Const Pointers and Pointers to Const

The placement of const in a pointer declaration can be confusing, but it's important to understand the difference:

1. A pointer to const (e.g., const int*) is a pointer that points to a const value. You cannot change the value that the pointer points to, but you can change the pointer itself to point to another value.
2. A const pointer (e.g., int* const) is a pointer that cannot be changed to point to another value. However, the value that the pointer points to can be changed.

Here's an example:

1int main() {
2  int x = 10;
3  int y = 20;
4
5  const int* ptr1 = &x;
6  // Error: cannot change value through ptr1
7  *ptr1 = 30; 
8  // Okay: can change to point to another value
9  ptr1 = &y;
10
11  int* const ptr2 = &x;
12  // Okay: can change value through ptr2
13  *ptr2 = 30;
14  // Error: cannot change to point to another value
15  ptr2 = &y; 
16}

You can also have a const pointer to a const value (e.g., const int* const), which means that neither the pointer nor the value it points to can be changed.