C++ FAQ / Section 18 / FAQ 18.16

 E-mail the author
[ Subject index | About the author | © ]
Revised Jul 4, 2012

[18.16] Does "Fred const* p" mean that *p can't change?

No! (This is related to the FAQ about aliasing of int pointers.)

"Fred const* p" means that the Fred can't be changed via pointer p, but there might be other ways to get at the object without going through a const (such as an aliased non-const pointer such as a Fred*). For example, if you have two pointers "Fred const* p" and "Fred* q" that point to the same Fred object (aliasing), pointer q can be used to change the Fred object but pointer p cannot.

class Fred {
public:
  void inspect() const;   // A const member function
  void mutate();          // A non-const member function
};

int main()
{
  Fred f;
  Fred const* p = &f;
  Fred*       q = &f;

  p->inspect();    // OK: No change to *p
  p->mutate();     // Error: Can't change *p via p

  q->inspect();    // OK: q is allowed to inspect the object
  q->mutate();     // OK: q is allowed to mutate the object

  f.inspect();     // OK: f is allowed to inspect the object
  f.mutate();      // OK: f is allowed to mutate the object

  ...
}