如果一个类继承 NSObject 会产生多少开销

Question

I have a lightweight c++ class needed to be ported to Objective-C to exploit the facilities provided by NSObject. There will be thousands of instances of the C++ class, and i dont want to add to much overhead by wrapping it in a NSObject derived class.

how big is NSObject?

Answer 1

First of all, you should descend from NSObject . There is no tangible benefit to not doing so. NSObject adds nothing in terms of storage over the lowest level representation of a class but adds an awful lot of behaviour that will allow your classes to act correctly with the runloop, with the normal collections (eg, NSArray or NSDictionary ) and with just about everything else.

To answer your question specifically:

#import <objc/runtime.h>

...

NSLog(@"obj size: %zd", class_getInstanceSize([NSObject class]));

Outputs '4' on iOS and '8' on 64-bit OS X. So the answer you're looking for is 'the natural size of an integer'. NSProxy is the same size, UIView (on iOS) is 84 bytes.

Documentation for class_getInstanceSize is here — it explicitly returns "[t]he size in bytes of instances of the class cls", as the name says.

Answer 2

in the apple documentation it says here that

The root class of all Objective-C classes is NSObject, which is part of the Foundation framework. All objects in a Cocoa or Cocoa Touch application ultimately inherit from NSObject. This class is the primary access point whereby other classes interact with the Objective-C runtime

So I'd say that every objective-c class ultimately derives from NSObject anyway, so you have no choice. Please, correct me if I'm wrong.

EDIT: What I've learned now is that NSObject is one root class , which implements the basic behaviour expected from every objective-c class. So basically you could write an objective-c class which implements the protocol of NSObject (I think that is objective-c speak for interface) and implement basic stuff like alloc and init yourself. Be sure to check out this thread: How to instantiate a class in Objective-C that don't inherit from NSObject

EDIT: Some more insight into root classes in objective-c , very interesting!