从Objective-C开始：ARC还是ARC？

Question

According to Apple's ARC Documentation , there are a fairly significant number of changes to the way that one develops software when using ARC.

As a complete beginner to Objective-C, would it be better to start off with ARC disabled, with the idea that it would give me a better low-level understanding of what is going on behind the scenes? Or has ARC essentially deprecated the 'old way' of doing things, to the point that it's not really worth spending time learning?

Answer 1

This is basically an opinion question, and is therefore fairly dangerous.

My Opinion is a qualified yes. It is worth learning basic memory management. The qualification being don't get bogged down in it. Learn what ARC is doing for you under the hood with some very simple projects. Once you have a basic understanding of how to handle memory management, ie how to avoid retain cycles(as jemmons alluded to they can still be a problem with ARC). Once you have a basic grasp of memory management. Start using ARC.

Also as Jason Coco pointed out ARC handles memory management for (to put it simply) NSObject subclasses. So all of the CF Objects you will still be handling yourself, if you need to use them.

An excellent explanation about what ARC is doing for you under the hood can be found in the WWDC2011 Session 323 - Introducing Automatic Reference Counting.

But there are some other considerations that might steer your decision.

What devices do you need to target?

If you plan to target iOS 4.3 and up ARC effectively handles memory management for you.(of NSObject subclasses)

If you plan to target iOS 4.2 then you will not be able to use weak references(you will use unsafe_unretained). iPhone 3g? & iPod touch 2nd gen are stuck at this OS level, because there are many of these devices still in service many developers are still targeting them.

If you plan to target iOS s earlier than 4.2(This would be rare) you will definitely need to learn MRC(Manual Reference Counting).

If you plan to build Mac Apps, there is a garbage collector available on that platform. ARC is also an option(full ARC 10.7, no weak support 10.6).

Answer 2

It's worth noting that ARC is checked by default when you start a new project in Xcode. This is as good a sign as any that the old retain / release way of doing things is deprecated and Apple sees ARC as the future. Your first lesson as a new ObjC developer might be that it never pays to swim up-stream against Apple.

Also, while it's fairly easy to convert old retain / release samples to ARC (for the most part, just drop any retain s, release s, and autorelease s), the inverse is not true. And I've already seen a lot of sample code cropping up that's written ARC-style. So as someone just starting out, it'd pay more to learn the ARC way.

Note this doesn't mean you don't have to understand reference counting. It's still an important part of an object's life cycle and you still need to be aware of such things (if only to know when to use weak or strong references). But when it comes time to write code, write it with ARC on.

Answer 3

The "old" style is simply just reference counting to manage your object's lifetime. There's not really all that much too it, but it can be error-prone and cause all kinds of grief. If you're just starting, I'd personally suggest you just learn to program using ARC. You'll still get to deal with reference counting when you need to use the C-library objects, like CoreFoundation or CoreGraphics.

Answer 4

Apple itself recommends ARC for new projects. http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/MemoryMgmt/Articles/MemoryMgmt.html#//apple_ref/doc/uid/10000011i

read point 2 in At a Glance

There is sooo much more interesting to learn in Objective C & iOS apart from the memory management. My advice : Don't bother about MRR

Answer 5

It would be a great idea just to go over it and understand what's going on, but for development it's not essential, and as you can see most if not all developers have moved their deployment targets to iOS 5.0+, so you are likely not to develop under manual reference counting.

However if you plan to use non ROP -retainable object pointers- in your code like CFStringRef, you might want to really look at non ARC so you can understand things like bridge, because you can combine ARC and non ARC code in one project.