Pointers are very interesting aspects of C++ language. If you master this concept, it is useful in all the other topics, specially data structures and algorithms. An interesting part though, is the interaction of pointers with const keyword.


POINTERS & CONST

We have learnt to use const keyword for declaring symbolic constants. Using the same keyword, we can also declare constant pointers or pointers to constants.

 

A constant pointer means that the pointer in consideration will always point to the same address. Its address (to which it is pointing to) cannot be modified. A constant pointer must be initialized at time of creation. The address stored in that pointer at time of creation becomes its final value and it cannot be changed further. But the value at that location can be modified.

General syntax  :

int * const iptr = new int ();

See the use of keyword const and also that it is mandatory to initialize the constant pointer the moment it is created.

 

A pointer to a constant refers to a pointer which is pointing to a symbolic constant. Using a pointer to a constant, the constant value (to which this pointer is pointing to) can not be modified, however, the pointer can be made to point at some other address.

General syntax :

const int * iptr;

iptr = new int ();

See the use of keyword const. Unlike the previous case, a pointer to a constant need not be initialized at time of creation.

 

 

The following examples will make it clearer.


In the following lines, we shall declare four variables:

a pointer called ptr

a constant pointer called cptr

a pointer named ptrC, pointing to constant

a constant pointer cptrC pointing to a constant

 

int n = 44;                                    // an int

int *ptr = &n;                              // a pointer to an int

++(*ptr);                                     // ok : increment int*ptr

int *const cptr = &n;                // a const pointer to an int

++ (*cptr);                                 // ok: increments the contents

++ cptr;                                      // illegal : pointer cptr is constant

const int kn = 88;                     // a const int

const int *ptrC = &kn;            // a pointer to a const int

++ (*ptrC);                                // illegal : int *ptrC is constant

++ ptrC;                                     // ok : increments pointer ptrC

const int *const cptrC = &k;  // a const pointer to a const int   

++(*cptrC);                               // illegal : int *cptrC is const

++cptrC;                                    // illegal : pointer cptrC is const

 

 

 

 

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Related Topics :

Introduction to pointers

Static & Dynamic Allocation of memory

Declaration & Initialization of pointers

Pointer Arithmetic

Dynamic Allocation Operators

Memory Leaks