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- Rust allows you to
define structure types

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or structs as they're known.

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In a structure, you can
define related fields,

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and related functionality,

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and also you can implement traits.

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Remember in Rust, a trait
is like an interface.

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So, this would be equivalent

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to some of the concepts of
classes in other languages.

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In other languages, like Java
and C Sharp, C++ and Python,

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you can define classes.

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In Rust, they're called structures.

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In Rust, we have traits,

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in other languages, we have interfaces.

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So, the concepts are quite similar in Rust

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but the terminology and
the syntax is different

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probably from what you're
gonna be familiar with

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in those other languages.

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So, to define a structure type,

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you use the struct keyword,

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you give your structure type a name,

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starting with capital letter.

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Rust is quite fussy about that.

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And then you define the
field names and types.

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You can also define
functionality for structure,

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like you could for a
class in other languages.

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We'll have a look at
that in the next lesson.

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Right. Here's a simple example.

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I've defined a structure
type called Employee.

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An Employee has a field called name,

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which is a string,
salary is an unsigned 64.

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That's ambitious, isn't it?

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And then a fulltime boolean field as well.

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I haven't actually
created any objects yet,

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I've just defined the shape
of the structure in memory.

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Now, by default, a structure
and its fields are private.

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You can only access the
structure and its fields

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within the module, within
the file they're defined.

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Okay. And that might be fine,

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but if you want a structure
to be accessed elsewhere,

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then you have to marked it as public.

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You have to use the pub
keyword on the structure type.

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You also have to do that with the fields.

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If you want the fields to be public,

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then you have to say pub
on each field as well.

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Otherwise, the fields would
be private just to that file.

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Now, it's quite common to put a structure

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into a separate module file,
so it can then be reused

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across the entire application.

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So that's what we are going to do.

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In order for that structure to
be visible outside that file,

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we'd have to make it public.

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And I'm gonna do that.

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They're gonna have a public
structure and public fields,

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and I'm gonna put it into a
separate file called mytypes.rs.

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You can call the file
anything you want, okay?

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It's not like Java,

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where you have to have a file
name called employee.java.

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You can call it anything you want to,

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and you can have as many
structure types as you want

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in a given file.

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So mytypes.rs, it defines
a public structure Employee

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with public fields.

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At the moment, it's just a file.

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In order to introduce
that file as a module

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in your application, then in main.rs,

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you'd have to declare it as a module.

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Okay? So remember this is important.

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We need to declare a module
in Rust called mytpes.

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It will go and look for
a file called mytypes.rs

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in the current folder by default.

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And it'll drag that code and
effectively input that code

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into your application.

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If you forgot to say mod mytypes,

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the compiler would
completely ignore this file

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and it wouldn't be part of your code base.

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So, don't forget to declare the module

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to effectively import the
code into your application.

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So, let's say we've done that.

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Let's say, we declared the module mytypes

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and I want to actually
create an employee object.

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Well, you could use fully
qualified syntax like this.

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We haven't seen the syntax before.

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So let me explain.

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Effectively in Rust, you
have kind of like a module,

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a hierarchy, whenever your
project might be called,

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there's a logical namespace
at the top called crate, okay?

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That represents the root
namespace in your project.

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And then for each module,
each module or submodule

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is considered to be like a child.

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So we have a hierarchy of names, mytypes,

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and in mytypes, I've defined
a structure called Employee.

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Okay? So in anywhere in your application,

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if you want to use the Employee structure,

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you could say effectively
crate is a keyword,

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go to the root of my namespace,

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here, underneath go to the
submodule called mytypes,

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which is here.

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And then in there, you'll find
the Employee structure type,

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which is there.

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So that's a fully qualified syntax.

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You could use the syntax anywhere

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you wanted to use Employee
in your application.

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It would get quite tiresome though,

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have to say crate, colon, colon,

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mytypes, colon, colon, employee.

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Simpler approach that
people usually go for,

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is to use the used statement.

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Remember, what the used statement does,

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it just brings something into scope, okay?

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So the Employee structure
type already exists.

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All you're doing is introducing
it into the current scope.

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So, when you say use
crate mytypes Employee,

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it means you can then use this symbol name

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directly in your application rather than

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have to use the fully qualified
name like we did before.

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Okay. Right.

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Well, we'll have a look at an example.

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For this lesson, the project is

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lesson10_structs_getting_started.

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Let me just quickly show you that.

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All right, so in the rustdev demos folder,

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lesson10_structs_getting_started.

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So, if you open that
project in code editor,

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and here we are, the files

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that we're going to look at up here are.

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Well, first of all
mytypes.rs as advertised,

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it defines a public
structure called Employee

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with public fields.

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And then in the main code,

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I declare it as a module,

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technically that's how you'd say it.

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I declare it as a module that causes

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that file to be processed.

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I've got some other modules as well.

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Each of these other
modules is illustrating

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different techniques per
section in the lesson.

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So, as well as importing
the mytypes module

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which has got my Employee structure.

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I've also imported, for
example, demo_accessing_struct.

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And that's here.

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And in this example,

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I've imported from the top
level namespace in my crate.

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I've imported the mytypes module.

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I've imported the Employee structure name.

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Okay. I could still use
the fully qualified name

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if I want to.

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I've declared a variable here called e1.

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Notice the underscore
because I'm not actually

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gonna be using the variable just yet.

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So, I've got an underscore here

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to avoid getting compiler warnings.

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Anyway, e1 is off type
crate, colon, colon,

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mytypes, colon, colon, Employee, right?

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So that would've worked, even
if I hadn't imported the name.

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But since as I have
imported the name Employee,

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I could just use the
employee name directly,

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and of course that would
be a much better approach.

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So, really here I wouldn't bother

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using the fully qualified name,

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I would just declare that
as an Employee as well.

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I was just put in there
just to remind you.

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Wouldn't it be interesting to see

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how many bytes an Employee
object would take?

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Well, in the standard crate,
there's the mem module,

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which has a size of
function, which is generic,

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you can give it a type

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and it'll tell you how
many bytes that type is,

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what's the size of that type in bytes.

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So, I thought it would be
quite interesting to see.

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Right. Well, in my main code,

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let me just run the
demo_accessing_instruct.

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That's the code we just saw

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and it'll tell us how big the
structure type is in memory.

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We haven't actually
created any instances yet,

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but in theory, how big is
the structure type itself?

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Cargo run.

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So, it turns out the size of
a structure of an Employee

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is 40 bytes.

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Okay. So,

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a u64 is eight bytes.

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A boolean is probably one or two bytes.

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A string is probably
about 24 bytes, I think.

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So, there's pretty some
alignment in there,

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just to make the fields align nicely.

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But anyway, it tells me one of those

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will be 40 bytes in size.

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So, we've seen how to
define structured types,

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and basically how to organize
it into separate modules.

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What we're gonna see in the next demo

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is how to actually create employees

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and actually start accessing
the fields inside them.