jargon, node: fall through

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fall through

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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