Dan P <dan### [at] yahoocom> wrote:
> Let's think of it in more familiar terms because computer-science is
> confusing. When mom squirted you out, you became an object derived from
> attributes of your mom and your dad.

  You clearly need a lot more studying of OOP. You are confusing the
word "inheritance" as used in colloquial language with what it means
in object-oriented programming.

  Base classes are more abstract/generic than derived classes. When you
create a class derived from another class, you are making a specialization
of that more abstract class. The derived class *is* of the same type as
the base class, but more specialized.

  The above paragraph probably doesn't tell much, so let's look at an
example:

  Suppose you have a class called LivingEntity which has certain properties
(such as whether the current instance is alive or dead).

  Now you can derive for example two classes from it: Plant and Animal.
They implement everyting LivingEntity implements (eg. the knowledge of
being alive or dead) plus more. 'Plant' has specific information common
to all plants and 'Animal' common to all animals.
  'Plant' *is a* LivingEntity because it implements everything the latter
does and can be handled as an insteance of the latter. The same goes
for 'Animal'. 'LivingEntity' is more abstract than either one because
it does not specify as much as them.

  Now suppose you inherit the classes 'Mammal' and 'Fish' from 'Animal'.
  'Fish' *is an* 'Animal', and it also *is a* 'LivingEntity' now. It's more
specific than either one because it contains more specialized info common
to all fish (but not common to eg. mammals).

  Now suppose you inherit 'Dog' from 'Mammal'.
  'Dog' is a 'Mammal', an 'Animal' and a 'LivingEntity'. 'Dog' is
more specific than 'Mammal' because it contains specific info common
to all dogs.

  If you have some function which for example handles LivingEntities,
you can give it a 'Dog' because 'Dog' *is a* 'LivingEntity'. You can also
give it a 'Fish'.

  If you have a function which handles Mammals you can also give it a 'Dog'
because 'Dog' *is a* 'Mammal'. However, you can't give it a 'Fish' because
it's not a 'Mammal'.

  'Dog' is a very specialized version of 'LivingEntity' (etc. all the
way down in the inheritance hierarchy).

  Now if you have a "mom" Dog, you can't inherit a "child" Dog from it
because the "child" Dog *is not* a "mom" Dog.
  You can create a new 'Dog' instance using the info in "mom" Dog, but
that's not inheritance.

  Inheritance should always happen like that, from a more abstract
concept to a more specialized concept. You should always be able
to say that the derived class "is a" base class. If you can't say
that, then you have a flaw in your OO design.

  For example, if you want a 'Car' class having a 'Motor', inheriting
'Car' from 'Motor' would be a flawed OO design because 'Car' *is not*
a 'Motor'.
  In this case 'Car' *has a* 'Motor', so compositionality is the tool
to use.

  Of course all this is very basic OO stuff which should be clear to
everyone making object-oriented design and programs.

-- 
#macro M(A,N,D,L)plane{-z,-9pigment{mandel L*9translate N color_map{[0rgb x]
[1rgb 9]}scale<D,D*3D>*1e3}rotate y*A*8}#end M(-3<1.206434.28623>70,7)M(
-1<.7438.1795>1,20)M(1<.77595.13699>30,20)M(3<.75923.07145>80,99)// - Warp -

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