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- It's quite common to return
a structure from a function.

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It lets you return a lot of
data as a cohesive bundle

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in a single object and that can be useful

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to the calling function.

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You return a lot of data back
to the caller in one object,

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they can then use that data as they like.

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So there were two ways
to return a structure

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from a function.

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You can either return by value,

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which we'll have a look at now,

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or you can return by reference,

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which we'll have a look at a bit later.

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So have a look at this function.

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It's called build_employee.

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I've given it name build_employee,

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and it takes some parameters as input.

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It takes in the person's
name as a parameter,

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and the salary and a
full-time status of boolean.

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And it returns an employee,

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it returns an employee object by value.

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And that's what this does.

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Remember when you have a function,

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if you have an expression
at the end of the function,

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then it's like having a return statement.

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This is returning an employee

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whose name field is set
to the name parameter

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and whose salary field is
set to the salary parameter

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and whose full-time status
is set to full-time status.

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Okay, so you pass three
parameters into this function.

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What you'll get back is a
properly initialized employee.

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These are the names of the fields

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and these are the parameters

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that we're going to
use to initialize them.

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So this is a builder function.

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It encapsulates creating
a structure instance.

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It returns the structure
back to the caller.

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Here's the employee
that's just been built.

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Okay, so it's quite common

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to like build a functions like this.

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As an aside, the syntax
that we've got here

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where we kind of manually
initialize the name field

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to be the value of the name
parameter and the salary field

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to be the salary and the full-time field

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to be the full-time status like that,

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this syntax can be
simplified to this syntax,

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if the field name is the
same as the value name,

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if you like, if the
field is called salary,

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and the parameter is called salary,

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then instead of saying name equals name,

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salary equals salary,
full-time equals full-time,

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you can just use this syntax,
this abbreviated syntax is

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what you'd actually use in practice.

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It only works if the name
of the field is the same

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as the parameter.

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Okay, it'll use the name
parameter to initialize the name,

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the salary parameter to
initialize the salary

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and the full-time parameter

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to initialize the full-time field.

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Okay, so obviously that makes sense,

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just make sure that when
you name these parameters,

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they have the same name as the fields

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and they will automatically be transferred

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into each field
respectively, so that syntax,

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you would agree, is a lot
simpler than that syntax.

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So let's go for this.

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It means that when we call the function,

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we'll get back an employee.

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So let's have a look at how that works.

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We can mark the employee as
mutable if we want to or not.

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If we don't,

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so I've called the builder
function build_employee.

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I've passed in three parameters,

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a string for the person's
name, their salary,

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and their full-time status.

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This will gimme back an employee object,

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which in this first case is not mutable.

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In the second case I call the
build_employee function again,

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I pass in different parameters.

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I get back another employee object

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which I assign to e2 and e2 is mutable,

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which means I can change
anything in the employee object.

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So having these builder
functions is convenient

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because it kind of encapsulates
the creation of the object

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and the passing in of
fields into the object.

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It makes it a neater syntax.

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00:04:01,560 --> 00:04:02,940
Right, so let's have a look.

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00:04:02,940 --> 00:04:04,800
Usual project.

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The files we're going to look at,

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obviously mytypes.rs defines
the employee structure.

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main.rs coordinates the whole program

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and the demo file for
this part of the example,

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demo_struck_return_value and
then we'll run it at the end.

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So let's have a look.

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Okay, so mytypes.rs, that's
the employee structure.

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We've seen that many times now.

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My main code,

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I'm going to un-comment
demo_struck_return_value,

98
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and let's have a look at it here.

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00:04:35,940 --> 00:04:40,940
All right, so what have
we got going on here then.

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I've got a builder function,
which when you call it

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with a person's name and
salary and full-time status,

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it returns an employee object.

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Let's have a look.

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Okay, so this one is using
the kind of verbose syntax

105
00:04:57,000 --> 00:05:01,050
where it takes the parameter
name and stores it in name,

106
00:05:01,050 --> 00:05:03,810
it takes the salary
parameter and transfers it

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00:05:03,810 --> 00:05:07,740
to the salary field,
and the full-time flag

108
00:05:07,740 --> 00:05:10,440
and passes it in here,
so in this case here,

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I could have called these parameters n,

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or anything really, an s, an f.

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00:05:16,920 --> 00:05:19,080
Okay and then here I'd have said

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the name field is initialized with n,

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the salary field is initialized with s,

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and the full-time field
is initialized with f.

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00:05:27,390 --> 00:05:30,090
Okay, it's effectively doing that,

116
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but obviously it makes a lot more sense

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to give the parameters the
same names as the fields.

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I've called this build_employee_v2,

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and it's much better.

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So there it basically
creates an employee object

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and implicitly initializes the
name with the name parameter,

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the salary with a salary parameter,

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and the full-time with
a full-time parameter.

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And this only works because
the fields have the same name

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as the parameters.

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If this parameter was
called n and s and f,

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then this wouldn't work.

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It would say, I'm looking

129
00:06:06,319 --> 00:06:09,750
for like an external variable
called name, where is it?

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You'd have to use name, colon, n,

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and then we're back to square one.

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So let's not do that.

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Let's name the prompters
the same as the field names,

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00:06:19,470 --> 00:06:22,260
and then they will automatically
be transferred here.

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00:06:22,260 --> 00:06:24,870
And like I was saying,
when you have a function

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with an expression at the end,

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then it's implicitly the return
value, so if I wanted to,

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I could have said return
employee semicolon,

139
00:06:34,440 --> 00:06:36,210
but if it's the last expression,

140
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then it's implicitly the the return value.

141
00:06:38,400 --> 00:06:42,420
So that's the way Crusty the
Crustacean would write it.

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00:06:42,420 --> 00:06:44,430
Oh and then finally, I've got a function

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00:06:44,430 --> 00:06:46,590
that takes an employee by reference,

144
00:06:46,590 --> 00:06:48,960
and it prints the data for the employee,

145
00:06:48,960 --> 00:06:52,470
the name and the salary
and the full-time status.

146
00:06:52,470 --> 00:06:55,323
Okay, so let's just run the code up here.

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00:06:56,310 --> 00:06:59,700
So I create an employee
using the first version

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00:06:59,700 --> 00:07:03,900
of the builder, and e1 isn't mutable.

149
00:07:03,900 --> 00:07:05,880
I print the employee details

150
00:07:05,880 --> 00:07:09,750
and then I create employee
two using the second version

151
00:07:09,750 --> 00:07:11,130
of my builder function,

152
00:07:11,130 --> 00:07:15,240
the improved version with the
more concise syntax down here.

153
00:07:15,240 --> 00:07:17,700
And that employee object would be John.

154
00:07:17,700 --> 00:07:20,310
And I've decided to make
that employee mutable,

155
00:07:20,310 --> 00:07:23,523
which means I can employee salary rise,

156
00:07:24,537 --> 00:07:27,030
and then finally I print
employee two as well.

157
00:07:27,030 --> 00:07:29,880
So I feel like we should just
run this just to make sure.

158
00:07:35,950 --> 00:07:40,200
Okay, so I built employee
one, Jane, 1000, true.

159
00:07:40,200 --> 00:07:44,610
And then I print employee
one, Jane, 1000, true.

160
00:07:44,610 --> 00:07:47,310
And then I create a mutable employee,

161
00:07:47,310 --> 00:07:50,970
John, 1000, false, give
them a thousand pay rise

162
00:07:50,970 --> 00:07:54,210
and then print John's details
and John's details are here.

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00:07:54,210 --> 00:07:56,880
So this is a really widely used mechanism.

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00:07:56,880 --> 00:07:59,850
We'll take it a step
further in the next lesson

165
00:07:59,850 --> 00:08:01,920
and I'll show you what that means later.

166
00:08:01,920 --> 00:08:05,580
But having a function
whose responsibility is

167
00:08:05,580 --> 00:08:08,580
to create and return an object

168
00:08:08,580 --> 00:08:10,920
with the initial values
is a very useful thing

169
00:08:10,920 --> 00:08:11,880
because it means your code

170
00:08:11,880 --> 00:08:14,130
up here is just a simple function call.

171
00:08:14,130 --> 00:08:15,540
And you know, rather than having

172
00:08:15,540 --> 00:08:17,310
to create the employee explicitly,

173
00:08:17,310 --> 00:08:19,680
you get the employee
back already initialized

174
00:08:19,680 --> 00:08:21,120
through the field so it's quite elegant

175
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and that's the way to do it.