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Pointer and const

Discussion in 'C' started by Rajesh M. Kanojia, Dec 9, 2012.

  1. Rajesh M. Kanojia

    Rajesh M. Kanojia New Member

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    Dear members,
    can any one tell me what exactly meaning of:-
    • Variable constant( const <data type> variable_name);
    • pointer to a constant
    • constant pointer
     
  2. IndiraP

    IndiraP New Member

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    constant variable
    say
    const int a=10;
    then u cant change a to any other value... ie.,
    a=11; would be an error..

    constant pointer

    int i=10,j;
    int *const a=&i;
    then that pointer is a constant pointer to that memory location..so that pointer can't be used to point to other location...
    a=&j; //would be an error

    pointer to a constant

    int i=10;
    const int *p=&i;
    then the value pointed to by the pointer cant be changed.
    *p=11; would be an error..
     
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  3. Rajesh M. Kanojia

    Rajesh M. Kanojia New Member

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    dear IndiraP,
    i agree with u what u are saying about constant variable, but i get confused when i access constant variable through a pointer, it not only allow me to access the value but through this i can also modify the value of constant variable.

    main()
    {
    const int a=10;
    a=11; // generates error
    int *ptr;
    ptr =&a;
    *ptr =11;
    printf("\n Now value of constant variable a is : %d",a);
    getch();
    return 0;
    }

    plz tell me how variable a become constant.
     
  4. IndiraP

    IndiraP New Member

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    see...wat do u mean by constant variable first of all???

    u r actually making a variable hold a constant value in it instead of allowing it to handle multiple varibale values..rite..???

    now in the program u mentioned..u made "a" variable constant..means "a" can only hold 10...so it can't take any other value not even 11..!!!

    n when u created a pointer for it...look carefully...u created a pointer to "a" which is constant n which holds only 10 ..rite???

    it means.. a pointer to a constant...so wat happens..even the pointer can't change the value that it is pointing to...afterall "a"-->can only hold 10....n when u r manipulating *p..u r actually trying to change "a'"s value ..so will it allow.??? no ...rite??

    n "a" is nothing but the name of the memory location where 10 resides...so p=&a..makes p point to the memory location of "a"...n *p is value at &a...where that memory is locked to hold only 10 by specifying it constant...so *p=11 is error..


    n refer to pointer to constant..for any better understanding..

    got it???
     
    Last edited: Dec 12, 2012
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  5. Rajesh M. Kanojia

    Rajesh M. Kanojia New Member

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    Dear Indira P
    I got u and also agree with u but first copy the below program and run then see what i am asking:-
    Code:
    #include<stdio.h>
    #include<conio.h>
    int main()
    {
     	const int a=10;
     	int *ptr;
     	ptr=&a;
     	printf("\n Now the value at memory : %u  is : %d  ",&a,a); 	
     	*ptr=11;
     	printf("\n Now the value at memory : %u  is : %d  ",&a,a);
     	getch();
     	return 0 ;
    }
     
    Last edited by a moderator: Dec 13, 2012
  6. IndiraP

    IndiraP New Member

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    yeah..i did run it..n u get error at p=&a; stating that "const int * can't be converted into int * "
    rite???..
    it's because...tha variable is constant...so the pointer that it points to becomes pointer to a constant..rite???

    but *p is declared as normal integer...so in order to eliminate the error make *p a const int *p;

    and coming to *p=11...u can't change the value anyway..!!!
     

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