All language subtitles for 012 Constructor Functions_en

In the last section, we used our person class to create a new instance of a person which we assigned

to the variable lowercase P person then on the line.

After that, we set a value to the first name property on that person instance.

At the very end of that section, we had said that it sure seems kind of awkward to have to first declare

the new instance of the person and then set the first name property after that.

It sure seems to me like every person who exists inside of our application should always have a first

name associated with them.

So in this section, we're going to add a little bit of code to make sure that anyone who wants to use

this person class right here has to provide a first name property to the class the instant that the

class is created.

So the instant we call a new person right here.

To do this kind of required setup or required configuration of our class, whenever we create a new

instance of it, we're going to add something called a constructor function.

A constructor function is going to be another function or method added on to this class right here.

This constructor function gets executed automatically any time we create a new instance of the person

class.

And so it makes it a perfect location to do a little bit of initial setup or initialization of our class.

So to just kind of put this into diagram format, here's what's going to go on.

Any time you and I create a new instance of a class, the constructor function is going to be called

with any arguments that we provided.

When we try to create a new instance, that constructor function will then be executed and it's going

to do some initial setup for us.

And then after all, that runs our new instance of the class, then gets returned and then assigned

to this person variable over here.

So let's get to it.

Let's define the constructor on our person class.

To define a constructor, I'm going to add a little bit of space here underneath our field of first

name, and then I'm going to define a new function with a very special name.

So the function name is going to be a person with a capital P like.

So so notice how the name of this function right here is absolutely identical to the name of the class,

because these two names are completely identical in case and spelling.

So they both have a capital P.

This person function right here will be used as the constructor function.

And so this function will be called automatically any time we try to create a new instance of a person.

And again, that makes it a perfect location to do some initial configuration of our instance.

Now, as we were saying just a moment ago, we kind of want to add on a requirement to our application.

We want to say that any time you try to create a person instance, you have to immediately provide a

first name.

In other words, we should not have any people inside of our application who do not have a first name.

So to add in that requirement, we can specify an argument inside this constructor functions argument

list or the set of parentheses right here.

So inside the set of parentheses, I'm going to write out a name like so.

When as soon as I type that in, you'll notice that we get an error appearing on the right hand side

over here and if you click on it, it highlights where we are trying to create that new person instance

up here.

So as soon as we start to add in an argument to a function that means that anywhere we are calling that

function from needs to also provide the correct number and type of arguments as well.

So in order to make sure that every person gets created with a first name, we just had to add in the

argument right here and then, poof, everything else inside of our code base now says, hey, if you're

going to try to create a person, you have to pass along some argument as well.

Now, we'll fix up this kind of new instance creation right here in just a moment.

But first, let's finish off our constructor.

So as soon as someone provides a name property to this constructor function, we probably want to take

that provided name in, assign it to the first name property.

So to do so, we can write out first name equals name like so.

Now, one thing I want to point out here is that my choice of variable names is a little bit unclear.

I provided a variable name or an argument name here of simply name.

And so it's not super clear inside my constructor if this is supposed to be like the last name or is

it supposed to be the first name of the person?

It's not super clear.

And so you might think, well, it would be a little bit more clear if we just wrote out first name.

But then that makes this line of code a little bit less clear because this would technically be first

name and now we are in a state where things are super unclear.

We've got an argument name, a first name, but our property name is first name as well.

So Darte understands that this right here is probably a line of code that you're going to want to write

all the time, it's very common to have constructor arguments with names that are identical to the property

names that you're attempting to sign them to assign them to.

So rather than making you write out some very awkward code like this right here, Darte has a built

in shortcut that kind of takes care of this entire situation for us.

So let's write out this shortcut, and I think you'll be somewhat happy with how this turns out.

So I'm going to take the set of curly braces here.

I'm going to highlight everything in between.

I'm going to delete it all.

And then going to place a semicolon.

After that person function and then to first name right here, I'm going to add on this dot like so.

OK, so let's talk about what's going on here.

So this is some pretty crazy syntax, but here's what's going on now.

Any time we create a new instance of person and pass in a first argument to that creation, Dart is

going to automatically take that first argument in, assign it to the first name property of the instance

that is created.

So normally when we see this set of parentheses after a function name, we think of it as the argument

list, but DART is instead taking this argument list and it's kind of doing a little shortcut for you.

And it's saying, oh, well, since you referred to this argument as this first name, I'm just going

to take whatever argument was passed to this new person when it was created and automatically assign

it to the first name property for you.

Now, I'm using a lot of words to describe this, but it might not be super clear, so let's kind of

complete this example and I think it'll get a little bit more obvious as time goes on.

It's now back up inside of my main function.

I'm going to find the person first name property Simon right here, and I'm going to delete the entire

thing.

And then going to find where we try to create a instance of a person right here and I'm going to make

sure that I pass in my first name property directly to this constructor function.

So to the thing I'm going to add a string of Steven like so.

Now, you'll notice that all the air messages go away, and if I try to run this thing.

Over here in the console, I'll see Stephen appear again.

All right, let's take a quick pause right here, and when we come back, the next section, we're going

to review what we've spoken about with Constructor's and we'll review this little shortcut of property

assignment right here.

So quick break and I'll see you in just a minute.