Objective-C Fundamentals

Expressions
297
Table B.3
Relational operators available in C
Operator
Description
== Equal to
!= Not equal to
> Greater than
< Less than
>= Greater than or equal to
25 &&
weight < 180. The ! is a special logical operator in that it requires only one Boolean
value and it reverses it (true becomes false, false becomes true).
Although it makes little difference to the expression just shown, it’s important to
note that C implements short-circuit Boolean evaluation: an expression will stop
being evaluated as soon as the end result can be determined without any level of
doubt. The following logical expression involving the && operator demonstrates how
this feature can make the difference between your application running correctly or
randomly crashing:
struct box *p = get_some_pointer_value();
BOOL result = (p != NULL) && (p->width == 20);
The expression on the second line evaluates to true if p is a non-NULL pointer and the
width of the box it points to has a width of 20.
Without short-circuit Boolean evaluation, the expression would crash whenever p
was NULL. This would occur because after evaluating the left side of the && operator (p
!= NULL), execution would then evaluate the right side (p->width == 20) and crash
while attempting to dereference the NULL pointer. The right side is evaluated even
though the final result of the statement is known to be false because false && x for
any value of x will always be false.