Comp210 Lab 14: Passing Pointers

Comp 210 Lab 14: Passing Pointers

If you are using non-extended C, and are forced to manually use * and & to effect pass-by-reference, here are some notes to help you avoid bugs.

// swap
// Exchange the values of the ints *x and *y, as passed-by-reference.
//
void swap( int *x, int *y ) {
  int tmp;
  tmp = *x;
  *x = *y;
  *y = tmp;
  }

int main() {
  int myScore = 23, yourScore = 92;
  swap( &myScore, &yourScore );        // cheating on the exam?

  cout << "Hey I got " << myScore << ", you got " << yourScore << ".\n";

  swap( &100, &myScore );              // ERROR: &100 doesn't make sense!

  return 0;
  }

While it's nice to have seen the pointer perspective on things, you don't need to think that much to think of passing-by-reference. Whenever possible, avoid thinking of the pointer details; they'll only confuse you when you start thinking about pointers-to-pointers-to-int, and so on.

When writing the function, consider the star * as part of the variable name. Never ever refer to the parameter without using the star. (The reason the variable-name has that funny star is solely to remind you that any changes to the variable are seen by the caller.)
When calling the function, just include the ampersand & which serves as a reminder that this variable might be modified during the function call.

Again, these tips are intended to allow you to forget the fact that you're messing with pointers, and are only necessary if you are using straight C. C++'s ``referent-to-int'' int& type was created for this exact same reason.

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