The Increment/Decrement Operators and Pointers

You can use increment operators with pointers as well as with basic variables. Recall that adding an increment operator to a pointer increases its value by the number of bytes in the type it points to. The same rule holds for incrementing and decrementing pointers:

double arr[5] = {21.1, 32.8, 23.4, 45.2, 37.4};
double *pt = arr;  // pt points to arr[0], i.e. to 21.1
++pt;              // pt points to arr[1], i.e. to 32.8

You can also use these operators to change the quantity a pointer points to by using them in conjunction with the * operator. Applying both * and ++ to a pointer raises the questions of what gets dereferenced and what gets incremented. Those actions are determined by the placement and precedence of the operators. The prefix increment, prefix decrement, and dereferencing operators all have the same precedence and associate from right to left. The postfix increment and decrement operators both have the same precedence, which is higher than the prefix precedence. These two operators associate from left to right.

The right-to-left association rule for prefix operators implies that *++pt means first apply ++ to pt (because the ++ is to the right of the *) and then apply * to the new value of pt:

double x = *++pt;  // increment pointer, take the value; i.e., arr[2], or 23.4

On the other hand, ++*pt means obtain the value that pt points to and then increment that value:

++*pt;       // increment the pointed to value; i.e., change 23.4 to 24.4

Here, pt remains pointing to arr[2].

Next, consider this combination:

(*pt)++;     // increment pointed-to value

The parentheses indicate that first the pointer is dereferenced, yielding 24.4. Then the ++ operator increments that value to 25.4; pt remains pointing at arr[2].

Finally, consider this combination:

x = *pt++;       // dereference original location, then increment pointer

The higher precedence of the postfix ++ operator means the ++ operator operates on pt, not on *pt, so the pointer is incremented. But the fact that the postfix operator is used means that the address that gets dereferenced is the original address, &arr[2], not the new address. Thus, the value of *pt++ is arr[2], or 25.4, but the value of pt after the statement completes is the address of arr[3].