Constant Pointers

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Constant Pointer

File: const1.cpp

#include<stdio.h>

int main(void)
{
    int var1 = 0, var2 = 20;
    //Constant Pointer
    //The ptr is a constant . I cannot change the value of the pointer.
    int* const ptr1 = &var1;
    //Can't do this
    //ptr1 = &var2;
    //or this
    //int *const ptr2   ;
    //Can change the value of what the pointer points to

    *ptr1 = 100 ;
    printf("%d  %d\n", *ptr1, var1 );
    return 0;}
$ g++ const1.cpp ; ./a.exe
100  100

We have the concept of a constant pointer. It is defined as:

int* const ptr1

What this is saying is that the variable "ptr1" is a
constant and we cannot change it's value but we can change
the value of what it's pointing to.

So we cannot do this:

ptr1 = &var2

but we can do

*ptr1 = 100

Let us build a RAM diagram:

                                      RAM
                    var1 10            0     -->But we can change this value
                   ptr1  100          10    --> Can't change this value

We can't do :

int* const ptr2 ;

Since the variable ptr2 is a constant we must assign a value to
it once we define it. We can also have the other case of having
the ability to change a pointer but not change the value of
what it is pointing to.

Pointer to Constant

File: const2.cpp

#include<stdio.h>
int main(void)
{
    int var1 = 0, var2 = 20;
    const int* ptr1 = &var1;
    //Not allowed
    //*ptr1 = 1;
    //Allowed
    ptr1 = &var2 ;
    printf("%d\n", *ptr1) ;
    return 0;
}

The const is to the left of the pointer. This means the variable "ptr1" can be changed but the contents cannot be changed. Finally we have the case where the pointer or what the pointer points to cannot be changed.

Constant Pointer to Constant

File: const3.cpp

#include<stdio.h>

int main(void)
{

    int var1 = 0, var2 = 20;
    const int* const ptr1 = &var1;
    //Not allowed
    //*ptr1 = 1;
    //  Not Allowed
    //    ptr1 = &var2 ;
    return 0 ;

}

We cannot change the value of what the pointer points to :

*ptr1 =  1

and we cannot change the value of the pointer:

ptr1 = &var2

Constant pointers are usually used in arguments to
functions when we do not want the function to change
the original value of what the pointer points to.

An easy to remember the const and pointers is to read things from right to left.
const int* const ptr1
The above will read as constant pointer ( int* const ) to a constant. Constant pointers are usually used in arguments to functions when we do not want the function to change the original value of what the pointer points to.
File: const4.cpp

#include<stdio.h>
#include<iostream>
using namespace std ;

void function1( const char* str1 )
{     
cout << str1 << endl ;    
*str1 = "Something Else." ;
}

int main(void)
{   
char buffer[] = "Testing" ;  
function1( buffer ) ;
return 0;

}
const4.cpp: In function ‘void function1(const char*)’:
const4.cpp:8:7: error: assignment of read-only location ‘* str1’
    8 | *str1 = "Something Else." ;