Why Can't We Convert a Pointer-to-Pointer-to-Non-const to a Pointer-to-Pointer-to-const in C ?

Converting Pointer to Pointer to Const: Restrictions and Considerations

In C , while it is permitted to cast a pointer-to-non-const to a pointer-to-const, the reverse operation seems to be illegal. This raises the question: why is it not possible to convert a pointer-to-pointer-to-non-const to a pointer-to-pointer-to-const?

The answer lies in the semantics of pointer types. A pointer-to-const denotes a constant variable that cannot be modified. When we change the value of a pointer-to-pointer-to-non-const, we are effectively changing the value of the innermost pointer, which points to a non-const object. This would allow us to modify a const object indirectly, violating its immutability.

For example, the following code snippet illustrates the issue:

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

When we try to assign a pointer-to-pointer-to-a to a pointer-to-pointer-to-const, the compiler raises an error because the pointer arithmetic and dereferencing operations would allow us to modify a constant object. In the example above, the line *pc = 'C'; would modify the value of the string that is pointed to by s1, which is a const object.

To prevent such undesirable behavior, the C standard disallows the conversion of a pointer-to-pointer-to-non-const to a pointer-to-pointer-to-const. This ensures that the immutability of const objects is maintained and unintentional modifications are prevented.

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