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

Read it backwards (as driven by Clockwise/Spiral Rule):

• `int*` – pointer to int
• `int const *` – pointer to const int
• `int * const` – const pointer to int
• `int const * const` – const pointer to const int

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

• `const int *` == `int const *`
• `const int * const` == `int const * const`

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

• `int **` – pointer to pointer to int
• `int ** const` – a const pointer to a pointer to an int
• `int * const *` – a pointer to a const pointer to an int
• `int const **` – a pointer to a pointer to a const int
• `int * const * const` – a const pointer to a const pointer to an int

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

``````int a = 5, b = 10, c = 15;

const int* foo;     // pointer to constant int.
foo = &a;           // assignment to where foo points to.

/* dummy statement*/
*foo = 6;           // the value of a canĀ“t get changed through the pointer.

foo = &b;           // the pointer foo can be changed.

int *const bar = &c;  // constant pointer to int
// note, you actually need to set the pointer
// here because you can't change it later ;)

*bar = 16;            // the value of c can be changed through the pointer.

/* dummy statement*/
bar = &a;             // not possible because bar is a constant pointer.
``````

`foo` is a variable pointer to a constant integer. This lets you change what you point to but not the value that you point to. Most often this is seen with C-style strings 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.

`bar` is a constant 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.