Difference between ++*p, *p++ and *++p in C

In C programming, *p represents the value stored at the address a pointer points to. ++ is the increment operator (used in both prefix and postfix forms), and * is the dereference operator. Understanding how these combine requires knowing their precedence and associativity −

• Prefix ++ and * have the same precedence and are right-to-left associative.
• Postfix ++ has higher precedence than both and is left-to-right associative.

The Three Expressions

Example

The following program demonstrates all three expressions using an array ?

#include <stdio.h>

int main() {
    int arr[] = {20, 30, 40};
    int *p = arr;
    int q;

    // ++*p : increment the VALUE at p (20 becomes 21)
    // pointer still points to arr[0]
    q = ++*p;
    printf("arr[0] = %d, arr[1] = %d, *p = %d, q = %d<br>",
        arr[0], arr[1], *p, q);

    // *p++ : return VALUE at p (21), then move pointer to arr[1]
    q = *p++;
    printf("arr[0] = %d, arr[1] = %d, *p = %d, q = %d<br>",
        arr[0], arr[1], *p, q);

    // *++p : move pointer to arr[2] first, then return VALUE (40)
    q = *++p;
    printf("arr[0] = %d, arr[1] = %d, *p = %d, q = %d<br>",
        arr[0], arr[1], *p, q);

    return 0;
}

The output of the above code is ?

arr[0] = 21, arr[1] = 30, *p = 21, q = 21
arr[0] = 21, arr[1] = 30, *p = 30, q = 21
arr[0] = 21, arr[1] = 30, *p = 40, q = 40

Step-by-Step Walkthrough

Step 1: q = ++*p − p points to arr[0] (value 20). Dereference gives 20, increment makes it 21. arr[0] is now 21. q = 21. Pointer still at arr[0].

Step 2: q = *p++ − p still points to arr[0] (value 21). Dereference gives 21, so q = 21. Then pointer moves to arr[1]. Now *p = 30.

Step 3: q = *++p − Pointer moves from arr[1] to arr[2] first. Then dereference gives 40. q = 40.

Conclusion

++*p increments the value the pointer points to. *p++ returns the current value and then moves the pointer forward. *++p moves the pointer forward first and then returns the value at the new location. The key is understanding that postfix ++ binds tighter than *, while prefix ++ and * share the same precedence with right-to-left associativity.