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Hello, this is a very advanced course.

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We'll focus on
how Pharo organizes classes

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and instance relationships
between classes.

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It's really aimed at enthusiasts.

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It's not essential to master,
since you already use Pharo without it.

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You can use Pharo for programming
without knowing how it works.

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But you may be unsatisfied if the course
didn't explain how it functions.

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Like explaining where "new" is defined,

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or what the class of a metaclass is.

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This video will explain all this stuff.

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It doesn't matter
if you don't understand straightaway.

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But if it really interests you,

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you can watch it several times.

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There is one fundamental key here.

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It's a fun aspect,
which this transparency should explain.

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The basic idea,

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which we keep reiterating in this MOOC,

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is that, as you should know,
when we send a message to an object,

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we search in the object's class
via the instantiation link.

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If it isn't there,
we follow the inheritance chain.

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We always start in the class,
then follow the inheritance chain.

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This rule must be followed
systematically.

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To explain metaclasses,

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we summarize them in seven points,
which are relatively simple.

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Every object is an instance of a class.

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That's difficult!
Look at this example.

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The object OrderedCollection
is an instance of this class.

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So far, it's clear.

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The second point is that every class
inherits from the Object class.

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As we see here,

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OrderedCollection and upwards
inherit ultimately from Object.

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Just a note for French speakers,

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"eventually" means "ultimately."

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It's confusing.

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So,

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we have our inheritance graph
that inevitably leads to Object.

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What is the responsibility of Object?
It represents object behavior

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common to all objects,
such as error handling,

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inserting a halt, and announcements.

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Object is the root
of the inheritance tree.

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Its key responsibility
is minimum behavior.

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Returning to our list, every class
is an instance of a metaclass.

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We explained this in an earlier course.

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A class X is the sole instance
of the metaclass named X class.

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We see that the OrderedCollection class

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is an instance of the class
OrderedCollection class.

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It works the same way
for the SequenceableCollection class,

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and the Object class
with the class Object class.

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All classes are instances
of another metaclass,

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which takes the same name
followed by "class."

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Metaclasses are created

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automatically when we create a class.

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We send a "new" message
to the metaclass to create the class.

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Next point.

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Metaclass hierarchy
parallels class hierarchy.

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Earlier, we explained that

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OrderedCollection
is an instance of its class,

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and likewise for SequenceableCollection.

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Now we need to specify
the relationship between the two.

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Since there's an inheritance
relationship here, there's also one here.

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In all cases.

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That's why we say
they are two parallel hierarchies.

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When do we use this hierarchy?

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If I send the "new" message
to OrderedCollection,

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where do I search?
In the class.

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I query in each class
whether or not "new" has been defined.

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Moving upwards, has it defined "new"?
Yes or no.

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Here we utilize the key
I mentioned at the beginning.

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To send a message, start with the class
then follow the inheritance chain.

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Up to this point,

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it may seem clear,
but now other questions come up.

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What is Object class an instance of?

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You might wonder.

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And what is the superclass
of Object class?

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Does it work
if we send a message to a metaclass?

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Or if we send a message
to whatever it is here?

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Let's have a look.

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The system says that every metaclass
inherits from Class,

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right up to Behavior.

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Let's look at that,
and answer the first question.

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Object class is a class,
hence it inherits from Class.

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Class inherits
from ClassDescription and Behavior.

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Unlike in Pharo,
in other systems such as Lisp

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these two elements are a single class.

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Pharo divides them for reuse.

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Metaclass can be attached
to ClassDescription

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to utilize ClassDescription
in two cases.

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We see that Behavior,
which represents the essence of a class,

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inherits from Object.

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So, if I send a message here,

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it will travel all the way up to here.

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It follows the inheritance tree
from the Class level.

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Now we want to know
where "new" is defined.

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The method for creating instances
is defined in Behavior.

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When I send the "new" message
to OrderedCollection, what happens?

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First, I search in the class.

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Suppose that "new" is not redefined
in the inheritance,

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so I search all of the superclasses

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until we find "new."

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In a lookup, we search for a method
and execute it on the receiver.

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So, I find the "new" method
and execute it on the receiver,

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the class OrderedCollection,
which creates a new instance

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that we'll name 3 4, for example.

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Okay?

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We're using the key I spoke about.

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We search the inheritance chain,
starting with the receiver's class.

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In a nutshell,
what does Behavior represent?

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It's the essence of an object
that can have instances.

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Objects with instances

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must include a superclass link,

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method dictionary,
and description of instances (format).

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The methods shown here include
examples such as new, basicNew and new:.

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The difference between new and basicNew

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is that you must never
redefine basicNew.

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As we explained
in the course on methods,

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if you redefine a method starting
with "basic," the method is inaccessible.

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The original method.

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There are other ways to access
the compiled methods, as shown here.

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Behavior is the essence of a class.

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What is ClassDescription?

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It's an abstract superclass
shared by Class and Metaclass.

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It adds facilities to Behavior,

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such as naming instance variables,
since class execution can be automatic.

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ClassDescription adds such things.

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Category organization concerns methods
in your browser showing the protocols.

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For example, a method can be stored
in the printing protocol.

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Also, the notion of a name.

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Plus the maintenance of Change sets
and the saving of changes in files.

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You can look at that yourself.

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This categorization exists because
use is shared by Class and Metaclass.

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The responsibility of Class

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is to represent the classes
we employ in Pharo.

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This includes representation
for classVariable names

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and a better way of naming
and compiling elements.

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Once again, refer to the code.

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Classes are instances of metaclasses.

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If Object is an instance
of a class named Object class,

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and likewise for OrderedCollection,

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the same applies to Class,
ClassDescription, and Behavior.

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So, Class is an instance of Class class

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and ClassDescription is an instance
of ClassDescription class.

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The same applies to Behavior.

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We must also look at
the inheritance chain.

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If there's an inheritance relationship
between these two elements,

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the same relationship exists
here on the right.

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As you see here,
we can trace the inheritance chain.

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Behavior class therefore inherits
from Object class.

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Now for the second last point.

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Since every class is an instance
of a metaclass, a question arises.

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What is OrderedCollection class
an instance of?

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Since everything is an object in Pharo.

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OrderedCollection class
is therefore an instance of Metaclass.

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Object class and Class class
are instances of Metaclass.

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All the metaclasses in the system
are instances of Metaclass.

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Furthermore, Metaclass inherits
from ClassDescription.

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It's an unusual class
because it only has one instance.

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And you can't name it because its name
is defined by its instance.

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That may seem barbaric.

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So, we see that OrderedCollection

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is an instance of OrderedCollection
class, which takes its name.

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All metaclasses
are instances of Metaclass.

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In the code,
the main responsibility of Metaclass

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is to create and initialize
a single instance of itself.

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There's only one question
left to answer.

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What is Metaclass an instance of?

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Because Metaclass is a class,
like OrderedCollection,

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it is an instance
of the class Metaclass class.

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You'll see that Metaclass
inherits from ClassDescription,

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therefore Metaclass class
inherits from ClassDescription class.

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OrderedCollection class
inherits from Object class

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as OrderedCollection
inherits from Object.

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So, Metaclass is an instance
of Metaclass class.

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OrderedCollection is an instance
of OrderedCollection class.

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And Metaclass class
is an instance of Metaclass,

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just as Class class and Object class
are instances of Metaclass.

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This loop may seem bizarre,
but it makes sense in context.

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You don't need this to use Pharo,
so feel free to overlook it.

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But as this graph shows,

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the key we spoke about earlier
works perfectly.

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Every time we send a message
to an object, we look at its class

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and its inheritance tree,
as shown in these examples.

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If I send a message to this instance,
it leads me to Object.

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If I send a message to the class,

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I go to the metaclass
and follow the inheritance.

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If I send a message to a metaclass,
where should I look?

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In Metaclass, since the metaclass
is an instance of Metaclass.

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I follow the instantiation link
to the end.

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This is when a message is not understood
or a method is only defined in Object.

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Now, what if I send a message
to Metaclass itself?

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Where do I search?

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First in Metaclass class,
then I follow the inheritance chain.

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So... oops!

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It gets more complicated here.

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Similarly, if I send a message
to Metaclass class,

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or to OrderedCollection class,

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I look in the instance

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of Metaclass class, which is Metaclass.

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Like for OrderedCollection class,
I follow this path.

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You will see
that this graph is entirely logical.

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We cannot have an inconsistent graph

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since the virtual machine
only does one thing.

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It looks for messages in the class
and follows the inheritance chain.

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This comprehensive graph
is consistent with that process.

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I find this exciting because
I wondered about it myself.

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So, to sum up, classes are objects
and can receive messages.

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The procedure is the same
for all objects,

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even if there are metaclasses
with a sole instance,

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which are not explicitly named.

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But it's not essential to know that.