WEBVTT

00:00:00.440 --> 00:00:05.120 align:middle
In this video, we'll explain a point
rarely covered in Pharo courses.

00:00:05.320 --> 00:00:08.040 align:middle
How to define variable size objects.

00:00:09.360 --> 00:00:13.760 align:middle
You'll notice that we already use
objects of different sizes.

00:00:13.920 --> 00:00:18.040 align:middle
Here I'll create an array
using this expression.

00:00:18.240 --> 00:00:21.680 align:middle
It contains ten elements,
all set at nil.

00:00:21.920 --> 00:00:25.480 align:middle
Using the same expression,
I now create an array

00:00:26.040 --> 00:00:28.640 align:middle
that contains five elements.

00:00:29.160 --> 00:00:33.720 align:middle
We see that the Array class
creates instances of variable sizes.

00:00:34.120 --> 00:00:38.480 align:middle
But until now, we haven't explained
how you can do this.

00:00:39.040 --> 00:00:40.040 align:middle
So,

00:00:40.440 --> 00:00:45.720 align:middle
here we'll show you how to define
classes with variable size instances,

00:00:46.040 --> 00:00:49.080 align:middle
how to instantiate these objects,

00:00:49.240 --> 00:00:52.280 align:middle
and how to access these variable sizes.

00:00:52.800 --> 00:00:54.360 align:middle
Now for the first point.

00:00:55.600 --> 00:00:56.800 align:middle
It's very simple.

00:00:57.600 --> 00:01:02.280 align:middle
The definition uses the message
variableSubclass: instead of subclass:.

00:01:02.600 --> 00:01:05.040 align:middle
We copy the Array class definition.

00:01:05.280 --> 00:01:09.120 align:middle
Note that we use variableSubclass,
instanceVariableNames,

00:01:09.600 --> 00:01:12.240 align:middle
and so on, to create the class.

00:01:12.400 --> 00:01:15.760 align:middle
Whereas usually,
when we define the Pointer class,

00:01:15.920 --> 00:01:18.800 align:middle
Counter class, or Dice class,

00:01:19.280 --> 00:01:21.520 align:middle
we would only use Subclass.

00:01:21.960 --> 00:01:26.800 align:middle
Using this definition, the instances
we create have variable sizes.

00:01:27.200 --> 00:01:31.320 align:middle
Using this example,
if we want to make a "strange" point,

00:01:31.520 --> 00:01:34.640 align:middle
using two instance variables, x and y,

00:01:34.800 --> 00:01:38.600 align:middle
as a point and a variable zone,
we use this definition.

00:01:40.000 --> 00:01:44.640 align:middle
This creates objects that look like this,
with a value for x,

00:01:45.480 --> 00:01:49.600 align:middle
a value for y,
and one variable zone per instance.

00:01:49.760 --> 00:01:52.160 align:middle
We could also have another instance,

00:01:52.320 --> 00:01:56.000 align:middle
which also has x and y,
with 10 here and 20 here,

00:01:56.160 --> 00:01:59.080 align:middle
and, for example, 100 here and 200 here,

00:01:59.760 --> 00:02:03.800 align:middle
with a variable zone
that can contain different objects.

00:02:04.800 --> 00:02:09.600 align:middle
So, the instances of a class
that can have variable size instances

00:02:09.800 --> 00:02:15.400 align:middle
have an indexed zone that is added
after named variables, like x and y.

00:02:15.680 --> 00:02:18.680 align:middle
It's implicit
and there's only one per class,

00:02:18.880 --> 00:02:22.360 align:middle
to avoid implementation problems
when expanding it.

00:02:22.520 --> 00:02:25.520 align:middle
Now we've seen how to create classes

00:02:25.680 --> 00:02:28.240 align:middle
that have different sized instances.

00:02:28.400 --> 00:02:32.120 align:middle
Now we'll go back
to how we can instantiate them

00:02:32.320 --> 00:02:34.960 align:middle
and how to access this variable size.

00:02:36.120 --> 00:02:38.480 align:middle
The way we instantiate them

00:02:38.720 --> 00:02:42.880 align:middle
is to use the message "new:"
noting the use of the colon.

00:02:43.080 --> 00:02:46.520 align:middle
We discussed this
in the course on collections.

00:02:46.760 --> 00:02:49.920 align:middle
Here we want to create
a variable size object.

00:02:50.080 --> 00:02:54.480 align:middle
The variable size of the object
is specified by its "max" value.

00:02:54.680 --> 00:02:58.280 align:middle
So, we create an object,
an array with four elements.

00:02:58.880 --> 00:03:01.320 align:middle
This shows the four elements.

00:03:01.960 --> 00:03:04.400 align:middle
How do we access this variable size?

00:03:04.760 --> 00:03:09.400 align:middle
We use the messages at: and at:put:,
which you saw in the arrays.

00:03:09.680 --> 00:03:13.880 align:middle
This works for all objects
with a variable size.

00:03:14.280 --> 00:03:18.400 align:middle
So, if we write at: 2 here, we add 'lulu'.

00:03:20.480 --> 00:03:25.240 align:middle
Then if we access value 1,
it will logically return nil.

00:03:25.440 --> 00:03:29.680 align:middle
And if we look at value 2,
it will give us 'lulu.'

00:03:30.080 --> 00:03:33.320 align:middle
In fact, a mapping is applied here

00:03:33.520 --> 00:03:38.000 align:middle
between arrays and variable size objects
using at:put: and 'lulu.'

00:03:38.160 --> 00:03:41.440 align:middle
This API protocol functions for arrays

00:03:42.560 --> 00:03:47.320 align:middle
and for all objects
we want to have a variable size.

00:03:49.680 --> 00:03:52.440 align:middle
I haven't explained a great deal here,

00:03:52.600 --> 00:03:56.840 align:middle
only how to define Array class
and extend it to other objects,

00:03:57.000 --> 00:03:58.960 align:middle
since you know the protocol.

00:03:59.840 --> 00:04:03.480 align:middle
So, in Pharo
there are objects, or classes,

00:04:03.760 --> 00:04:05.880 align:middle
that take different forms.

00:04:06.080 --> 00:04:10.320 align:middle
There are classes
that only have named instance variables,

00:04:10.600 --> 00:04:15.160 align:middle
like Counter class with count
or Dice class with the number of sides.

00:04:15.480 --> 00:04:18.880 align:middle
Then we have classes that only have

00:04:19.200 --> 00:04:22.640 align:middle
a variable or indexed zone,
like Array class.

00:04:22.800 --> 00:04:26.240 align:middle
Then we have other classes
that are a mix of both.

00:04:26.960 --> 00:04:32.000 align:middle
However, there are some constraints,
it's more subtle than it seems.

00:04:32.160 --> 00:04:35.680 align:middle
We can now decide
on the type of class variable.

00:04:35.840 --> 00:04:38.280 align:middle
I'll just show you the intention.

00:04:38.440 --> 00:04:42.400 align:middle
This variable aspect we're talking about

00:04:43.240 --> 00:04:47.840 align:middle
can simply be object pointers,
or perhaps Byte or Word variables.

00:04:48.440 --> 00:04:51.520 align:middle
It's used to interface
with the outside world

00:04:51.680 --> 00:04:54.160 align:middle
when we need to specify a size,

00:04:54.360 --> 00:04:58.840 align:middle
and within it we put pointers
to objects with a specific form.

00:04:59.240 --> 00:05:03.480 align:middle
This is an advanced feature,
but I wanted to show it to you.

00:05:03.680 --> 00:05:06.760 align:middle
You'll usually only need
to use "Subclass."

00:05:06.920 --> 00:05:10.280 align:middle
Sometimes you'll use "variableSubclass"

00:05:10.680 --> 00:05:14.080 align:middle
and "Bitmap,"
which represents the screen.

00:05:14.280 --> 00:05:17.280 align:middle
Bitmap is an object

00:05:17.840 --> 00:05:20.440 align:middle
that has a variable zone, named Words.

00:05:21.280 --> 00:05:23.280 align:middle
Just so you know it exists.

00:05:23.680 --> 00:05:27.600 align:middle
Since Pharo is a reflective system,

00:05:27.760 --> 00:05:29.800 align:middle
we can ask it questions.

00:05:29.960 --> 00:05:33.680 align:middle
So, using these methods,
we can make queries.

00:05:33.840 --> 00:05:37.840 align:middle
Such as whether the class size
is fixed or variable.

00:05:38.000 --> 00:05:41.560 align:middle
Or whether it stores pointers or bytes,
for example.

00:05:41.760 --> 00:05:46.160 align:middle
We have the entire API
to enquire about the type of class.

00:05:46.400 --> 00:05:50.120 align:middle
This list was created
by making queries to the system.

00:05:51.240 --> 00:05:52.640 align:middle
A constraint applies.

00:05:53.640 --> 00:05:58.480 align:middle
Classes that are defined using subclass

00:05:58.880 --> 00:06:01.080 align:middle
can have any type of subclass.

00:06:01.280 --> 00:06:04.040 align:middle
If we have points using x and y,

00:06:04.280 --> 00:06:07.600 align:middle
we'll be able to add a subclass named z

00:06:07.840 --> 00:06:12.440 align:middle
or a subclass named xy,
plus a variable zone.

00:06:14.520 --> 00:06:20.520 align:middle
But if you want to create a subclass
of a class that already has a variable,

00:06:20.720 --> 00:06:25.560 align:middle
you can only use variables
from classes that are variable.

00:06:25.760 --> 00:06:30.680 align:middle
Otherwise, the system that constructs
classes says it's incompatible.

00:06:30.840 --> 00:06:35.800 align:middle
It needs to know that there is one zone
to handle the constraints.

00:06:36.000 --> 00:06:40.280 align:middle
In any case, the system will inform you
if it's impossible.

00:06:41.000 --> 00:06:43.200 align:middle
So, what do you need to know?

00:06:43.440 --> 00:06:46.920 align:middle
You should now know
how to define classes

00:06:47.080 --> 00:06:50.520 align:middle
that have instances of variable sizes

00:06:50.680 --> 00:06:56.520 align:middle
and how to instantiate them using "new:"

00:06:56.840 --> 00:07:02.320 align:middle
and how to access them
using the protocol seen in the arrays.