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In this video, we'll learn
how we can call c++ functions

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using default argument values.

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As we've seen, when we call a
function, we must provide all

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the arguments that, that function
requires, and they must be in the

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right order and of the right type.

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But sometimes when we call
functions some of the argument

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values that we pass in tend to be
the same values most of the time.

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For example, if we have a
function that calculates the cost

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of an item, we can provide the
function with the base cost of

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the item and the sales tax rate.

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Suppose that 98% of our customers live
in a region where the tax rate is 6%,

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that means that we have to provide the
tax rate in every function call even

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though it will almost always be 6%.

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C++ allows us to provide
default values for arguments.

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So in the case of the sales tax,
if we omit the argument from the

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function call, then the c++
compiler will automatically replace

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it with the default value of 6%.

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However, if we have a customer
whose sales tax rate is 8%, then

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we can provide it explicitly
and override the default value.

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As we'll see in the next few slides,
we can add default arguments to

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the function prototype or the
function definition but not both.

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But best practice is to do
it in the function prototype.

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Default arguments must appear at
the tail end of the parameter list.

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We can also have multiple
default arguments.

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Let's see an example that
demonstrates default arguments.

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In this example, we have a
function with no default arguments.

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In the next slide, we'll modify it
so it does have a default argument.

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The function is called calculate
cost and it expects the base

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cost of an item and the tax rate.

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The function calculates the tax amount
and adds that to the base costs and

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returns that value to the caller.

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In the main function, we're
calling the function with 100.0

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as the base cost and 0.06 or
6% percent as the tax rate.

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In this case, 106.0 is returned from
the function and assigned to cost.

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That's pretty easy.

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But if 98% of our customers will have
a 6% tax rate, I'm forced to supply

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this every time I call the function.

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Why not let the compiler
default the tax rate to 6%

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unless I tell it otherwise.

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Let's do that in the next slide.

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Here, we have the same function
except that the function prototype

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has been modified to provide a
default value for the tax rate.

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Notice that the parameter
tax rate in the function

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prototype now has the default
initialization to 0.06 added to it.

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This effectively tells the
compiler that if this argument

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is not provided by the function
call, then use the value 0.06

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for tax rate, simple as that.

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Now notice the two calls
in the main function.

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The first call simply calls calc cost
with 200.0 which is the base cost.

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Since the argument for tax rate
is not provided, the compiler

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will use 6% for the tax rate.

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In the second call, we are
providing the tax rate.

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In this case, the compiler will use
that and ignore the default value.

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This can be pretty handy.

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Suppose we have a function that
prints documents and we have to

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supply the document name, the
printer to print to, the paper size,

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the resolution and so forth.

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Again, most of the time we
print to the default printer

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with default settings.

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But sometimes, we need higher
resolution or a different

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printer, you get the idea.

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In this case, we can provide
multiple default parameters for

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everything except the document
name which is always required.

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This makes writing
the code much simpler.

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Let's add another default
parameter to the calc cost function.

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In this example, we'll calculate
the cost of an item based on the

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base cost of the item, the sales
tax and the shipping charge.

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As before, most of the times
the sales tax will be 6%.

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And in this example, the standard
typical shipping charge is $3.50.

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So most customers will have a 6% tax
rate and a $3.50 shipping charge.

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We can easily add a second
default argument to this function.

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Notice that in the function
prototype, we added another

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parameter named shipping and
we default this value to $3.50.

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The function body now calculates
the tax for the item, adds the

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shipping charge, adds this to
the original cost of the item and

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returns the result to the caller.

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In the main function, we're calling
calc cost in three different ways.

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In the first call, we're providing
all three arguments to the function.

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In this case, no default values
will be used and the tax rate will

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be 8% and shipping will be $4.25.

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In the second call, we're
providing the cost of the

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item and the tax rate of 8%.

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In this case, the default value
for shipping will be used.

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But the tax rate will
use the 8% we provided.

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In the final example, we're only
providing the cost of the item.

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So in this case, both
default values are used.

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The tax rate will default to 6%
and the shipping charge to $3.50.

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As you can see, default argument
values give us the ability to

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make our code less verbose and
potentially prevent errors by using

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default values rather than having
to supply all of the arguments

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to the function all the time.

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In the next video, we'll learn
about function overloading.

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Before we talk about function
overloading in the next video,

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let me go over default arguments.

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I'm in the section 11 workspace
in the default arguments project.

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And this is the same example
you saw on the slides.

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Here, I've got a function called
calculate cost that expects $3.

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The base cost, a tax rate, which is
a default argument at 0.06 or 6% and

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the default shipping which is $3.50.

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Now if those arguments are not
supplied, then the defaults kick in.

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It's really pretty simple.

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And here's the calculation.

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Notice, I don't have to include
the function default arguments

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here in the function definition,
only in the function prototype and

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that's usually where you see them.

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So the code here says base cost plus
equals base cost times the tax rate,

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so I'm figuring out what's my tax.

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And I'm adding the shipping
to that and then I'm adding

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the whole thing to base cost.

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So I'm not taxing the shipping.

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And then we've got some sample calls.

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In this first example, I'm
calling calculate costs

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with all three arguments.

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So in this case, none of
the defaults will be used.

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I'll use 100 for the base cost, I'll
use 8% for the tax rate and I'll use

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four $4.25 for the shipping charge.

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I expect to get back 112 and 25.

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For this example, here, I'm
providing the first two arguments,

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and I'm not providing the shipping.

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So in this case, it's going to
use a 100 as the base cost, 8%

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for the tax rate and it'll default
to 350 here for the shipping.

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And then this last example,

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I'm supplying the base cost only.

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So in this case, 6% and %3.50
are what're going to be used.

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So if I run this program,
you'll see that our results are

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exactly it was as we expected:
112.25, 111.50 and 215.50.

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Okay, so you can see default
arguments are pretty easy to use.

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I could also default the
base cost if I wanted to.

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I could say that the base cost is
a 100. Maybe that's the typical

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item that most people order.

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So in this case, I could just simply
just say cost is calc cost, right,

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with nothing in the parentheses.

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So I'm not supplying
any arguments here.

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That's it.

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So what's going to happen
here is it's going to use

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all three default arguments.

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So let me print that out real quick.

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So in this case, we expect the same
result as -- well, if we would have

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done 106, so we'd have 100 is the base
cost that's being used by default.

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We're using a 6% tax rate
so that's another $6 and

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our shipping would be 3.50.

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So in this case, we expect 109.50.

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So let's give that a shot, and that's
what we're getting here, 109.50.

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This is pretty straightforward.

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The only rule with default arguments
is that once you create a parameter

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that has a default value, then you've
got to default the rest of them.

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So in other words, I can't say
that base cost is 100 and then

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not have one here because that's
going to give me an error, right.

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Once you start, they all
have to be on the tail end.

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So in this case, I
could just say 0.06.

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So that's really the only rule there.

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And I'll run that again just
to make sure I've got no

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errors, and there you go.

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So let's add another default type
function to this application.

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How about a greeting function?

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So let's do something like this.

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Let's create a function
prototype here for a function

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called greeting that expects a
string, which is someone's name.

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Now let me include string
because I need the library.

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So that's a string name.

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Perfect.

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So that would be Frank Metropoulos,
Joe Miller, whatever you want.

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Then I've got another string,
which is the prefix that we

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want in front of their name.

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Mr., Dr., Mrs., whatever you like.

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And I'm going to prefix it to Mr.

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Let's say that most of
my customers are male.

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And I also want a suffix.

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And in this case, the suffix
I'm going to default it

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to just the empty string.

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Okay, so that's my function prototype.

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Let's implement this function now.

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We'll implement it right down here.

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And at this point, we don't
need those default arguments.

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We can just get rid of them
from here, and it cleans

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up the code a little bit.

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All right, so what do
we want to do here.

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We just really want to
display some stuff, right.

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So let's just say I want an output
statement, and I'm going to say hello,

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followed by a space, followed by
the prefix and what we could do here

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is we could append a space, right.

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We can use concatenation here, and the
name and another space and the suffix.

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Okay, so we're just using
concatenation here, just like

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we've learned in string class.

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And then we'll use the
endline to finish it off.

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Okay, so let's run that, and
give it a few example runs here.

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So let's say how about -- let's do a
really simple one, how about greeting.

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And I'm going to supply
all of these arguments.

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So I'm going to say
Glenn Jones is the name.

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Okay, the prefix will be Dr.

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and the suffix will be M.D.

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Okay, so if we run that, we expect
this to display Dr Glenn Jones M.D.

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Okay, so let's give it a shot.

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And I forgot the semicolon.

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I'll fix that real quick.

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And there you go.

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Hello Dr Glenn Jones M.D.

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Perfect, now let's run some of
the other examples.

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So how about we do something
like greeting James Rogers.

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How about professor and
Ph.D for the suffix.

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Okay.

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We'll run that, and now we'll
say hello Professor James

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Rogers Ph.D, pretty easy.

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Now let's use some defaults.

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In this case, we'll say greeting,
let's say, Frank Miller.

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And Dr.

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No suffix in this case.

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So it'll default to the
blank or the empty space.

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So in this case, it'll just say Dr
Frank Miller, which is what I want.

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And then let's do one more,
and we'll just say greeting,

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let's say, William Smith.

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So in this case, it's going
to use all the default values.

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So it's going to use Mr William
Smith followed by the empty

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string right there for the suffix.

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Okay, so in this case, we're just
going to see Mr William Smith.

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There you go.

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If we have Mary Howard,
and she would be Mrs.

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And we can give her a title
as well, whatever we like.

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There you go.

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So now we've got Mrs Mary
Howard Ph.D instead of Dr.

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We could have said
doctor here as well.

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You can see all the
different combinations.

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This can come in pretty handy.

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I used to work with someone who used
to say that if something is always

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required, then it should never be
required, which makes a lot of sense.

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And I think that's part of
the idea behind using default

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arguments and using overloading
which we'll talk about next.