CODEWITHSUNDEEP
objective-c
Amogh Talpallikar
Amogh Talpallikar

Reputation: 12184

Why does calling a property on NSObject pointer gives build errors?

I have an NSMutableArray which returns me some object. The object which I added had properties name,age.

now when I use these properties on the Object returned (obj.name or obj.age ), Compiler says, no such member, use (->) instead of (.)

I understand that NSObject wont have these members and hence it wont understand the property.

But If i use setters, and getters as method ([obj name] or [obj age]) syntax instead of this properties, I dont get any errors.

But using property means calling a setter or getter only ?
ad Objective C is suppose to be dynamic language, right ?

Upvotes: 0

Views: 178

Answers (2)

Pooria Azimi
Pooria Azimi

Reputation: 8633

Do you cast the returned object to your object type (MyObject)?

You should do something like:

((MyObject*)[mutableArray objectAtIndex:0]).age = 20;

The reason you're not getting any errors when using [[mutableArray objectAtIndex:0] name] syntax is that you're calling a method on the returned object (which is of type id), and id s tend to not choke in the compile-time if you call a (yet) non-existant method on them. At the run-time, [mutableArray objectAtIndex:0] might resolve to type MyObject an in that case, the message [obj name] has a proper implementation (IMP). If it doesn't resolve to MyObject, your app will crash.

And note that the reason you're not even getting a compile-time warning is that Xcode knows that there is at least 1 class in your codebase that implements the method name, and it trusts you with calling this method only on instances of that class. if you do something like ((MyObject*)[mutableArray objectAtIndex:0]).ageeeeee = 20;, it'll give you a warning as there's a very good chance that it'll crash (no class in your app implements the method ageeeeee statically).

The type id does not have a property name, and that's why you can't use dot syntax.

Actually, this incident shows perfectly why ObjC is called a dynamic language!

Upvotes: 3

sigman
sigman

Reputation: 1301

That's right - dot syntax is not supported in such case.

You need to cast a pointer to the actual class:

((MyObject*)[array objectAtIndex: 0]).name = @"Bill";

Upvotes: 1

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