Contents page

Index (83KB)

fall through

   1. To exit a loop by exhaustion, i.e., by having fulfilled its exit
   condition rather than via a break or exception condition that exits
   from the middle of it.  This usage appears to be *really* old,
   dating from the 1940s and 1950s.  2. To fail a test that would have
   passed control to a subroutine or some other distant portion of
   code.  3. In C, `fall-through' occurs when the flow of execution in
   a switch statement reaches a `case' label other than by
   jumping there from the switch header, passing a point where one
   would normally expect to find a `break'.  A trivial example:


     switch (color)
     {
     case GREEN:
        do_green();
        break;
     case PINK:
        do_pink();
        /* FALL THROUGH */
     case RED:
        do_red();
        break;
     default:
        do_blue();
        break;
     }


   The variant spelling `/* FALL THRU */' is also common.


   The effect of the above code is to `do_green()' when color is
   `GREEN', `do_red()' when color is `RED',
   `do_blue()' on any other color other than `PINK', and
   (and this is the important part) `do_pink()' *and then*
   `do_red()' when color is `PINK'.  Fall-through is
   considered harmful by some, though there are contexts (such as
   the coding of state machines) in which it is natural; it is
   generally considered good practice to include a comment
   highlighting the fall-through where one would normally expect a
   break.  See also Duff's Device.