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

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

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…..

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.

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

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?

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.

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

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

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

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

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”

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

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.

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?

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

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

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;

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?

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

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();

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

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.

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.

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

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.

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

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

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

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