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Automatic Reference Counting (ARC) is a memory management feature of the Clangcompiler providing automatic reference counting for the Objective-C and Swiftprogramming languages. At compile time, it inserts into the object codemessagesretain and release^[1][2] which increase and decrease the reference count at run time, marking for deallocation those objects when the number of references to them reaches zero.

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Mac OS X Snow Leopard Sticky Keys enables the user to enter key combinations by pressing keys in sequence rather than simultaneously. This will benefit users who are unable to press or have difficulty pressing shortcut key combinations. Turning On Sticky Keys. By default, Sticky Keys may be turned on in the Universal Access panel of System. Firstly, try to look for the Power key on your Mac. It is located on the top right corner of the keyboard in laptops while on desktops, it is at the back of the screen. Press the Power key and keep holding it for the next 5-10 seconds. Let it go as you hear a click sound that would turn your Mac off. Press your Mac's ⏏ Eject button. This key is in the top-right corner of your Mac's keyboard. If the CD tray/slot is working correctly, the disk should slide right out. The disk may take a few seconds to eject if you were using it immediately before pressing the Eject button. WebKit, the underpinnings of Safari, was made open source on 6 June 2005. When Safari was run with this latest version of WebKit, it passed the Acid2 test. 31 October 2005 Safari 2.0.2 official release: Included in Mac OS X 10.4.3. First officially released web browser to pass test. 29 November 2005 Konqueror 3.5 official release.

ARC differs from tracing garbage collection in that there is no background process that deallocates the objects asynchronously at runtime.^[3] Unlike tracing garbage collection, ARC does not handle reference cycles automatically. This means that as long as there are 'strong' references to an object, it will not be deallocated. Strong cross-references can accordingly create deadlocks and memory leaks. It is up to the developer to break cycles by using weak references.^[4]

Apple Inc. deploys ARC in their operating systems, such as macOS (OS X) and iOS. Limited support (ARCLite)^[5] has been available since Mac OS X Snow Leopard and iOS 4, with complete support following in Mac OS XLion and iOS 5.^[6] Garbage collection was declared deprecated in OS X Mountain Lion, in favor of ARC, and removed from the Objective-C runtime library in macOS Sierra.^[7][8]

Objective-C[edit]

The following rules are enforced by the compiler when ARC is turned on:

• retain, release, retainCount, autorelease or dealloc cannot be sent to objects. Instead, the compiler inserts these messages at compile time automatically, including [super dealloc] when dealloc is overridden.^[9]
• Programs cannot cast directly between id and void *.^[9] This includes casting between Foundation objects and Core Foundation objects. Programs must use special casts, or calls to special functions, to tell the compiler more information about an object's lifetime.
• An autorelease pool can be used to allocate objects temporarily and retain them in memory until the pool is 'drained'. Without ARC, an NSAutoreleasePool object can be created for this purpose. ARC uses @autoreleasepool blocks instead, which encapsulate the allocation of the temporary objects and deallocates them when the end of the block is reached.^[9]
• Programs cannot call the functions NSAllocateObject and NSDeallocateObject^[9]
• Programs cannot use object pointers in C structures (structs)^[9]
• Programs cannot use memory zones (NSZone)^[9]
• To properly cooperate with non-ARC code, programs must use no method or declared property (unless explicitly choosing a different getter) that starts with new.^[9]

Property declarations[edit]

ARC introduces some new property declaration attributes, some of which replace the old attributes.

1. ^ARCLite is ARC but without zeroing weak references (used when deploying to a less-capable operating environment than ARC requires).

Zeroing weak references[edit]

Zeroing weak references is a feature in Objective-C ARC that automatically clears (sets to nil) weak-reference local variables, instance variables, and declared properties immediately before the object being pointed to starts deallocating. This ensures that the pointer goes to either a valid object or nil, and avoids dangling pointers. Prior to the introduction of this feature, 'weak references' referred to references that were not retaining, but were not set to nil when the object they pointed to was deallocated (equivalent to unsafe_unretained in ARC), thus possibly leading to a dangling pointer. The programmer typically had to ensure that all possible weak references to an object were set to nil manually when it was being deallocated. Zeroing weak references obviates the need to do this.

Zeroing weak references are indicated by using the declared property attributeweak or by using the variable attribute __weak.

Zeroing weak references are only available in Mac OS X Lion (10.7) or later and iOS 5 or later, because they require additional support from the Objective-C runtime. However, some OS X classes do not currently support weak references.^[9] Code that uses ARC but needs to support versions of the OS older than those above cannot use zeroing weak references, and therefore must use unsafe_unretained weak references. There exists a third-party library called PLWeakCompatibility [1] that allows one to use zeroing weak references even on these older OS versions.

Converting to[edit]

Xcode 4.2 or later provides a way to convert code to ARC.^[10] As of Xcode 4.5, it is found by choosing Edit > Refactor > Convert to Objective-C ARC... Although Xcode will automatically convert most code, some code may have to be converted manually. Xcode will inform the developer when more complex use cases arise, such as when a variable is declared inside an autorelease pool and used outside it or when two objects need to be toll-free bridged with special casts.

Swift[edit]

In Swift, references to objects are strong, unless they are declared weak or unowned. Swift requires explicit handling of nil with the Optional type: a value type that can either have a value or be nil. An Optional type must be handled by 'unwrapping' it with a conditional statement, allowing safe usage of the value, if present. Conversely, any non-Optional type will always have a value and cannot be nil.

Accordingly, a strong reference to an object cannot be of type Optional, as the object will be kept in the heap until the reference itself is deallocated. A weak reference is of type Optional, as the object can be deallocated and the reference be set to nil. Unowned references fall in-between; they are neither strong nor of type Optional. Instead, the compiler assumes that the object to which an unowned reference points is not deallocated as long the reference itself remains allocated. This is typically used in situations where the target object itself holds a reference to the object that holds the unowned reference.

Swift also differs from Objective-C in its usage and encouragement of value types instead of reference types. Most types in the Swift standard library are value types and they are copied by value, whereas classes and closures are reference types and passed by reference. Because value types are copied when passed around, they are deallocated automatically with the reference that created them.^[11]

See also[edit]

References[edit]

1. ^Siracusa, John (July 20, 2011). 'Mac OS X 10.7 Lion: the Ars Technica review'. Ars Technica. Ars Technica. At section 'Automatic Reference Counting'. Retrieved November 17, 2016.CS1 maint: discouraged parameter (link)
2. ^Kochan, Stephen G. (2011). Programming in Objective-C (4th ed.). Boston, Mass.: Addison-Wesley. pp. 408. ISBN978-0321811905.
3. ^Hoffman, Kevin (2012). Sams teach yourself Mac OS X Lion app development in 24 hours. Indianapolis, Ind.: Sams. pp. 73. ISBN9780672335815.
4. ^'General'. Automatic Reference Counting. LLVM.org. Retrieved 15 August 2012.CS1 maint: discouraged parameter (link)
5. ^'Objective-C Feature Availability Index'. Apple, Inc. Retrieved 2013-10-14.CS1 maint: discouraged parameter (link)
6. ^Sakamoto, Kazuki (2012). Pro Multithreading and Memory Management for iOS and OS X with ARC, Grand Central Dispatch and Blocks. Apress. pp. xii. ISBN978-1430241164.
7. ^Siracusa, John (July 25, 2012). 'OS X 10.8 Mountain Lion: the Ars Technica review'. Ars Technica. At section 'Objective-C enhancements'. Retrieved November 17, 2016.
8. ^'Xcode 8 Release Notes'. Apple Developer. October 27, 2016. Archived from the original on March 19, 2017. Retrieved March 19, 2017.
9. ^ ^abcdefgh'Transitioning to ARC Release Notes'. Retrieved 14 September 2012.CS1 maint: discouraged parameter (link)
10. ^'What's New in Xcode 4.2 – Automatic Reference Counting'. Apple Inc. Archived from the original on 20 August 2012. Retrieved 3 October 2012.CS1 maint: discouraged parameter (link)
11. ^'Value and Reference Types'. Apple Developer. August 15, 2014. Retrieved November 17, 2016.

External links[edit]

• 'Automatic Reference Counting' in The Swift Programming Language

Start up from macOS Recovery

Determine whether you're using a Mac with Apple silicon, then follow the appropriate steps:

Apple silicon

Turn on your Mac and continue to press and hold the power button until you see the startup options window. Click the gear icon labeled Options, then click Continue.

Intel processor

Make sure that your Mac has a connection to the internet. Then turn on your Mac and immediately press and hold Command (⌘)-R until you see an Apple logo or other image.

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If you're asked to select a user you know the password for, select the user, click Next, then enter their administrator password.

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Reinstall macOS

Select Reinstall macOS from the utilities window in macOS Recovery, then click Continue and follow the onscreen instructions.

Follow these guidelines during installation:

• If the installer asks to unlock your disk, enter the password you use to log in to your Mac.
• If the installer doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk first.
• If the installer offers you the choice between installing on Macintosh HD or Macintosh HD - Data, choose Macintosh HD.
• Allow installation to complete without putting your Mac to sleep or closing its lid. Your Mac might restart and show a progress bar several times, and the screen might be empty for minutes at a time.

After installation is complete, your Mac might restart to a setup assistant. If you're selling, trading in, or giving away your Mac, press Command-Q to quit the assistant without completing setup. Then click Shut Down. When the new owner starts up the Mac, they can use their own information to complete setup.

Other macOS installation options

When you install macOS from Recovery, you get the current version of the most recently installed macOS, with some exceptions:

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• If the Mac logic board was just replaced, you may be offered only the latest macOS that is compatible with your Mac. If you just erased your entire startup disk, you may be offered only the macOS that came with your Mac, or the closest version still available.

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You can also use these methods to install macOS, if the macOS is compatible with your Mac:

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• Use a USB flash drive or other secondary volume to create a bootable installer.

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