On 24 Feb 2012, at 8:50 AM, Oleg Krupnov wrote:
> An interesting question. The following samples are equivalent in terms
> of compiled code, but which one is more correct from the language's
> point of view?
>
> self = [super init];
> if (self)
> {
> }
> return self;
>
> self = [super init];
> if (self != nil)
> {
> }
> return self;
>
> The Xcode samples promote the first variant, but I'm wondering if the
> second one is more correct?
It's a matter of style; neither is "more correct." C has always guaranteed that
in source (if not necessarily in the target architecture), NULL pointers can be
treated as 0.
if (self)
and
if (self != nil)
should produce identical code. Some people find the second easier to read. I
find the first to be so fundamental an idiom that anyone who can't understand
it has no business working with C.
> So basically, nil is of type "void*", so the expression "self != nil"
> compares two pointers and the result is "boolean", which is perfect
> for testing in the "if" statement. But the "self" alone is of type
> "pointer" and so when it is tested by the "if" statement, it's
> implicitly cast to the type "boolean".
C, earlier than C99, has no concept of a Boolean. Conditionals are not false or
true, but zero (or NULL) or non-zero. C99 has a bool (_Bool) type, but has to
conform to the 0/non-0 rule. Boolean operators reduce true expressions to 1,
but code that relies on 1 (or true, or YES) being the only non-false value is
incorrect.
— F
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