Question

How to enforce immutability through pointers to prevent the modification of values in C

Answer 1

#include <stdio.h>

// pointer can change, but the data cannot.

int main() {
    const int value = 5;      // immutable data
    const int *ptr = &value;  // pointer to const int (data cannot change)

    printf("Value through ptr = %d\n", *ptr);

    // *ptr = 10;  // ERROR: cannot modify data through pointer

    int other = 20;
    ptr = &other;  // pointer itself can change

    printf("Now ptr points to %d\n", *ptr);

    return 0;
}



/*
run:

Value through ptr = 5
Now ptr points to 20

*/

 

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Answer 2

#include <stdio.h>

// pointer cannot change, but the data can.

int main() {
    int value = 5;
    int * const ptr = &value;  // const pointer to int (pointer cannot change)

    printf("Value = %d\n", *ptr);

    *ptr = 10;  // ✔ allowed: data is mutable
    printf("Modified value = %d\n", *ptr);

    // ptr = NULL;  // ERROR: cannot change a const pointer

    return 0;
}



/*
run:

Value = 5
Modified value = 10

*/

 

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Answer 3

#include <stdio.h>

// Neither the pointer nor the data can change.

int main() {
    const int value = 5;

    // Fully immutable: pointer cannot change, data cannot change
    const int * const ptr = &value;

    printf("Value = %d\n", *ptr);

    // *ptr = 10;  // ERROR: data is read-only
    // ptr = NULL; // ERROR: pointer is read-only

    return 0;
}


/*
run:

Value = 5

*/

 

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