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fall through v.
(n. `fallthrough', var.
`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.
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