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- It's very common to
pass structure objects

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into a function.

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This enables you to pass a
whole object into a function,

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a lot of data all in one go,

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much better than passing

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a whole load of individual parameters.

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You can just pass a single structure,

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and all of its data
immediately gets passed

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into the other function in one fell swoop.

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So that's quite handy.

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So when you pass values into a function,

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you've got two different ways to do it.

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You can pass by value, which
is what we'll discuss now,

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or you can pass by reference,

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which we'll discuss a bit later.

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So let's imagine we have a function.

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This function receives a
structure parameter by value.

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There's no ampersands, it's
not receiving a reference,

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it's receiving a structure by value,

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and it prints the name and the salary

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and the fulltime flag.

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Now, the name of the function,

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I've called it consume employee,

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gives you an indication
of what's gonna happen

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when we call the function.

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So let's see some high level function.

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Here we are.

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And in here, I declare e1.

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So e1 is my employee,

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the name, and the salary, and the region.

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And then I pass e1 basically
up into the function above.

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Okay, so e1 gets passed up into here.

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So what happens now
when you pass by value?

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Remember, this is tricky in Rust.

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When you pass something
by value, what happens?

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It depends on whether the type

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implements the Copy trait or not.

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So what I'm telling you
for a fact at the moment,

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the structure that we've got,

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the employee doesn't
implement the Copy trait.

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We haven't really seen how to do that yet.

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We can and we will, but
we haven't done it yet.

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So at the moment,

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the Employee structure doesn't
implement the Copy trait.

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So what's gonna happen is it will copy,

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it will still copy the data
into this employee here, e.

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Okay, so the data will
be copied into there,

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the name, and the salary,
and the fulltime status.

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A move occurs, basically.

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The employee object here
used to own the data.

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That data is moved into the
other structure up here.

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The data was moved into this
structure object up here.

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And this structure basically goes away.

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It becomes vacated, a move occurs.

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So our function down here
loses ownership of the data.

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It's gone effectively.

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That data has been moved up to here.

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So we can use that data in the function.

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We can access the name
and the salary and so on.

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But of course, when this
structure goes at the scope,

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it will be popped off the
stack and it disappears, okay?

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So if you do pass a structure

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and the structure doesn't implement Copy,

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then by passing the
structure into a function,

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you basically lose that object.

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You pass the object into the function,

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that receiving function
acquires ownership.

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And at the end of that function,

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the object is dropped, it's destroyed.

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It means you can't then use
the object anymore down here.

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So to summarize the rules,

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what happens when you pass
a structure into a function?

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It depends on whether
the structure implements

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the Copy trait or not.

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If it did, then it would
behave more like an integer.

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A bitwise copy of the structure

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would be passed into the function.

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The original function retains ownership

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and can use the structure afterwards

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as if you were passing integer,

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which implement the Copy trait.

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If you don't implement the
Copy trait for the structure,

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and many structures don't implement Copy,

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then ownership of the structure is moved

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into the called function,

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and the caller loses ownership

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and can't use the object anymore.

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So you have to be careful here.

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Right, well, let's have
a look at the example.

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Same project as before.

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The files involved, obviously, mytypes,

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which defines the Employee structure.

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The main code, which
calls my demo function.

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And my demo function this time is

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in demo_struct_pass_value, pass by value.

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Okay, so we'll run the project

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once we've had a look at the code.

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Let's have a look at the code.

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So quick reminder, mytypes,

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my Employee structure doesn't
implement the Copy trait.

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We haven't seen how to
do it yet, so it doesn't.

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We will see that later.

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But at the moment, our structure
doesn't implement Copy.

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So when we pass by
value, a move will occur.

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In my main code,

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let's uncomment the demo for
this part of the example,

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demo_struct_pass_value, and it's here.

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So I have a function
called consume_employee.

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It receives an employee
and acquires ownership,

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accesses the data.

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And then at the end of the function,

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the employee object will
be dropped and it's gone.

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It'll be popped off the stack
and that's the end of that.

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So my calling function creates
the original employee, e1,

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and it's John again, good old John.

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When I pass the object
into the function by value,

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my code loses ownership.

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That employee is now
owned by this function,

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which uses it and then loses it.

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Up here, I can't use the employee again.

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I've lost ownership.

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The object has moved away
into this variable here.

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I can't use the original
variable up here, okay?

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It's defunct.

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If I try to, I'm gonna get an error.

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It'll say you can't try
to borrow this object

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because you've moved ownership
away into that function.

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It's gone now.

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So if I ran the code now,

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I'd get a compiler error on line 17.

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Let's see, cargo check.

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Right.

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So I've created an employee.

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Employee doesn't implement the Copy trait,

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meaning when you pass by value,

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it will be moved away from you.

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You'll lose it.

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So it's saying, "What?"

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Bear that in mind.

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So when you pass e1 into the
consume_employee function,

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the function did consume
it, hence the name.

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So the value was moved away
from you here into the function.

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You don't own it anymore.

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So then we need to access
the name here on line 17,

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you can't try to access the name.

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The print function is trying
to borrow the employee,

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trying to reference it.

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It can't because the object
has been moved previously.

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You can't use the value
after it's been moved away.

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So, I mean, obviously,
one solution would be

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to implement the Copy trait on employee.

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Another solution would
be to pass by reference,

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and we'll look at that later.

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But if you do pass by value
and it doesn't implement Copy,

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you will lose the object when
you pass the value in, right?

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So let me comment out this code for now

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and I'll just try running
the application as it stands,

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just to confirm that
it does actually work.

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Yes, that compiles.

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Let's run it.

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Okay, so from the top, I'm
in demo_struct_pass_value.

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Here I am.

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And I create my employee, I pass it,

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I move it into that function.

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It prints the employee details.

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John earns 1K, doesn't say per annum,

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maybe that's per minute, and
he's not a full-time employee.

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The employee object will be dropped here.

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So it's gone now,

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which means we couldn't access it anymore.

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Right, so pass by value for structures

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is potentially fraught

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because you could and
you will lose the object

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if the object doesn't
implement the Copy trait.

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So, maybe you can pass
it by reference instead

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so that you don't lose ownership.

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That's what we're going to
look at in the next demo.