[Objective-C] 從“引用計數”過渡到“自動引用計數”（ARC）

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

Transitioning to ARC Release Notes

Automatic Reference Counting (ARC) is a compiler feature that provides automatic memory management of Objective-C objects. Rather than having to think about retain and release operations, ARC allows you to concentrate on the interesting code, the object graphs, and the relationships between objects in your application.

Summary

ARC works by adding code at compile time to ensure that objects live as long as necessary, but no longer. Conceptually, it follows the same memory management conventions as manual reference counting (described in Advanced Memory Management Programming Guide) by adding the appropriate memory management calls for you.

In order for the compiler to generate correct code, ARC restricts the methods you can use and how you use toll-free bridging (see Toll-Free Bridged Types). ARC also introduces new lifetime qualifiers for object references and declared properties.

ARC is supported in Xcode 4.2 for OS X v10.6 and v10.7 (64-bit applications) and for iOS 4 and iOS 5. Weak references are not supported in OS X v10.6 and iOS 4.

Xcode provides a tool that automates the mechanical parts of the ARC conversion (such as removing retain and release calls) and helps you to fix issues the migrator can’t handle automatically (choose Edit > Refactor > Convert to Objective-C ARC). The migration tool converts all files in a project to use ARC. You can also choose to use ARC on a per-file basis if it’s more convenient for you to use manual reference counting for some files.

See also:

Advanced Memory Management Programming Guide

Memory Management Programming Guide for Core Foundation ARC Overview

Instead of you having to remember when to use retain, release, and autorelease, ARC evaluates the lifetime requirements of your objects and automatically inserts appropriate memory management calls for you at compile time. The compiler also generates appropriate dealloc methods for you. In general, if you’re only using ARC the traditional Cocoa naming conventions are important only if you need to interoperate with code that uses manual reference counting.

A complete and correct implementation of a Person class might look like this:

@interface Person : NSObject

@property NSString *firstName;

@property NSString *lastName;

@property NSNumber *yearOfBirth;

@property Person *spouse;

@end

@implementation Person

@end

(Object properties are strong by default; the strong attribute is described in ARC Introduces New Lifetime Qualifiers.)

Using ARC, you could implement a contrived method like this:

- (void)contrived {

Person *aPerson = [[Person alloc] init];

[aPerson setFirstName:@"William"];

[aPerson setLastName:@"Dudney"];

[aPerson setYearOfBirth:[[NSNumber alloc] initWithInteger:2011]];

NSLog(@"aPerson: %@", aPerson);

}

ARC takes care of memory management so that neither the Person nor the NSNumber objects are leaked.

You could also safely implement a takeLastNameFrom: method of Person like this:

- (void)takeLastNameFrom:(Person *)person {

NSString *oldLastname = [self lastName];

[self setLastName:[person lastName]];

NSLog(@"Lastname changed from %@ to %@", oldLastname, [self lastName]);

}

ARC ensures that oldLastName is not deallocated before the NSLog statement. ARC Enforces New Rules

To work, ARC imposes some new rules that are not present when using other compiler modes. The rules are intended to provide a fully reliable memory management model; in some cases, they simply enforce best practice, in some others they simplify your code or are obvious corollaries of your not having to deal with memory management. If you violate these rules, you get an immediate compile-time error, not a subtle bug that may become apparent at runtime.

You cannot explicitly invoke dealloc, or implement or invoke retain, release, retainCount, or autorelease.

The prohibition extends to using @selector(retain), @selector(release), and so on.

You may implement a dealloc method if you need to manage resources other than releasing instance variables. You do not have to (indeed you cannot) release instance variables, but you may need to invoke [systemClassInstance setDelegate:nil] on system classes and other code that isn’t compiled using ARC.

Custom dealloc methods in ARC do not require a call to [super dealloc] (it actually results in a compiler error). The chaining to super is automated and enforced by the compiler.

You can still use CFRetain, CFRelease, and other related functions with Core Foundation-style objects (see Managing Toll-Free Bridging).

You cannot use NSAllocateObject or NSDeallocateObject.

You create objects using alloc; the runtime takes care of deallocating objects.

You cannot use object pointers in C structures.

Rather than using a struct, you can create an Objective-C class to manage the data instead.

There is no casual casting between id and void *.

You must use special casts that tell the compiler about object lifetime. You need to do this to cast between Objective-C objects and Core Foundation types that you pass as function arguments. For more details, see Managing Toll-Free Bridging.

You cannot use NSAutoreleasePool objects.

ARC provides @autoreleasepool blocks instead. These have an advantage of being more efficient than NSAutoreleasePool.

You cannot use memory zones.

There is no need to use NSZone any more—they are ignored by the modern Objective-C runtime anyway.

To allow interoperation with manual retain-release code, ARC imposes a constraint on method naming:

You cannot give an accessor a name that begins with new. This in turn means that you can’t, for example, declare a property whose name begins with new unless you specify a different getter:

// Won't work:

@property NSString *newTitle;

// Works:

@property (getter=theNewTitle) NSString *newTitle;

ARC Introduces New Lifetime Qualifiers

ARC introduces several new lifetime qualifiers for objects, and weak references. A weak reference does not extend the lifetime of the object it points to, and automatically becomesnil when there are no strong references to the object.

You should take advantage of these qualifiers to manage the object graphs in your program. In particular, ARC does not guard against strong reference cycles (previously known as retain cycles—see