Pointer

To create linked list in C/C++ we must have a clear understanding about pointer. Now I will explain in brief what is pointer and how it works.

A pointer is a variable that contains the address of a variable. The question is why we need pointer? Or why it is so powerful? The answer is they have been part of the C/C++ language and so we have to use it. Using pointer we can pass argument to the functions. Generally we pass them by value as a copy. So we cannot change them. But if we pass argument using pointer, we can modify them. To understand about pointers, we must know how computer store variable and its value. Now, I will show it here in a very simple way.

Let us imagine that a computer memory is a long array and every array location has a distinct memory location.

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int a = 50 // initialize variable a 

Figure: Variable value store inside an array

It is like a house which has an address and this house has only one room. So the full address is-

Name of the house: a

Name of the person/value who live here is: 50

House Number: 4010

If we want to change the person/value of this house, the conventional way is, type this code line

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a = 100    // new initialization 

But using pointer we can directly go to the memory location of 'a' and change the person/value of this house without disturbing ‘a’. This is the main point about pointer.

Now the question is how we can use pointer. Type this code line:

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int *b;    // declare pointer b

We transfer the memory location of a to b.

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b = &a;    // the unary operator & gives the address of an object

Figure: Integer pointer b store the address of the integer variable a

Now, we can change the value of a without accessing a.

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*b = 100;  // change the value of 'a' using pointer ‘b’
  cout<<a;  // show the output of 'a'

When you order the computer to access to access *b, it reads the content inside b, which is actually the address of a then it will follow the address and comes to the house of a and read a`s content which is 50.

Now the question is, if it is possible to read and change the content of b without accessing b? The answer is affirmative. We can create a pointer of pointer.

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int **c;   //declare a pointer to a pointer
 c = &b;    //transfer the address of ‘b’ to ‘c’

So, we can change the value of a without disturbing variable a and pointer b.

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**c = 200;  // change the value of ‘a’ using pointer to a pointer ‘c’
  cout<<a;  // show the output of a