What is the difference between Constant pointer and pointer on a constant value?

This article explores the understanding of pointers in C/C++, particularly the distinction between a pointer to constant and a constant pointer. Conceptually, they differ in functionality, primarily in terms of what they point to and whether the pointer itself can be reassigned.

1. Pointer to Constant: A pointer to constant is a pointer that points to a constant, meaning the data it points to cannot be modified through this pointer, but the pointer itself can be reassigned to point to other addresses. This type of pointer is commonly used for function parameters to ensure that the function does not modify the input data.

Example:

cpp
int value = 10;
int anotherValue = 20;
const int* ptr = &value;
// *ptr = 20; // Error: cannot modify data through constant pointer
ptr = &anotherValue; // Correct: constant pointer can point to another address

2. Constant Pointer: A constant pointer is a pointer whose value (i.e., the stored address) cannot be changed, but the data it points to can be modified. This type of pointer is suitable for scenarios where the pointer must be fixed to a specific data structure, but the content of that structure may change.

Example:

cpp
int value = 10;
int anotherValue = 20;
int* const ptr = &value;
*ptr = 20; // Correct: constant pointer can modify data
// ptr = &anotherValue; // Error: cannot reassign constant pointer

In summary, a pointer to constant protects the data content from modification, while a constant pointer protects the pointer from reassignment. In practical development, selecting between these based on whether you need to protect the data content or the pointer target can improve program stability and readability.