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- [Instructor] Passing functions in C++.

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Here's our use case.

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We've been studying hash tables,

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and soon we're going to
implement hash tables in C++.

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And for the upcoming
project on hash tables,

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you'll be asked to experiment
with different hash functions

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to see how behavior changes

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when we change our hash function.

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Now, in order to do that,

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you could implement two
different hash table classes,

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hash table A

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and hash table B,

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each with a different hash function.

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But that's gonna lead to
a lot of code duplication

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because the only thing that's changing

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is the hash function itself.

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The rest of the code

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for your hash table's gonna be the same.

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So, instead what we're gonna do

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is we're gonna pass a hash function

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to the hash table constructor.

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And then the hash table
object thus instantiated

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will include the hash
function we passed to it

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when we created it.

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Now in order to do this,

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first we need to learn

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how to pass a function in C++.

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And C++ allows us to do that.

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Just like we can pass a
variable to a constructor,

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we can pass a value to a constructor,

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we can also pass a function.

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And then that function
will be incorporated

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into the object itself
when it's instantiated.

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So let's first,

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like I said,

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see how this passing of functions works.

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So here we are back in CLion,

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and I'm going to create

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a new file

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and I'm gonna call it
passingFunctions.cpp.

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And then we're going to use a new include

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that we haven't seen before.

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Include functional.

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And this header gives us the ability

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to pass functions around in C++.

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Now we're also,

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for the purposes of this demo,

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gonna use IO stream.

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And we're gonna use string.

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And now I'm gonna do
something that I'll...

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It's cheating a little bit.

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It's not a good practice

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but for the purposes of this
very short demo it's okay.

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I'm gonna define

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and implement a class
right here in this file.

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Ideally, this should go in a header file.

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So do as I say not as I do.

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So without further ado,

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we're gonna create a class.

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We'll call it very creatively, MyClass.

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And then we're going to
have a private member.

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And this is gonna be the
function that we pass in.

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So how do we do this?

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Well, whenever we create a private member

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we have to give the type.

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And so the type here is standard function.

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And that's provided by

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the functional library
that we just included.

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And this function is
going to return a string.

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And it's gonna take as
a parameter an integer.

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And we'll just call it func.

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So here we have a private member,

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it's a function,

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it's gonna return a string,

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it's gonna take an integer

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and it's called func.

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And now we need to
implement our constructor,

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public.

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And here's our constructor,

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MyClass.

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And we're going to pass in a function.

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So it's gonna take a
function as a parameter.

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Again, we have to give the type.

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It's a function,

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that takes a string

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or that returns a string I should say.

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It takes an int

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as a parameter itself, func.

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And then a very simple constructor indeed

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this func = func.

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So here we've defined this private member.

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It's a function.

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When we construct our object,

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we're going to pass in a function

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and then it's gonna be assigned
to this private member func.

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And then we need some way
to exercise that code.

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So I'm gonna create another function here.

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A member function that we
can call standard string.

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This is gonna return a string demo,

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gonna pass in an int,

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call it X.

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And then what we're gonna
do is we're gonna return

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X applied to that function.

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So when we call this function,

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when we call demo in our driver code,

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we're gonna pass in an integer

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and then it's going to pass that integer

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to whatever function
it is that we passed in

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and is stored in this member.

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So now that's our class.

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Now let's create a function,

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standard string.

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Because remember our
function returns a string

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and it's gonna take an integer

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and we'll just call it a function.

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again very creative.

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Int X, so it's passing in an integer X,

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and then let's make it
very very simple function.

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If X modular two,

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then return, and it's
gotta return a string,

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"Number is odd."

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Otherwise, return

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"Number is even."

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I gotta put semi-colons there.

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So there's our function.

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Takes in an integer.

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If the integer is odd,

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it returns the string "Number is odd."

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If the input integer is even

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it returns the string "Number is even."

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And that's it.

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So that's the function
that we're going to pass

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to our constructor when we instantiate

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an object of my class type.

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Now the last thing we need to do is

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implement our main function int main,

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and that's gonna return zero.

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And then we're gonna
instantiate my class object.

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My class, we'll call it MyClassObject.

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And we have to pass in the functions.

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Now here we go, a function.

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So we're not passing in a variable.

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We're not passing in an integer

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or a string,

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we're passing in this function.

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And so now,

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what did I do here?

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I don't need those braces.

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Silly me. Sorry.

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So now let's exercise our code.

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Standard string S equals
MyClassObject demo.

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I'm gonna call that demo function.

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And demo, as you remember,

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is just gonna return

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the value that we pass in
applied to function, so 42.

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And that should return,

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"Number is even."

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So string will become "Number is even."

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And then we'll print that out, std cout,

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std endl.

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And then let's pass it an odd number

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and verify that it returns
the string "Number is odd."

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So just gonna do odd number three.

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So when we run this program,

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it should print out,

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"Number is even,"

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"Number is odd."

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And now let's run pass function.

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Oh, dear.

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I have a typo. Sorry.

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I see where it is right away.

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I need another pointy bit here.

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All right. Sorry.

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Just run it again.

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Oh, dear.

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Redefinition of S,

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look at me making mistakes
all over the place.

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Sorry about this.

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All right. Finally.

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"Number's even," "Number's odd."

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So again, we've created a class.

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The class has a private
member that holds a function.

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We pass a function to the constructor

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that becomes incorporated
into our class object

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when we instantiate.

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We can use that function
anywhere within our class

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if we like,

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and that's what we're gonna do
when we come to hash tables.

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We can also call it from
the outside to test it.

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Here's the function that we passed in

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and here's our code to exercise it.

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That's all there is to it.

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And we'll use this when
we implement hash tables.

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That's all for now.