Lazy Instantiation in Objective C

Lazy instantiation is a relatively cheap way of allocating memory to component objects as it’s required instead of up-front in a constructor.

{% highlight objectivec %} @interface MyStack() @property (nonatomic, strong) NSMutableArray *stack; @end

@implementation MyStack

@synthesize stack = _stack;

• (NSMutableArray*)stack { if (_stack == nil) { _stack = [[NSMutableArray alloc] init]; } return _stack; }
• (void)push:(double)operand { [self.stack addObject: [NSNumber numberWithDouble:operand]]; } @end {% endhighlight %}

This implementation overrides the synthesized accessor and adds an alloc-init call if the storage hasn’t yet been initialized.

Initialization happens at line 12, when the first message is sent to addObject: by push:, which means that no storage is allocated until it’s required by the application.