Basic of Pointers - Objective C

Pointers in Objective C are easy and fun to learn. Some Objective C programming tasks are performed more easily with pointers, and other tasks, such as dynamic memory allocation, cannot be performed without using pointers. So it becomes necessary to learn pointers to become a perfect Objective-C programmer. Let's start learning them in simple and easy steps.

As you know, ever variable is a memory location and every memory location has its address defined which can be accessed using ampersand(&) operator, which denotes an address in memory. Consider the following example, which will print the address of the variables defined.

#import <Foundation/Foundation.h>
int main()
{
      int var1;
      char var2[10];
      NSLog(@"Address of var1 variable:%x\n", &var1 );
      NSLog(@"Address of var2 variable :%x\n", &var2 );
      return 0;
}

So, you understood what is memory address and how to access it, so base of the concept is over. Now let us see what is a pointer.

What Are Pointer?

A pointer is a variable whose value is the address of another variable, i.e. direct address of the memory location. Like  any variable or constant, you must declare a pointer before you can use it to store any variable address. The general form of a pointer variable declaration is:
  

type *var-name;

Here, type is the pointer's base type, it must be a valid Objective C data type and var-name is the name of the pointer variable. The  asterisk (* ) you used to declare a pointer is the same asterisk that you use for multiplication. However,in this statement the asterisk is being used to designate a variable as a pointer. Following are the valid pointer declaration:

#import <Foundation/Foundation.h>
int  *ip;          // pointer to an integer

double *dp;   //pointer to a double

float *fp;       //pointer to a float

char *ch;      // pointer to an character

The actual data type of the value of all pointers, whether integer, float, character or otherwise, is the same, as long hexadecimal number that represents a memory address. The only difference between pointer of different data type is the data type of the variable or constant that the points to.

How to use Pointers?

There are few important operations, which we will do with the pointers very frequently. 

• We define a pointer variable.
• assign the address of a variable to a pointer and ,
• finally access the value at the address available in the pointer variable

 This is done by using unary operator * that return the value of the variable located at the address specified by its operand. Following makes use of these operations:

#import <Foundation/Foundation.h>
int main()
{
      int var = 20;    //actual variable declration
      int *ip ;           //pointer variable declaration
      ip = &var ;      //store address of var in pointer variable

      NSLog(@"Address of var variable:%x\n", &var );

      //address stored in pointer variable
      NSLog(@"Address stored in ip variable :%x\n", ip);

      //access the value using the pointer
      NSLog(@"Value of *ip variable :%d\n", *ip );
      return 0;
}

NULL Pointer in Objective C 

It is always a good practice to assign a NULL value to a pointer variable in case you do not  have exact address to be assigned. This is done at the time of variable declaration. A pointer that is assigned NULL is called a null pointer.

The NULL pointer is a constant with a value of zero defined in several standard libraries. Consider the program:

#import <Foundation/Foundation.h>
int main()
{
      int  *ptr1 = NULL;
  
      NSLog(@"The value of ptr is :%x\n", ptr );

     return 0;
}

 On the most operating systems, programs are not permitted to access at address 0 because that memory is reserved by the operating system. However the memory address 0 has special significance, it signals that the pointer is not intended to point to an accessible memory location.But by convention, if a pointer contains the null value, it is assumed to point to nothing.

To check for a null pointer, you can use an if statement as follows:

if ( ptr )            //succeeds if p is not null
if ( !ptr )          //succeedes if p is null

 Objective C pointer in detail in next post....