Question

The book I am reading says that after allocating an object I need to call the init function on the instance. However, I did not see any differences in the functionality of the class when I don’t call the init method. So what exactly is the init function doing ? Below is the interface for the simple class:

@interface Fraction: NSObject
{
   int numerator;

   int denominator;
}
-(void) print;

-(void) setNumerator: (int) num;

-(void) setDenominator:(int) den ;

@end

Answer 1

init is meant to initialize all necessary instance variables so that the object is in a defined state. After alloc, all ivars are set to 0/nil/NULL, which might not be a permitted state for your object (for example, denominator should never be 0).

The init method your class inherited from NSObject does nothing so if your class does not need to initialize any ivars you don’t have to implement your own. But it is good custom to always call init right after alloc, even if you know the method does nothing (who says this will remain true forever?).