CPP08 - Pointers

Introduction
• One of the trickiest things in C++ to get your heads around is
the topic of pointers.
• I want to introduce this subject early so it doesn’t become a huge
problem down the line.
• We don’t need to do an awful lot of pointer work at the
moment.
• We’ll need to do more in the future.
• Pointers eventually become second nature.
• Honest

Cube Calculator
#include <iostream>
using namespace std;
void calculate_cube (int num) {
num = num * num * num;
}
int main() {
int num = 5;
calculate_cube(num);
cout << "Cube is " << num << endl;
return 1;
}

Cube Calculator
• What will the output of this program be?
• We create a variable
• We pass that variable to our function
• We make that variable equal the cube of itself.
• It seems like the answer should be 125…
• … but it won’t be.
• Why?

Functions and Variables
• We have spoken about functions in an earlier lecture.
• And how we can provide information to functions by passing
parameters.
• When we do this, what we pass into the function is a copy of
the data we have.
• So we really have two versions of the same data in the program at
any one time.

Functions and Variables
• In our calculate_cube function, we are working with a copy of
num.
• We are never directly manipulating the value of num itself.
• This is known as passing by value.
• We pass the value of a variable, not the variable itself.
• Every variable in C++ is stored in the computer’s memory.
• Sometimes we want to be able to get to that memory location.
• We want to pass things by reference.

Pointers
• Here is where the pointer enters our discussion.
• It points to a memory location.
• Pointers come with two new syntax elements to learn.
• & which is the reference operator
• * which is the dereference operator.
• We use these to swap between the memory location and the
value of a memory location.

Reference Operator
• The & symbol is our reference operator.
• You can think of it as meaning ‘the address of’
• We can use this to say ‘I want to send a memory location into
this function’:
int main() {
int num = 5;
calculate_cube(&num);
return 1;
}

Dereference Operator
• * is the dereference operator
• Think of it as ‘the value of’
• We also use this to indicate that a variable is going to be a
pointer.
• When we accept the parameter we are given, we want to be
able to indicate we are working with a pointer.
• We need to change our function a little to accommodate that:

Our Program With Pointers
#include <iostream>
void calculate_cube (int *num) {
*num = *num * *num * *num;
}
int main() {
int num = 5;
calculate_cube(&num);
return 1;
}

How Do They Work?
• Every variable takes up a certain amount of memory.
• This varies from type to type.
• Each variable has a memory address.
• That’s how the computer knows where to find it in memory.
• A pointer is a location in memory that contains the memory
address of another location.
• This is known as indirection.

Alternative Syntax
#include <iostream>
void calculate_square (int &num) {
num = num * num;
}
int main() {
int num = 5;
int num2 = 10;
calculate_square (num2);
cout << "Square is " << num2 << endl;
return 1;
}

Do We Want Them?
• Yes and no.
• They are invaluable for doing certain things.
• They are overly complex for many requirements.
• Far better method in most cases to avoid the use of pointers
and make use of return values.
• This is not possible in all situations.
• When in doubt, try to think of a way to avoid using a pointer.

Why Use Pointers?
• Very efficient.
• Copying values, especially things like objects, is very costly.
• More on this in a later lecture.
• Permits functions that return more than one value.
• Canonical example of this is a function that swaps the contents of
two variables.
• Resolves some issues of data persistance.
• Introduces others along the way…

Why Not Use Pointers?
• Additional complexity of code.
• Code that uses pointers is almost always more complex.
• Easy to make mistakes.
• Pointers let you work with the live ammo of memory addresses.
• You can easily manipulate the memory location rather than the
memory contents.
• Permits unintended side effects.
• Largely negates the issue of scope.

Alas…
• Sometimes, we have to use pointers.
• Because C++ is built around the assumption we will be.
• We’ll see them used relatively freely when we talk about
objects in a couple of weeks.
• For various reasons, passing by value is not really appropriate for
dealing with objects.

Pointer Notation
• When we want to represent pointers in diagram form, we use
the notation below.
• This is a simple representation.
• We don’t worry about memory locations
• Just the relationship between variables.
*ptr Var

Pointers Example 1
int main() {
int *yPtr, *zPtr;
int y, z;
y = 10;
z = 5;
zPtr = &z;
yPtr = &y;
cout << "Y is " << y << endl;
cout << "yPtr is " << yPtr<< endl;
cout << "Z is " << z<< endl;
cout << "zPtr is " << zPtr<< endl;
return 1;
}

Pointers Example 2
int main() {
int *yPtr, *zPtr;
int y, z;
y = 10;
z = 5;
zPtr = &z;
yPtr = &y;
cout << "Y is " << &y << endl;
cout << "yPtr is " << *yPtr<< endl;
cout << "Z is " << &z << endl;
cout << "zPtr is " << *zPtr<< endl;
return 1;
}

Pointers Example 3
int main() {
int *yPtr, *zPtr;
int y, z;
y = 10;
z = 5;
zPtr = &z;
yPtr = zPtr;
*zPtr = 20;
cout << "yPtr is " << *yPtr<< endl;
cout << "zPtr is " << *zPtr<< endl;
return 1;
}

When Working With Pointers
• Always be clear of the relationship between variables.
• Draw diagrams if needed
• Always be sure of where your pointers are actually pointing.
• Remember, the * means ‘the value of my current memory
location’
• Be wary of side effects
• Very easy to introduce!

Summary
• C++ has pointers.
• Don’t sweat it too much just now…
• They are extremely powerful
• They let you do things you couldn’t really otherwise do.
• They are extremely dangerous.
• It’s very easy to lose control of your program by using them.
• Be wary!

CPP08 - Pointers

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CPP08 - Pointers