Pointers and Constants: A Comprehensive Guide to Pointer Constants and Constant Pointers in C

Introduction:

In the intricate world of C programming, pointers and constants play pivotal roles. However, distinguishing between pointer constants and constant pointers can be a perplexing endeavor. This article aims to shed light on the differences between these two constructs, empowering you with a profound understanding of their usage and application.

Pointer Constant:

A pointer constant, denoted as int *const p;, signifies a constant pointer variable. This means that the pointer itself cannot be reassigned to point to a different memory location, even though the value stored at the pointed memory address can be modified.

Key Points:

• The asterisk (*) precedes the variable name, indicating that it is a pointer.
• The const keyword follows the asterisk, making the pointer constant.
• The value pointed to by the constant pointer can still be altered.

Constant Pointer:

A constant pointer, written as const int *p;, denotes a pointer to a constant variable. This implies that the value stored at the memory address pointed to by the pointer cannot be modified, although the pointer itself can be reassigned to point to a different constant variable.

Key Points:

• The const keyword precedes the asterisk, modifying the variable pointed to.
• The asterisk follows the const keyword, indicating that it is a pointer.
• The pointer can be reassigned to point to a different constant variable.

Remembering the Difference:

A simple mnemonic to remember the difference is:

• Pointer Constant: Constant pointer, variable value (the pointer cannot change, but the pointed value can).
• Constant Pointer: Variable pointer, constant value (the pointer can change, but the pointed value cannot).

Example:

// Pointer Constant
int *const p = #
*p = 20; // Allowed (modifies the pointed value)
p = &num2; // Error (replaces the pointer)

// Constant Pointer
const int *p = #
*p = 20; // Error (modifies the pointed value)
p = &num2; // Allowed (reassigns to a different constant variable)

Conclusion

Mastering the distinction between pointer constants and constant pointers is essential for navigating the complexities of C programming. By understanding their individual characteristics and applying them judiciously, you can harness the power of pointers to manipulate data and memory with precision and confidence.