Are instance variables set to nil by default in Objective-C?

Question

Hi there,

I'm sorting out some memory issues with my iPhone app and I've just been thinking about some basics. If I setup an ivar and never end up using it in the lifespan of my object, when I call dealloc on it, will that cause a problem? E.g.

@interface testClass {
    id myobject;
}
@property (nonatomic, retain) id myobject;
@end

@implementation testClass
@synthesize myobject;
- (id)init {
    ...
    // Do I have to set myobject to nil here?
    // So if myobject isn't used the dealloc call to nil
    // will be okay? Or can you release the variable without
    // having set every object to nil that you may may not use 
    ...
}

...

// Somewhere in the code, myobject may be set to
// an instance of an object via self.myobject = [AnObject grabAnObject]
// but the object may be left alone

...

- (void)dealloc {
    [myobject release];
    [super dealloc];
}
@end

Answer 1

I find that it is good practice to always set those ivars to nil in the init method. That way, you are absolutely sure that your call to release in the destructor can not cause problems.

If it turns out that Objective-C does automatically set them to nil, and for some reason you find yourself with a speed bottleneck that can be improved upon by removing those assignments (highly unlikely), then you can worray about removing them. In the meantime, set theem all to nil and sleep easier :)

update: BJ Homer and Chuck have pointed out that the ivars will automatically be set to zero, so now it comes down to a decision on style.

Answer 2

Yes, ivars are always initialized to 0/nil/NULL/NO/etc.

However, explicitly doing it is good practice, anyway. The performance impact is extremely minimal, and then you'll be explicit about what you're doing.

Answer 3

Instance variables are initialized to 0 before your initializer runs..