I've heard these terms used, but I don't now what they mean: contravariant
and covariant type.  What do they mean?

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Ilya Vinarsky When smashing monuments, save the pedestals -
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In article <3ip2e1$c65@agate.berkeley.edu>,
Ilya Vinarsky <ivin@corvus.EECS.Berkeley.EDU> wrote:
>I've heard these terms used, but I don't now what they mean: contravariant
>and covariant type.  What do they mean?

These terms come from typing theory and apply equally well to
inheritance:

Covariant means restriction. Generally covariance must be obeyed in
the attributes when inheriting. That means the set of values an
attribute of a Derived class may have is equal or less to it's base
class. this is the same of saying the set of instances of the derived
class is a subset of the set of all instances of the Base Class.

Contravariance is the opposite: as you descend in your inheritance
tree the set of values may be expanded.

Oversimplified: in OOP covariance must apply to the return types of
methods. Contravariance applies to the parameters passed to the
methods. This means you may substitute a method of a derived class for
a method of a base class (and thereby substitute an instance of a
derived class for one of a base class). This enables polymorphism is
one of the key elements to Object Oriented programming.

Think your question is more appropriate for comp.object so I have
redirected follow-ups to that group.

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