pointers and const variable

Question

int *const plz;

means that I will not change where the pointer is pointing to, (ie increment or decrement the address)

const int *plz

means I will not change the variable the pointer is pointing to through the pointer

const int* const *plz

means both

I got a question

I just saw a function which looks like this

check_plz(const int *const plz)

what exactly does this mean, other than that the address cannot be incremented or decremented, if it also means that I cannot change the variable, why is the second * operand missing? Thank you

const int* const *plz is a double pointer and const int *const plz is a single pointer. So these are very different indeed. const int* const *plz is the odd one out. You need to count the number of asterisks. — David Heffernan
– David Heffernan, Commented Jan 5, 2015 at 11:04
http://www.thegeekstuff.com/2012/06/c-constant-pointers/ refer this — Karthikeyan.R.S
– Karthikeyan.R.S, Commented Jan 5, 2015 at 11:04
@Dodu no that is not the same question, I also saw that question — Mcs
– Mcs, Commented Jan 5, 2015 at 11:07

David Ranieri · Accepted Answer · 2015-01-05 11:27:15Z

1

const int *const plz

Here plz is a constant pointer to a constant int variable

The below example might help you

const int *const *plz

Here plz is a double pointer so it can hold the address of a pointer.

#include <stdio.h>

int main(void) {
    const int a=10;
    const int *const p = &a;
    const int *const *q = &p; 
    printf("%d\n",*p);
    printf("%p\n",(void *)p);
    printf("%p\n",(void *)*q);
    printf("%d\n",**q);

    return 0;
}

So with this each of your used variables like p q a are just read-only now.

edited Jan 5, 2015 at 11:27

David Ranieri

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answered Jan 5, 2015 at 11:07

Gopi

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3 Comments

Mcs Over a year ago

so const int *const *ptr is a constant pointer to a variable but it is not allowed that we change the variable through the pointer. Const int const *ptr is a constant pointer to a constant variable

Dirk Koopman Over a year ago

Does one really need to use const with such gay abandon as in the examples above. We are writing in C, not C++. I cannot remember a time in the last 30 years when a const keyword would have saved me some debugging time, whereas overuse of const by others has caused much anquish (and occasionally #define const) to sort out problems.

Gopi Over a year ago

@DirkKoopman Yes coming to the usage of it I agree . But for understanding the above example is good IMO

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