Objective-C setter method

@property(nonatomic,retain) NSObject * exVal;

@synthesize exVal;

- (void)setExVal:(NSObject*) val {

   [exVal release]; 
   exVal = [val retain];
}

this is wrong. coz if val is equal to exVal, both exVal and val are released. and then nil retain.

- (void)setExVal:(NSObject*) val {

    [val retain];

    [exVal release];

    exVal = val;

}

this is right.