Objective-C内存管理指南

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"Memory Management Programming Guide.pdf 是一本专注于Objective-C编程中的内存管理的英文书籍，由Apple Inc.于2010年6月24日发布。书中详细介绍了如何有效地管理内存以提升程序性能，同时强调了版权保护，未经许可不得复制或用于商业目的。书中的内容可能包括对苹果技术的知识产权的描述，但不提供任何明确的许可证，仅供应用开发者参考以优化内存使用。" 内存管理是计算机科学中的关键概念，特别是在像Objective-C这样的面向对象编程语言中。Objective-C内存管理主要基于引用计数（Reference Counting）机制，这是苹果在早期iOS和Mac OS X开发中推荐的方法。这本书可能涵盖了以下几个核心知识点： 1. **引用计数**：每个对象都有一个引用计数，表示有多少个指针指向它。当对象的引用计数变为零时，系统会自动释放该对象的内存。了解何时增加和减少对象的引用计数是避免内存泄漏的关键。 2. **所有权规则**：Objective-C遵循所有权规则，即创建对象的对象负责销毁它。当你调用`alloc`, `new`, `copy`, 或 `retain`方法时，你成为对象的所有者，需要在不再使用时调用`release`或`autorelease`来减少引用计数。 3. **autorelease**：`autorelease`是延迟释放的一种方式，对象会在某个特定时间点（通常是下一次垃圾回收时）被释放。这对于跨越多个方法边界管理对象生命周期很有用。 4. **内存泄漏检测**：学习如何使用工具（如Instruments）检测并修复内存泄漏，确保程序的效率和稳定性。 5. **弱引用**：弱引用不会增加对象的引用计数，这在防止循环引用和保持对象生命周期独立性时非常有用。 6. **ARC（Automatic Reference Counting）**：虽然书可能在ARC出现之前出版，但理解ARC的引入如何简化内存管理也是重要的。ARC是Objective-C的一个特性，自动处理对象的引用计数，减少了手动管理的需要。 7. **内存区域**：理解堆（Heap）和栈（Stack）的区别，以及对象如何在这些区域之间分配，有助于优化内存使用。 8. **内存对齐与块分配**：了解内存是如何在物理层面分配和对齐的，以及如何影响程序的性能。 9. **性能优化**：书中可能会讨论如何通过有效的内存管理策略提高程序运行速度，例如减少不必要的内存分配和释放，以及避免过度的内存碎片。 10. **错误处理**：在处理内存问题时，错误处理是必不可少的。学会识别和处理内存相关错误，如`EXC_BAD_ACCESS`，可以帮助开发者调试和修复程序。通过深入学习和实践《Memory Management Programming Guide》，开发者将能够掌握Objective-C中的内存管理技巧，从而编写出更高效、更稳定的应用程序。

This article summarizes the rules for memory management in Objective-C.

This is the fundamental rule:

■ You only release or autorelease objects you own.

❏

You take ownership of an object if you create it using a method whose name begins with “alloc” or

“new” or contains “copy” (for example, alloc, newObject, or mutableCopy), or if you send it a

retain message.

❏

You use release or autorelease to relinquish ownership of an object. autorelease just means

“send a release message in the future” (to understand when this will be, see “Autorelease

Pools” (page 29)).

The following rules derive from the fundamental rule, or cope with edge cases:

■ As a corollary of the fundamental rule, if you need to store a received object as a property in an instance

variable, you must retain or copy it. (This is not true for weak references, described at “Weak References

to Objects” (page 17), but these are typically rare.) Generally you devolve responsibility for doing this

to accessor methods (see “Accessor Methods” (page 35)).

■ A received object is normally guaranteed to remain valid within the method it was received in, and that

method may also safely return the object to its invoker. Exceptions include multithreaded applications

and some Distributed Objects situations, although you must also take care if you modify an object from

which you received another object (see “Validity of Shared Objects” (page 14)). Use retain in combination

with release or autorelease when needed to prevent an object from being invalidated as a normal

side-effect of a message.

The reasoning behind these rules is discussed in “Object Ownership and Disposal” (page 11).

Important: There are similar memory management rules for Core Foundation objects (see Memory

Management Programming Guide for Core Foundation). The naming conventions for Cocoa and Core Foundation,

however, are different. In particular, Core Foundation’s “create rule in Memory Management Programming

Guide for Core Foundation” does not apply to methods that return Objective-C objects. For example, in the

following code fragment, you are not responsible for relinquishing ownership of myInstance:

MyClass *myInstance = [MyClass createInstance];

See also “Memory Management of Core Foundation Objects in Cocoa” (page 45).

Memory Management Rules

It is good practice for a program to use as little memory as possible. Environments therefore define mechanisms

and policies to allow you to manage your program’s memory. Although you can consider memory management

of an Objective-C program in terms of the underlying implementation (described in “Behind the Scenes:

Retain Counts” (page 12)) is typically easier to think of it in terms of object ownership.

In an Objective-C program, objects are created and destroyed. To ensure that your application does not use

more memory than necessary, objects should be destroyed when they are no longer needed. It is important

of course, however, that objects are not destroyed if they are still needed. To support these requirements,

Cocoa defines a mechanism, object ownership, by which you can specify when you need an object and when

you have finished with it.

To fully understand how the object ownership policy is implemented in Cocoa, you must also read “Autorelease

Pools” (page 29).

Object Ownership Policy

Any object may have one or more owner. As long as an object has at least one owner, it continues to exist.

If an object has no owners, the runtime system destroys it automatically (see “Deallocating an Object” (page

15)). To make sure it is clear when you own an object and when you do not, Cocoa sets the following policy:

■ You own any object you create.

You “create” an object using a method whose name begins with “alloc” or “new” or contains “copy” (for

example, alloc, newObject, or mutableCopy).

■ You can take ownership of an object using retain.

Remember that an object may have more than one owner. Taking ownership of an object is your way

of saying that you need it to be kept alive. (This is discussed in more detail in “Accessor Methods” (page

15).)

■ You must relinquish ownership of objects you own when you’re finished with them.

You relinquish ownership of an object by sending it a release message or an autorelease message

(autorelease is discussed in more detail in “Autorelease” (page 12)). In Cocoa terminology, relinquishing

ownership of an object is therefore typically referred to as “releasing” an object.

■ You must not relinquish ownership of an object you do not own.

This is primarily an implicit corollary of the previous policy rules, made explicit.

This policy applies both to GUI-based Cocoa applications and to command-line Foundation tools.

Consider the following code fragment:

{

Thingamajig *myThingamajig = [[Thingamajig alloc] init];

Object Ownership Policy 11

Object Ownership and Disposal

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