Question

I am crashing at runtime when i pop last element from NSMuttable array.I am getting all element fine but when i acess last element it will give error.

-(id)popOperand{
   id operandObject = [self.operandStack lastObject];
   if(operandObject) [self.operandStack removeLastObject];
   return operandObject;
}

For eg:- if in my stack i have “8”(Its NSNumber object) “+”(Its NSString object) 2(Its NSnumber Object).

NSNumber *rightOperand = [self popOperand];
NSString *operation = [self popOperand];
NSNumber *leftOperand = [self popOperand];//when i acess 8 it shows empty array other           element is getting fine;

My question is why i am not able to get last object while this function is working fine for other elememts.
EDIT : At run time when after poping 2 it show stack( 8 +) but after i poped + it show stack() empty.But The “leftOperand” is not getting value. I am not using ARC.

Please ellaborate it.
Thanks.

Answer 1

If you aren’t using ARC then you have memory management issues:

- (id)popOperand
{
   id operandObject = [self.operandStack lastObject];     // Still retained by operandStack
   if(operandObject) [self.operandStack removeLastObject];// Released by operandStack
   return operandObject;                                  // Could be dealloc'd any time now!
}

Try this version, which returns a retain‘d and autorelease‘d object, which conforms to the naming convention of the method:

- (id)popOperand
{
   id obj = nil;
   if ([self.operandStack count] > 0)
   {
       id obj = [[[self.operandStack lastObject] retain] autorelease];
       [self.operandStack removeLastObject];
   }
   return obj;
}