CS3240: Data Structures and Algorithms

Pointers

A pointer variable is a variable whose value is the address of a location in memory. To declare a pointer variable, you must specify the type of value that the pointer will point to. For example,

int*   ptr;   //ptr will hold the address of an int

char*  q;     //q will hold the address of a char

Getting Pointer to a Variable

The address of a non-array variable can be obtained by using the address-of operator &.

using namespace std;
int     x;
float   number;
char    ch;

cout << “Address of x is “ << &x << endl;

cout << “Address of number is “ << &number << endl;

cout << “Address of ch is “ << &ch << endl;

Array name is a Pointer

char msg [ 8 ];

msg is the base address of the array.

• We say msg is a pointer because its value is an address.
• It is a pointer constant because the value of msg itself cannot be changed by assignment. It “points” to the memory location of a char.

Using Pointers....An Example

int  x;
 x = 12;


 int*  ptr;
 ptr = &x;

 *ptr = 5;   // changes the value at adddress ptr to 5
           

Some Pointer Operations

Dangling Pointer

occurs when two pointers point to the same object and delete is applied to one of them

Example Allocation and Deallocation using new and delete

using namespace std;

Node *ptr = new Node; // where Node is a class.

...

delete ptr; // Later on we get rid of the allocated memory pointed to by ptr

PROBLEM -- you CAN NOT delete statically created items

Node abc; //this is "statically created in memory called the node constructor
Node *abc_ptr = &abc;

free(abc_ptr); //PROBLEM !! can not do

Special Note about new/delete and malloc/free

• new/delete is preferred to malloc()/free() because it can initialize the memory
  and it invokes the constructor for new objects.
• new/delete also point to the correct memory type.
• Note on mixing source code containing new/delete and malloc()/free(). This is not a problem with the GNU C++ compiler but this is not guaranteed for all C++ compilers. BUT--why do it, don't