Comment by wasmperson

Thinking of it as a "stopping condition" is backwards, that part of the loop is called the invariant:

https://en.wikipedia.org/wiki/Loop_invariant

You should think of it as the condition that's true for all iterations, not a one-time event that halts the loop. The loop is short for this:

  for(size_t i = size - 1; 0 <= i && i < size; i--){
  
  }

Which works for both signed and unsigned numbers. It just so happens that for unsigned numbers you can omit the left-hand side of the &&, and for signed numbers you can omit the right-hand side. To support arbitrary lower bounds, you omit neither.