Pointer in C++

Introduction to pointer

  • Pointers are variables used to store the address of another variable.
  • It is an extremely powerful programming tool.
  • It helps to improve program's efficiency and allows to handle unlimited amount of data.
Syntax of Pointer declaration:
datatype * ptr_name;

Address of operator (&)

  • Address of operator is called as the Referencing operator.
  • The ampersand (&) is the referencing operator that gives the address of that variable.
  • It returns the address of the variable associated with the operator.
  • Address is displayed in hexadecimal form.
  • Address of variable is the memory location number alloted to the variable.

    • Example:
    int *p , a;
    a=10;
    p=&a;

  • In the above example, &a returns the address of the variable a. p is a pointer which points to a variable a.
  • p=&a means "copy the address of the variable a in the pointer variable p".

Value of operator (*)

  • Value of operator is called as the De-referencing operator.
  • It returns the value stored in the variable pointed by the specified pointer.
  • The asterisk (*) is a dereference operator which means "value pointed to by".
  • In the above example, the *p will return the value of the variable pointed by the pointer p.
  • a=*p means that the value of the variable pointed by the pointer p is stored in the variable a.

Example : Demonstrating the referencing & de-referencing using pointers

#include <iostream>
using namespace std;
int main()
{
     int *p, a;
     a = 10;
     cout<<"Address of a (&a): "<<&a<<endl<<endl;
     cout<<"Value of a (a): "<<a<<endl<<endl;
     p = &a;         // Pointer p holds the memory address of variable a
     cout<<"Address that pointer p holds (p): "<<p<<endl<<endl;
     cout<<"Content of the address pointer p holds (*p): "<<*p<<endl;
     a = 20;          //The content inside memory address &a is changed from 10 to 20
     cout <<"Address pointer p holds (p): "<<p<<endl<<endl;
     cout <<"Content of the address pointer p holds (*p): "<< *p << endl;
     *p = 5;
     cout<<"Address of a (&a): "<<&a<<endl<<endl;
     cout<<"Value of a (a): "<<a<<endl<<endl;
     return 0;
}


Output:
Address of a (&a): 0x7ffc99a8e9a4
Value of a (a): 10
Address that pointer p holds (p): 0x7ffc99a8e9a4
Content of the address pointer p holds (*p): 10
Address pointer p holds (p): 0x7ffc99a8e9a4
Content of the address pointer p holds (*p): 20
Address of a (&a): 0x7ffc99a8e9a4
Value of a (a): 5

  • In the above program, when a=10; the value 10 is stored in the address of variable a.
  • When p=&a; the pointer p holds the address of a and the expression *p contains the value of that address which is 10;
  • When a=20; the address that pointer p holds is unchanged, but the expression *p is changed because the address &a (which is same as p) contains 20.
  • When *p=5; the content in the address p which is equal to &a is changed from 20 to 5. Since the pointer p and variable a has address, value of a is changed to 5.
  • 0x in the beginning represents the address is in hexadecimal form.
Note: You may not get the same address as an output on your system.
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