jbode

 Profile Answers by jbode Questions by jbode

• Dec 27th, 2010

 

The only well-defined way to change the value of a const-qualified variable is to also declare it volatile and let some external system change the value. The whole point behind declaring a variable as "const" is that you explicitly do not want the value to be changed by the code.

Tricks like

const int x = 5;
int *y = (int *) &x;
*y = 10;

Invoke undefined behavior; any result is possible. 
If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined. If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type, the behavior is undefined.

m_yesayan

 Profile Answers by m_yesayan 

• Dec 29th, 2010

 

I think like that

    const int a = 5;
    int * pA = (int *)&a;
    *pA = 0;

Jegan Chakkravarthy

 Profile Answers by Jegan Chakkravarthy Questions by Jegan Chakkravarthy

• Jan 7th, 2011

 

The aim of defining a constant value in C is to prevent changing its value. I don't think you can possibly change the value>

One abnormal case may be:

const int a=10;-->const int a=5;

ie change the value in the initialization statement!!!!

Jegan Chakkravarthy

 Profile Answers by Jegan Chakkravarthy Questions by Jegan Chakkravarthy

• Jan 7th, 2011

 

The value of a constant variable cannot be changed in the middle of the program. The only possible way is to change it is at the initialization statement. I don't think we can change it elsewhere!

Praneeth

• Aug 25th, 2011

 

No the constant can be changed by using the pointer. Initialize a pointer to point to the value of a and then change the value using the pointer. It works.....
Try this:

Code
#include <stdio.h>


int main()


{


    const int a = 5;


    printf("%d",a);


    int *k = (int *)&a;


    *k = 10;


    printf("%d",*k);


    printf("%d",a);


    return 0;


}

Praneeth

• Aug 25th, 2011

 

It works as said by the first two members. Try this :

Code
#include <stdio.h>


 


int main()


{


    const int a = 5;


    printf("%d",a);


    int *k = (int *)&a;


    *k = 10;


    printf("%d",*k);


    printf("%d",a);


    return 0;


}


 

Arup

• Aug 27th, 2011

 

@Praneeth,
how the line
int*k=(int *)&a;
executes....can u explain me...it'll be helpful for me??

OCEN Blue

 Profile Answers by OCEN Blue 

• Aug 29th, 2011

 

You can't assign new value but you can increment or decrement in this constant variable.

jbode

 Profile Answers by jbode Questions by jbode

• Aug 30th, 2011

 

@Praneeth: unfortunately, that code invokes undefined behavior, as per the following section from the C language standard:

6.7.3 Type Qualifiers
...
5 If an attempt is made to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type, the behavior is undefined. If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type, the behavior is undefined.115)

Furthermore, a const-qualified object may be placed in a read-only data segment, as per footnote 114:

114) The implementation may place a const object that is not volatile in a read-only region of storage. Moreover, the implementation need not allocate storage for such an object if its address is never used.

Your code may generate the result you expect on your system, but that's a matter of luck, not design. You can't guarantee that you'll get the same result on a different system.

Sudhir Venugopal

• Aug 31st, 2011

 

Thought so! You can change the memory value but you can not add the amount to a const variable

Code
#include <stdio.h>


 


Main{


const my_const = 1;


printf("%d


", my_const);


/*the constant has 1 as value; the value can not be changed, but the const can be traversed through various memory addresses to confuse the reader, programmer*/


my_const *int = (int *) my_const;


my_const++;


printf("%d


", my_const);


}

The first const value is 1;

The second const value is a memory address; Thanks

sapan

• Sep 8th, 2011

 

Code
#include<stdio.h>


#include<conio.h>


 


int main()


{


const int a=10;


int *p=&a;


printf("before modification a=%d


",a);


*p=12;


printf("after modification a=%d",a);


getch();


 


  return 0;


}


 

somgupta

 Profile Answers by somgupta 

• Sep 10th, 2011

 

we can not change the constant value