objective-C: Why interface type can't be statically allocated?

Question

NSString s1;   //compile error: interface type cannot be statically allocated
NSString *s2   //good

Statically would mean the memory for that object is allocated at compile-time? But what is the reason behind allocating all objects at runtime and access through pointers? I know that pointers allow a better use of memory, for example passing them as method's parameters but why forbid static allocation? I'm new to this language, I'd like to understand its vision.

Answer 1

In C++ you can't have polymorphic objects unless you have a pointer to that object. Objective-C doesn't let you open that can of worms. It's a dynamically typed language (with a lot of protection if you want) - and locking in your type at compile time goes against the philosophy of the language.

NSString *s2 = [[NSString alloc] initWith ???];

consider that line. You're allocating a string object (I left the details of the constructor blank on purpose). You won't get an actual instance of NSString - the type of your object will be a private subclass. This concept is called Class Clusters.

https://developer.apple.com/library/ios/documentation/general/conceptual/CocoaEncyclopedia/ClassClusters/ClassClusters.html

Depending on what constructor is run, and what parameters are passed in could affect what your runtime type is. This would not be possible with a statically allocated object.

There is of course, a lot more to your question, but consider all that is possible given every object is dynamic at runtime. Also consider the complexity of C++ that this avoids.