1. C++ Programming Concepts
Lecture 3
Pointers in C/C++

2. Introduction Basic Concepts Declaring, pointing and dereferencing.
“Ordinary” variables
Declaring, pointing and dereferencing.
Passing by value
Classes and Objects
Structures
Array
Linked List

3. Some Basic Concepts - 1
Should all be familiar with “ordinary” variables declarations.
Such as….
int nCount;
double dBalance;
char chLetter;
char arName[50];
and so on…..

4. Basic Concepts - 2 Should also be familiar with assignments.
Such as …..
nCount = 5;
dBalance = 12.36;
chLetter = ‘Z’; (note single quotes);
arName = “David D. Hodgkiss”; ??????
This is wrong – the compiler will complain
Discuss this later.

5. Declare and Assign Should also be familiar with….. int nCount = 4;
double dBalance = 12.36;
char cLetter = ‘Z’;
char arName[] = “David D. Hodgkiss”;

6. Why pointers? Consider this code (abridged)
int main(void) {
a = 1; b = 2;
swap(a, b);
cout << a << b << endl; }
void swap(int a, int b)
int temp;
temp = a;
a = b;
b = temp;
When ‘a’ and ‘b’ printed after
swap result will be….
a = 1;
b = 2;
Why?

7. Passing values. C does not pass the variable to functions.
It passes the value that the variable holds.
Within the function new variables are created.
This is known as “Pass by Value”
In “swap” – it is only the variables within swap() that have their values swapped.
Variables lost when function returns.

8. How can we swap? Using pointers. Pointers Declaration
Are themselves variables
That hold a memory address
instead of a variable value
Declaration
int *pInt;

9. Reading the declaration
int *pInt;
Break the declaration down
“pInt” The pointer’s name
“*” Indicating it is a pointer
“int” Type of pointer
So….
“pInt” is a pointer to an integer

10. Reading the declaration
int *pInt;
Break the declaration down
“pInt” The pointers name
“*” Indicating it is a pointer
“int” Type of pointer
So….
“pInt” is a pointer to an integer

11. Other declaration examples
double *pDouble;
char *pChar;
int *pMyPointer;
long *pYourPointer;
and so on

12. Using pointers The pointer variable holds an address location.
From whence do’th this address derive?
Consider….
int nCount; // an integer variable
int *pInt; // a pointer to an integer
Requirement – make pInt “point” at “nCount”

13. Finding the address We can use the “&” character (address of) So….
pInt = & nCount; // spaces added for clarity
pInt equals the address of nCount

14. Getting at the value How do we get a value via the pointer?
Using the ‘*’ character again.
To “dereference” the pointer.
cout << *pInt << endl;
output the value at which “pInt” is pointing.

15. More accessing myInt = *pInt; arInts[5] = *pInt; *pInt = nCount;
*pA = *pB; //is this the same as pA = pB?
You should experiment with these concepts
myInt = nInt; // What would happen here?

16. Back to swap How can me make swap work correctly? By using pointers!
Instead of swap receiving variable values
Pass pointer values to it.

17. Swapping with pointers
swap(&a, &b); // this is in “main”
void swap(int *pA, int *pB) { int temp; temp = *pA; *pA = *pB; *pB = temp; }

18. What about classes Consider that we have a class called “Account”
Could instantiate using
class Account Fred;
Could also use pointers
First create a pointer to a class of type Account
class Account *pAcc;
Now instantiate
pAcc = new Account;

19. pAcc = new Account; “pAcc” is a pointer “new” is a C++ keyword So
It allocates a block of memory
and
passes (returns) the location (address) of that block (containing an Account Object) to pAcc So
pAcc is pointing at the Account object.
what if “new” fails to allocate?

20. Interacting with the object
When using a pointer we do not use the “dot” notation.
We use an arrow “->”
That consists of ….
a “dash”
followed by a
“greater than” symbol

21. Interacting with the object
Remember pAcc = new Account;
To interact use……
pAcc->SetBalance(12.36);
pAcc->SetIntRate(3.4);
dBal = pAcc->GetBalance();
dInt = pAcc->GetIntRate();

22. Big deal – what good is it
Let’s have an array
of pointers
Account *arAccounts[10];
An array called “arAccounts”
Has 10 elements
Each of which is……
A pointer to……
An object of type Account

23. Handling objects via an array
Some code …..
Account *arAccounts[10];
for(nCount = 0; nCount < 10; nCount++)
arAccounts[nCount] = new Account;
That will create an array of pointers to ten separate Account objects.

24. Accessing via an array arAccounts[1]->SetBalance(12.36);
arAccounts[1]->SetIntRate(5.36);
dBal = arAccounts[1]->GetBalance():
dInt = arAccounts[1]->GetIntRate();
By using an array we need not find names for each instantiation.

25. What about strings char arName[50];
The actual array name is a pointer.
To copy one “string” to another.
E.g. contents of arName[] to arCustomer[]
?? arCustomer = arName
?? arCustomer[] = arName[] Neither of these will work

26. Copying and manipulating strings
C provides a number of functions to copy strings
Copy strcpy(……..)
Concatenate strcat(…….)
Compare strcmp(……)
Length strlen(……) All of these use pointers.

27. C++ & Strings C++ can handle strings as objects
We will be looking at string handling classes in a later lecture.
It hides the implementation!

28. Linked Lists Excellent for handling an unknown number of objects.
ptrHead
prtObject
ptrNext
Account Object
prtObject
ptrNext
Account Object
prtObject
ptrNext
Account Object
???

29. Further investigation
“this” is a pointer
To what?
Background reading
Test by developing some code

30. Summary Some basic concepts Apply to pointer declaration
Finding the address of a variable
Dereferencing the pointer
Passing by reference – using pointers
Handling objects
Arrays
Linked Lists