Objective-C Tutorial - Lesson 18: How To Create An Object & Message It Video Lecture | Learn and Understand: Objective C (English) - Web Development

what's up guys jordan here with the
objective-c toros lesson 18 creating an
object and messaging it now objects are
very very important in objective-c
obviously you can tell that just from
the name i mean objective-c so anyways
we're going to go ahead and jump into
this lesson starting off on how to
create an object and that little snippet
of code there is how you create an
object and send message to it you have
the class then the object name and the
message so this creates an object by
sending a new message to the class that
this object is based on now a little bit
of specifics on messaging the object the
basic syntax of it is you have the
receiver so he's receiving this message
the actual message itself and then
arguments now the receiver can be a
class or an object so an objective c you
can send messages to a class and this is
pretty cool if you're coming from
another language like c++ if you're just
learning how to program this probably
isn't that special but not all classes
allow this so that's kind of a neat
little feature now continuing to go over
that message the budget class gets sent
the new message that's what's happening
there and there's two features of this
new method which is inherited from
nsobject which is the superclass of our
budget class and these two features are
that it holds memory or allocates it for
the ivars of the object and also sends
an init message which is the
initialization message to the new object
and another thing is each class at
runtime gets passed an object and this
object is a class object and it has the
ability to create new objects
now on to the name of the object as you
can see is not just a straight-up name
it's actually has an asterisk in front
of it and this is because it's a pointer
to the memory of the object so what's
returned is not the new objects actual
address and sending messages to the
object well the right there you see
three messages that we sent to this
Europe budget object and what we're
basically doing is we're just assigning
values to where the arguments were so
there were arguments the first one
created budget was then after that was a
budget and so we're just assigning a
thousand dollars and we're really
assigning it to the I bars because a
budget was assigned to budget so where's
Simon a thousand dollars to budget and
is kind of confusing a note because it's
in a keynote and you don't see the whole
code but what you see all the code don't
make a lot of sense and we did all this
assigning the numbers to like the budget
everything before using structs with the
dot syntax now accessing the objects
ivars how do you access the right
objects ivars because we're not using
that whole dot syntax any longer where
before we would use the struct variable
dot and then whatever that number was so
how do we access one objects I bars and
not in others well objective-c has a
very interesting way of solving this
problem when you send a message a hidden
argument called self which is a pointer
to the average is passed to the object
so on an example of a while I'm talking
about is when you wrote in the last
lesson that nslog when you typed budget
what the compiler actually sees itself
and then that arrow deference operator
than a budget so it says self so it
tells the compiler on what object it is
and then once the compiler knows which
object it knows which I've are to use
now we're going to go ahead and jump
into Xcode and actually create a couple
objects and send some messages okay so
here we are in Xcode working with the
program that we've been working with the
past couple of lessons and before we
actually start coding some objects I
have a typo i misspelled budget um and
it's one of the I've are so this program
obviously wouldn't work because all
these ivars would be undeclared so if
you did misspelled that also when you
were copying what I was coding go ahead
and change that and once that's all done
go ahead and jump down into main and
we're going to declare some local
variables so double number dollars in
Europe equals 100 and double number
euros equals 100 double number dollars
and England
and this is the exact the exact same
thing we did before so nothing different
here okay so now that we have that all
done we can go ahead and create our new
objects so it's going to be this object
is going to be based on the budget class
and it's going to be Europe budget and
then send it that a new message and
there you go you just created a new
object so now we're going to send it
some messages so Europe budget and then
create budget and we're going to create
this budget with a thousand dollars and
the exchange rate will be 1.25 hello and
there you go you just sent a message to
your newly created object so then we're
going to go through and send it two more
messages Oh spend dollars it will be a
number euros
our number dollars in Europe excuse me
there and then the last one will be
charged born currency can all be number
euros okay so now that we have that all
done we need to create a new object so
we're going to name this one England
budget send it the new message again and
there you've created your now second
object um oops England budget create
budget we're going to have two thousand
dollars in the England budget with the
exchange rate of 1 point 5 000 and then
spend dollars number dollars in England
and England budget charge foreign
currency number
pounds and there we go we now actually
have all the code for this program to
run right so if we build and run save
all okay now this is basically the exact
same results that we got last time in
Lesson 13 so I won't be going over them
but I will be going over exactly what
all is going on in this program if you
don't quite understand it so first off
we start off with the interface so we
create this class which is called budget
and spaced on nsobject and it has these
I box it has an exchange rate a budget
and exchange transaction and then we add
eclair these methods and I won't go over
them right now because they're right
down here below in the implementation so
now we jump down into the implementation
and the first method it creates a budget
and then has an argument for how much is
in the budget and then it also has an
exchange rate and has the argument for
the exchange rate and then it assigns
these arguments so whatever value is
assigned to the art of these arguments
and then that is assigned to the I bars
up here then we jump down to the spend
dollars method and as this argument in
dollars and it subtracts whatever value
is given two dollars and then it
subtracts and then reassigns that value
it subtracts the value from budget and
then reassigns it to budget their funny
got it said correctly and then it
displays this in an admin with an NS log
so converting whatever however much this
dollars was in the foreign currency
leaves and then the resulting amount of
the budget now we jump down to the
charge foreign currency method with
an argument for how much foreign
currency there is and then it assigns
the foreign currency the well however
much that is times the exchange rate and
it assigns all that to the exchange
transaction and then it subtracts that
from budget and then reassigns that
value to budget and then we display this
in NS with an NS log charging however
much the foreign currency um leaves and
then how much is left in the budget so
that's what all these methods do and
then down here this is where kind of
everything happens in the program we
declare these local variables number
dollars in Europe and number euro so I'm
just going to be going over the Europe
side of it and we assign both of them
the value of a hundred now we create
this object based on the budget class
and it's called Europe budget now we
send it three messages the first one is
its we say create a budget and since we
know that this needs an argument we say
hey create a budget with thousand
dollars and we also know that it has
another argument and so we give the
exchange rate we assign 1.25 as exchange
rate so then everything that's in this
gets run then we go to the next message
and it says hey spend dollars method and
since it recognizes that it has an
argument also it says dollars will be
assigned the value of a number of
dollars in Europe so that'll be a
hundred and then it goes through and
does everything that's in the spend
dollars method and then it also says
that this last message which is charged
foreign currency it recognizes again
that has an argument so it assigns the
value number the value that's in
number euros that's also hundred and
then it runs through these three steps
and that's all to the program so you
just assign the I bars up here you
declare first line the methods you put
all the action of the methods and then
you declare any local variables and then
you declare your objects and then you
send the messages to initiate these
methods and send it the values for the
arguments that it needs so a very basic
program for a object-oriented program
that is so anyways next lesson we'll be
splitting this program up into separate
files because an objective c program is
not all in one file like what we had
been working so we're going to split it
up and that's going to be the gist of
next lesson and then after that we're
going to be talking about naming
convention so some good stuff coming up
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