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Monday, June 04, 2007

Learn to use Pointers in C++

For those of you who do not know what pointers are, I give you a short definition. Pointer is a type of variable that stores memory addresses rather than simple data. Pointers also have Data Types associated with it, Ex. int pointers can only store memory address of int variables and similarly float pointers can only store memory address of float variables.

Pointers are one of the most important distinct features of C++. It gives the programmers power and control over several things.

Pointers have many uses; sometimes it is used as the faster means of accessing arrays while sometimes it is implemented so that the function in C++ can modify the parameters passed to it.

Pointers are extensively used in C++, this might not be obvious to you right now but as you start programming some complex program, you will notice how important pointers are.

This article would serve as a little introduction to pointers in C++, it would help you understand what pointers are and how it is used in C++.

Declaration of Pointer: Syntax: data_type *pointer_name; Please note that the * here is NOT the same as the 'value at' operator discussed below. Example: int *p;

The & Operator: As we know that every variable has two things associated with it 1)value 2) memory address of the variable. The & operator returns the memory address of a variable.It is known as the ‘address of’ operator. Example: int a; int *p; p=&a;//stores the memory address of variable 'a'

Increment/Decrement Operation on pointers: Like regular variables pointers can also be incremented/decremented If we increment a variable having value 10 then its new value will be 11, but if we increment a pointer it will point to the next memory location of its type. It is illustrated in the second program. The * Operator: It is known as the ‘value at’ operator. It returns the value at the memory address pointed by a pointer. The uses of these two operators are illustrated below:

   //C++ program to illustrate the use of * and & operators
   #include<iostream.h>
   void main(void)
   {
   int a;
   int *p;
   p=&a;//'&' is known as the 'address of'
   //operator it returns the memory address of
   //the variablein this case it is returning
   //the memory address of integer variable a
   cout<<"enter any number: ";
   cin>>a;
   cout<<endl<<"address of variable a: "<<p;
   cout<<endl<<"value at "<<p<<" "<<*p;
   cout<<endl<<"enter another number: ";
   cin>>*p;
   cout<<endl<<"value of *p: "<<*p;
   cout<<endl
   }

Have a look at the program below. In this program you’ll see how arrays are accessed using pointers.

    //C++ program to show how array elements
   //are accessed with the help of pointers
   #include<iostream.h>
   void main(void)
   {
   int arr[5]={4,8,9,2,0};
   int *p;
   p=&arr[0];//'&' is known as the 'address of'
   //operator it returns the memory address
   //of the variable in this case it is returning
   //the memory address of array arr[0]
   cout<<"address of arr[0]: "<<p;
   cout<<" value at address"<<p<<" "<<*p;
   cout<<endl;
   p++;//now 'p' is pointing to the next element
       //of the array i.e. 8
   cout<<"address of arr[1]: "<<p;
   cout<<" value at address"<<p<<" "<<*p;
   cout<<endl;
   p++;
   cout<<"address of arr[2]: "<<p;
   cout<<" value at address"<<p<<" "<<*p;
   cout<<endl;
   p++;
   cout<<"address of arr[3]: "<<p;
   cout<<" value at address"<<p<<" "<<*p;
   cout<<endl;
   p++;
   cout<<"address of arr[4]: "<<p;
   cout<<" value at address"<<p<<" "<<*p;
   cout<<endl;
   }

Pointer in C++ is a very vast topic (it has separate books written on it) so we will be discussing it in detail in the coming articles (knowing that the discussion on pointer once started, never gets end!)

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