WEBVTT

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Hello, everyone.

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This is the fifth sequence
on inheritance and lookup.

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We'll be covering
messaging

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and lookup algorithms
for metaclasses.

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In Pharo,

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everything is an object,

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and objects
can receive messages.

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So classes are objects, too.

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And since objects
can receive messages,

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classes can receive messages.

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The messaging mechanism
is exactly the same,

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whether the receiver is a class
or other object.

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There is only one way
to look up a method.

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This is the lookup algorithm
we've seen several times

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in earlier sessions.

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Now let's look at the example

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of messaging a class.

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Understand
the basic principle:

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Since classes are objects,

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and each object
is linked to a class,

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that is,
an instance of a class,

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aNode is an instance
of Node class.

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Since Node class
is an object,

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Node class is an instance
of another class.

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It is called Node class.

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So, aNode is an instance
of Node class,

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and Node class is an object
that is an instance of Node class class.

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If that's the way it is,
you may wonder

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Node class is a class; a class
is an object, an instance of a class.

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which class Node class
is an instance of.

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You will see that in the next sequence,
on metaclasses.

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Here are a few examples.

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Here, the object, aWorkstation,
is an instance of Workstation class.

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Workstation is a subclass of Node,
a subclass of Object.

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Here, you must
pay attention to the arrows.

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The arrow with a blank tip

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indicates a subclass relationship.

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Node is a subclass of Object.

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Workstation
is a subclass of Node.

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The arrow with a black point,

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is sometimes represented
as a simple two-line arrow, too.

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It indicates instanciation.

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aWorkstation is an instance
of Workstation.

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Workstation is an instance
of Workstation class.

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Likewise, Object
is an instance of Object class.

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If I message

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the object aWorkstation,

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the lookup algorithm searches
the aWorkstation class.

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Then it goes up
through superclasses

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until it finds
a matching method.

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The mechanism
is exactly the same

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when I message
the Workstation class.

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For example, if I send the message
"new"

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to Workstation,
the lookup algorithm

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searches Workstation class
for a method called "new."

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If it finds it here,
it executes it.

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If it does not find a match,
it moves up to the superclass,

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and then up
to the next superclass.

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And so on.
This hierarchy continues.

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I'll leave you with this slide
summing up messaging

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to standard objects
and classes as standard objects.

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Here's what to remember:

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A class is an object
like any other.

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Messages can be sent to objects,
and therefore to classes.

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The lookup mechanism
is exactly the same in both cases.

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It starts searching
in the class of the receiver,

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and then searches each superclass
in the inheritance chain

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until it finds a matching method.

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You'll learn more about metaclasses
in "Understanding Metaclasses."