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Mixing Managed Memory and Garbage Collection
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You may find yourself in a situation where you need to write Objective-C code that must function in both a managed memory and a garbage collection environment. It s possible to do this because the managed memory methods and garbage collection techniques have very little overlap. In a garbage collection environment, the messages that manage reference counting (-retain, -release, -autorelease) are ignored. In a managed memory environment, the compiler s support for garbage collection
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CHAPTER 24 MEMORY MANAGEMENT
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(__strong and __weak pointer types) is ignored. The end result is code that will operate correctly in either environment. When compiling code that will run in a mixed memory environment, the compiler s support for garbage collection should be made inclusive. In Xcode, set the Objective-C Garbage Collection build setting to Supported. This causes the compiler to include support for garbage collection, but also includes the code needed to support managed memory. When the Objective-C Garbage Collection build setting is set to Required (garbage collection only), the compiler actively strips out code that s only useful to managed memory. This makes the code more efficient, but sacrifices backward compatibility. Writing the code that will function in both environments is fairly straightforward. In general, code that correctly implements managed memory will also work in a garbage collection environment. For example, all of the code samples in this chapter would function correctly with garbage collection turned on. Table 24-2 lists the effects of each environment on your code. Table 24-2. Effects of Managed Memory vs. Garbage Collection
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-retain -release -autorelease -retainCount -dealloc -finalize __strong __weak -drain @property (assign) @property (retain)
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Retains an object Releases an object Adds object to autorelease pool Returns the current retain count Sent to an object to destroy it Never sent Ignored Ignored Drains the autorelease pool Unretained property Retained property
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Ignored Ignored Ignored Meaningless Runtime blocks this message from being sent Sent when an object is about to be collected Indicates a strong reference Indicates a weak reference Triggers garbage collection Strong reference Strong reference
NSAutoreleasePool s -drain method is about the only method that s functional in both environments. With garbage collection turned on, it hints to the garbage collector that it might want to start a garbage collection cycle.
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CHAPTER 24 MEMORY MANAGEMENT
Summary
Managed memory requires some additional effort, but it isn t onerous. Just remember these basic rules: When storing an object reference in a persistent variable, retain it. Release or autorelease any retained reference before discarding it. Write a -dealloc method to release all retained objects. Always return retained or autoreleased objects. Autoreleased objects won t be released until after the current method has returned.
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CHAPTER 25
Mixing C and Objective-C
One of the great strengths of Objective-C is that it allows seamless integration with C functions and variables. This gives you direct access to the vast wealth of existing C APIs. This chapter discusses the basic ways to mix C and Objective-C code in the same application, and covers Core Foundation, a C library that includes data structures that are interchangeable with Objective-C objects. Mixing languages creates some interesting memory management issues, which are explained toward the end of the chapter.
Using C in Objective-C
Using C in Objective-C is somewhat misleading, because Objective-C is a strict superset of the C language. You really can t use C in Objective-C, since Objective-C is C. The term is usually applied when you write code that uses C structures and calls C functions directly, instead of using Objective-C objects and messages.
Calling C Functions from Objective-C
There have been a number of code examples presented in the book that implement C functions and use them in Objective-C classes. These are standard C functions that can be called from C or Objective-C code. A C function with a static type limits its scope to the code in the module being compiled. Nonstatic functions are global and can be called from any module. (Remember that in this context, static means private, not static in the Java sense.) An example of a static C function is shown in Listing 251. Listing 25-1. C Function and Objective-C Method static NSInteger compareZombieBrains( id l, id r, void *ignored ) { float lSize = [l brainSize]; float rSize = [r brainSize]; if (lSize<rSize) return NSOrderedAscending; else if (lSize>rSize) return NSOrderedDescending; return NSOrderedSame; }
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