What equivalent code is synthesized for a declared property?

Question

How exactly getter and setter methods body looks like after they have been automatically synthesized ?

From official documentation I found so far only recommended im

Answer 1

When declaring a property as nonatomic, you'll get the following:

// .h
@property (nonatomic, retain) id ivar;

// .m
- (id)ivar {
    return ivar;
}

- (void)setIvar:(id)newValue {
    if (ivar != newValue) {  // this check is mandatory
        [ivar release];
        ivar = [newValue retain];
    }
}

Note the check ivar != newValue. If it was absent, ivar could be dealloc'ed after release, and the following retain would cause a memory access error.

When you declare your property with copy, the code will look almost the same, with retain replaced by copy.

For assign, it is even simpler:

- (void)setIvar:(id)newValue {
    ivar = newValue;
}

---

Now, when you declare your property as atomic (this one is the default), things get slightly more complicated. A snippet similar to the one below was posted by one of Apple's engineers on the development forums:

- (id)ivar {
    @synchronized (self) {
        return [[self->ivar retain] autorelease];
    }
}

- (void)setIvar:(id)newValue {
    @synchronized (self) {
        if (newValue != self->ivar) {
            [self->ivar release];
            self->ivar = newValue;
            [self->ivar retain];
        }
    }
}

Note the @synchronized block in both methods and additional retain-autorelease in the getter. Both those things ensure that you will either get the previous value (retained and autoreleased) or a new one in the case the value is changed by some thread while you are trying to read it.

Answer 2

It depends on the attributes you set, for instance with a RETAIN attribute...

- (void)setAProp:(AnotherClass *)value {
   [aProp release];
   aProp = value;
   [aProp retain];
}

For ASSIGN attribute....

- (void)setAProp:(AnotherClass *)value {
   aProp = value;
}

For COPY attribute (used with NSString)...

- (void)setAProp:(AnotherClass *)value {
   aProp = [value copy];
}