What is the difference between const int * and int const *?

One simple answer - read it backwards (as driven by Clockwise/Spiral Rule).

  • int * ptr - ptr is a pointer to int
  • int const * ptr - ptr is a pointer to constant int
  • int * const ptr - ptr is a constant pointer to int
  • const int * const ptr - ptr is a constant pointer to const int

Now the first const can be on either side of the type so:

  • const int * ptr equal to int const * ptr
  • const int * const ptr equal to int const * const ptr

If you want to go really crazy you can do things like this:

  • int ** ptr - ptr is a pointer to pointer to int
  • int ** const ptr - ptr is a const pointer to a pointer to an int
  • int * const * ptr - ptr is a pointer to a const pointer to an int
  • int const ** ptr - ptr is a pointer to a pointer to a const int
  • int * const * const ptr - ptr a const pointer to a const pointer to an int
  • or anything else

And to make sure we are clear on the meaning of const

const int* ptr1;  
int *const ptr2; //note, here you need to set the pointer here because you can't change it later  

ptr1 is a variable pointer to a constant int. This lets you change what you point to but not the value that you point to. Most often this is seen with cstrings where you have a pointer to a const char. You may change which string you point to but you can't change the content of these strings. This is important when the string itself is in the data segment of a program and shouldn't be changed.

ptr2 is a const or fixed pointer to a value that can be changed. This is like a reference without the extra syntactic sugar. Because of this fact, usually you would use a reference where you would use a T* const pointer unless you need to allow null pointers.

How to discover whether 'const' applies to pointer or to pointed data: split the statement at asterix sign, then, if the const keyword appears in the left part (like in const int * foo) - it belongs to pointed data, if it's in the right part (int * const bar) - it's about the pointer.