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Now that we know about references
and how to use them,

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let's look at l-values and r-values.

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L-values into r-values are something
you don't worry about too much

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when you're first learning c++.

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But as you learn more advanced
features of the language,

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they become very important.

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It's important that the fundamentals
make sense now so that when we

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move on to topics like c++11,
r-value references and move

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semantics, they'll be much clearer.

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So in this video, we'll do a
basic introduction to l-values,

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r-values and l-value references.

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So let's start with an l-value.

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An l-value is an object
that occupies a location in

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memory and is addressable.

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So in these examples, x
and name are l-values.

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They have names, and
they're addressable.

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We know that they're addressable
since we can use them on the left-hand

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side of an assignment statement.

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So what's not an l-value.

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In these examples, 100 is not
an l-value because it's not

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addressable, it's a literal.

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In fact, the compiler generates
a compiler error for the 100

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equals x statement, saying that
100 is not an assignable l-value.

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Similarly, in the assignment
statement 1000 plus 20 equals x,

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1000 plus 20 produces a temporary
variable with the value 1020, but

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that variable is not addressable.

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So we can't use it as an l-value.

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In the last assignment
statement, Frank equals name.

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Well, Frank is not an l-value
since it's not addressable.

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So these examples are of non-l-values.

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They're, in fact, called r-values.

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Let's talk a bit about r-values.

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We can define r-values by exclusion.

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So anything that's not
an l-value is an r-value.

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Usually, r-values on the right-hand
side of an assignment expression.

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R-values can also be temporary
values that the c++ compiler creates.

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We saw some examples of
r-values in the previous slide,

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let's see a few more here.

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In the first statement,
100 is an r-value.

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In the second statement, x plus
200 will produce a temporary

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variable and that variable is an
r-value since it's not addressable.

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The string literal Frank is also an
r-value, and max 2030 is an r-value

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because it returns a result that's
a temporary and not addressable.

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R-values can be assigned
to l-values explicitly.

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So l-values can appear both
on the left and right sides

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of an assignment statement.

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In this example, we see the first
assignment statement is using

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the r-value 100 on the right-hand
side of the assignment statement.

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And this will be assigned
to the l-value y.

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In the second example, the temporary
r-value with the sum x plus y

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will be assigned to the l-value x.

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Now let's look at references from the
perspective of l-values and r-values.

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The references we've used so far
have all been l-value references.

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That means that we're
referring to l-values.

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In the first reference
declaration, we're declaring

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ref1 as a reference to x.

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Notice that x is an l-value.

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This makes sense since we are
an alias for x, and we can

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use ref1 on the left-hand side
of an assignment statement

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since it refers to an l-value.

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But what happens when we declare
ref2 as a reference to 100.

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Well, 100 is an r-value, so
this isn't allowed, and the

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compiler will generate an error.

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The same applies when we pass
parameters by reference to functions.

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In this case, we define a
function square that expects

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an integer by reference.

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We have an integer num
initialized to 10, and then we

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call square and pass in num.

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This is fine because n is a reference
to an l-value and num is an l-value.

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However, if we pass in the
literal 5 to the function, we

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get a compiler error because 5 is
not an l-value, it's an r-value

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and we must reference l values.

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Don't worry if this is
a bit confusing now.

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As you'll learn more about c++
semantics, it'll make more sense,

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but it's important going forward
when we talk about move semantics.