class	O
Car	O
.	O
</s>
