Question

I have a very bad code written in my program, was just playing around as I am learning Objective C and iOS platform. What I did is,

I have created NSMutableArray like this,

placeInfo = [NSMutableArray array];

and than later in my code I am doing something like this, basically I am manipulating Google places api response(JSON).

NSDictionary *results = [responseString JSONValue];
placeInfo = [results objectForKey:@"result"];
self.phoneNumber = (NSString *)[placeInfo objectForKey:@"formatted_phone_number"]; // In this line compiler warns me that NSMutableArray might not response to this.

I checked documentation but I didn’t find objectForKey in NSMutableArray.

• So what could be the reason? Why my code isn’t crashing? Why it is returning phone number by “formatted_phone_number” key?

EDIT

After first answer I have edited my code and added type casting like this, but it still works.

NSDictionary *results = [responseString JSONValue];
placeInfo = (NSMutableArray *)[results objectForKey:@"result"];
self.phoneNumber = (NSString *)[placeInfo objectForKey:@"formatted_phone_number"];

Answer 1

I’ve never used the Google Places API, but I’d guess [results objectForKey:@"result"] actually returns another dictionary, so the objectForKey: works.

Because objective-c just uses pointers to refer to objects, it’s never actually being converted to an NSMutableArray. Also, objective-c doesn’t know at compile time if a method will exist, due to its dynamic nature (you can actually add methods and even whole classes at runtime).

Depending on the compiler settings, it may just show a warning that objectForKey: might not be found at runtime, and let it continue compiling anyway. It ends up working just fine if you actually passed it an NSDictionary.

Even when you put the (NSMutableArray *) cast in front of it, it won’t change anything. That simply casts the pointer type, and doesn’t actually change the object in any way.