Why is the non-const pointer being treated as a const when using typedef?

Question

typedef char* c;
const c ptr1 = "pointer";
++ptr1; /// error
const char* ptr2 = "pointer";
++ptr2; /// runs fine

Now ptr1 should be of type const char* and thus a non-const pointer, then why is it being treated as a constant pointer ?

Answer 1

They are not the same.

The first designates a const-pointer-to-char, the second is a pointer-to-const-char.

Try reading right to left:

const char *p;  // p is a pointer to char which is const
char const *p;  // p is a pointer to const char (same as previous)
char * const p; // p is a const pointer to char
char const * const p; // p is a const pointer to const char

By using the typedef typedef char* c you pack the meaning "pointer to char" into a single alias c:

const c p; // p is a const [c] --> p is a const [pointer to char]

---

Additional explanations:

Typedefs are not in-place-expanded like macros are, i.e.

const c p;

really becomes

const [char*] p;

it does not become

const char* p; // Nope.

Don't expand it like macros in your mind, with the typedefs, you've bound char and * together and formed an atom.

Answer 2

ptr1 is a const (char *), meaning the pointer itself is a const, whereas ptr2 is a (const char) *, meaning the target of the pointer is const.