fall thru

1 definition found

fall thru - Free On-line Dictionary of Computing (26 May 2007) :

  fall through
  fall thru
  
     <programming> (The American misspelling "fall thru" is
     also common)
  
     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 (colour)
     	 	case GREEN: 	 do_green(); 	 break; 	case PINK: 	 do_pink(); 	 /* FALL THROUGH */ 	case RED: 	 do_red(); 	 break; 	default: 	 do_blue(); 	 break; 	
                                                     
  
     The effect of the above code is to "do_green()" when colour is
     "GREEN", "do_red()" when colour is "RED", "do_blue()" on any
     other colour other than "PINK", and (and this is the important
     part) "do_pink()" __and then__ "do_red()" when colour 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.