Why Can't I Convert a Pointer-to-Pointer to Non-Const to a Pointer-to-Pointer to Const in C ?

Converting Pointers: Why Restrictions on Pointer-to-Pointer Conversions?

In C , it's generally permissible to cast a pointer to a non-const type to a pointer to a const type. This follows the principle of "const-correctness," allowing only constant data to be modified.

However, a peculiar exception arises when dealing with pointers to pointers. Unlike the aforementioned conversion, it's prohibited to convert a "pointer to pointer to non-const" to a "pointer to pointer to const."

Consider this code snippet:

char *s1 = 0;
const char *s2 = s1; // OK...
char *a[MAX]; // aka char **
const char **ps = a; // error!

Why is the assignment of a to ps illegal? The answer lies in the standard:

const char c = 'c';
char* pc;
const char** pcc = &pc;   // not allowed
*pcc = &c;
*pc = 'C';                // would allow to modify a const object

In this example:

1. c is a constant character.
2. pc is a non-const pointer to a character.
3. Attempting to initialize pcc (a pointer to a pointer to const) with &pc is disallowed.

If this conversion were allowed, it would enable modifications to constant objects. For instance, *pcc would point to a const character, but *pc would be modifiable. This violates the premise of const-correctness, as it potentially permits changes to constant data.

Therefore, the restriction on converting a pointer to pointer to non-const to a pointer to pointer to const stems from the need to maintain the integrity of constant objects and prevent unintended data modifications.

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