Const and pointers:

It is important to differentiate between a pointer to a constant and a
constant pointer to a variable.

Here is a pointer to a constant:

  const int* p;

The following statement is illegal, because it attempts to change the
value of a constant:

  *p = 3;

But this one is legal, because the pointer itself is not a constant:

  p = &x;

On the other hand, this declaration shows a constant pointer to a variable.

  int* const p;

In that case, the following statement is legal, as the variable can be changed:

  *p = 3;

but this one is not, because it attempts to change the value of a constant.

  p = &x;

It is possible to declare a constant pointer to a constant. In that
case neither the value of the pointer nor the value of the base data
item can be changed:

  const int* const p;


Here is one way to remember the syntax (from Bjarne Stroustrup):

Note that in const pointers, "const" always comes after the "*". For
example:

  int *const p1 = q;	// constant pointer to int variable
  int const* p2 = q;	// pointer to constant int
  const int* p3 = q;	// pointer to constant int

Another way is to think about '*' as capturing the largest leftward
unit that it can. 'Const' captures the smallest unit, left or right,
that is can. So think about the examples above as:

  (int*) const p1 = q;	// constant pointer to int variable
  (int const)* p2 = q;	// pointer to constant int
  (const int)* p3 = q;	// pointer to constant int

In other words, the qualifier immediately preceding the variable is
the first word of the definition.