Why is Casting a 'Pointer to Pointer to Non-Const' to a 'Pointer to Pointer to Const' Forbidden in C?

Pointer Promotion Puzzle: Why the Transformation from "Pointer to Pointer to Non-Const" to "Pointer to Pointer to Const" is Forbidden

In the world of C programming, where pointers play a crucial role, it is known that casting a pointer to non-const to a pointer to const is permissible. However, an attempt to perform the inverse operation – converting a "pointer to pointer to non-const" to a "pointer to pointer to const" – yields a compilation error. Why does this seemingly straightforward conversion fail?

To understand the reason behind this restriction, we turn to the C standard itself. The standard explicitly states that "it is not allowed" to cast a "const char*" to a type of "char". This constraint stems from the potential for a dangerous modification:

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

This example demonstrates how the illegal casting could lead to an unexpected change in the value of a const object. By initializing a char variable pc and assigning it the address of the const char variable c, the programmer intends to make pc point to an immutable value. However, if the casting to "const char*" were allowed, *pcc could be modified to point to another location in memory, allowing the contents of *pc to be altered. This violates the intended immutability of c.

Therefore, to safeguard against such modifications, the C standard prohibits the conversion from a "pointer to pointer to non-const" to a "pointer to pointer to const". This restriction ensures that const objects remain protected, preventing unexpected modifications and maintaining the integrity of the program's data.

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