15.1 const Pointers

Declaring constant pointers is a little tricky. For example, although the declaration:

const int result = 5;

tells C++ that result is a constant, so:

result = 10;      // Illegal

is illegal. The declaration:

const char *answer_ptr = "Forty-Two";

does not tell C++ that the variable answer_ptr is a constant. Instead it tells C++ that the data pointed to by answer_ptr is a constant. The data cannot be changed, but the pointer can. Again we need to make sure we know the difference between "things" and "pointers to things."

What's answer_ptr? A pointer. Can it be changed? Yes, it's just a pointer. What does it point to? A const char array. Can the data pointed to by answer_ptr be changed? No, it's constant.

Translating these rules into C++ syntax we get the following:

answer_ptr = "Fifty-One";     // Legal (answer_ptr is a variable)
*answer_ptr = 'X';            // Illegal (*answer_ptr is a constant)

If you put the const after the *, you tell C++ that the pointer is constant. For example:

char *const name_ptr = "Test";

What's name_ptr? A constant pointer. Can it be changed? No. What does it point to? A character. Can the data we pointed to by name_ptr be changed? Yes.

name_ptr = "New";           // Illegal (name_ptr is constant)
*name_ptr = 'B';            // Legal (*name_ptr is a char)

Finally, we put const in both places, creating a pointer that cannot be changed to a data item that cannot be changed:

const char *const title_ptr = "Title";

One way of remembering whether the const modifies the pointer or the value is to remember that *const reads "constant pointer" in English.