Question

I recently started a new project using Automatic Reference Counting (ARC).
When I assigned the contents of a CALayer:

UIView* view = ...
UIImage* image = ...
view.layer.contents = image.CGImage

I got an error

Implicit conversion of a non-Objective-C pointer type ‘CGImageRef’ to ‘id’ is disallowed with ARC

Simply casting the CGImageRef to id hides the error, but I was wondering if the ARC still functions correctly then?

Answer 1

You should really check out the ARC videos from WWDC 2011. They are available on the developer site and open through iTunes. Especially:

• Session 323 – Introducing Automatic Reference Counting

• Session 322 – Objective-C Advancements in Depth

Also, the ARC reference notes:

https://developer.apple.com/library/content/releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html

Both the reference notes and the videos discuss Core Graphics (et al) and how they work with ARC.

Specifically, look at the section called "Managing Toll-Free Bridging"

In many Cocoa applications, you need to use Core Foundation-style objects, whether from the Core Foundation framework itself (such as
CFArrayRef or CFMutableDictionaryRef) or from frameworks that adopt
Core Foundation conventions such as Core Graphics (you might use types
like CGColorSpaceRef and CGGradientRef).

The compiler does not automatically manage the lifetimes of Core
Foundation objects; you must call CFRetain and CFRelease (or the
corresponding type-specific variants) as dictated by the Core
Foundation memory management rules (see Memory Management Programming
Guide for Core Foundation).

If you cast between Objective-C and Core Foundation-style objects, you
need to tell the compiler about the ownership semantics of the object
using either a cast (defined in objc/runtime.h) or a Core
Foundation-style macro (defined in NSObject.h): […]

Jörg Jacobsen has a good summary overview of the bridging options as well: Managing Toll-free Bridging in an ARC’ed Environment.

__bridge_retained (n.b.: only use it when casting from object pointer to C type pointer): I (the programmer) need to reference this object
for some time in the dark world of C type pointers which is opaque to
you, ARC. So please, please do not release this object while I still
need it. I (the programmer) promise to release it myself (in the dark
world) when I’m done with it

__bridge_transfer (n.b.: only use it when casting from C type pointer to object pointer): I (the programmer) hand over to you, ARC, an
object that I own and that I am no longer interested in in the dark
world of C type pointers that is opaque to you. Whenever you, ARC, are
done with that object please release it yourself, because you know the
right time and thus save me some work not having to do it myself.

__bridge: ARC, you keep balancing out your retains and releases as I keep balancing out mine in the dark world of C type pointers which
is…. Whenever I need to hold on to an object in the dark world I will
retain it myself and release it when appropriate. I don’t need any
extra contract with you, ARC.