Cocoa Programming for Mac OS X 第四章（Memory Management）摘录

The other two choices will enable the garbage collector. You must recompile before the change takes effect. (The difference between Supported and required is interesting only if you are creating a framework or plug-in that will be used in another application.)

Your application will have an instance of NSGarbageCollector if and only if it is using the garbage collector. Add a line near the end of lottery.m:

[pool drain];NSLog(@"GC = %@",[NSGarbageCollector defaultCollector]);return 0;

Build and run the tool. Are you using the garbage collector?

If you are using the garbage collector,it is important that you not keep references to objects that you don't care about. In themain function of lotterynow and array pointers to nil when you lose interest in them. Add those lines now:

// Done with 'now'，now是一个局部变量

now = nil;

for (LotteryEntry *entryToPrint in array) {

        NSLog(@"%@",entryToPrint);

    }    

}  

// Done with 'array'，array是一个局部变量

array = nil;

[pool drain];

NSLog(@"GC = %@",[NSGarbageCollector defaultCollector]);

Now the garbage collector will know that it can deallocate the NSCalendarDate that now pointed to and the NSMutableArray thatarray pointed to. (In reality,the program almost certainly exits before the garbage collector gets around to deallocating those objects.)

三、living with Retain Counts

Creating autoreleased Objects

You created a description method that looks like this:

{

    Nsstring *result;

    result = [[Nsstring alloc] initWithFormat:@"%@ = %d and %d",

              [entryDate descriptionWithCalendarFormat:@"%b %d %Y"],monospace">              firstNumber,secondNumber];

    return result;

}

This code would work perfectly well but would result in an annoying memory leak. The alloc operation always yIElds an object with a retain count of 1; thus,the string being returned has a retain count of 1. Any object asking for the string would retain it. The string would then have a retain count of 2. When no longer interested in the string,the object would release it. The retain count would become 1. As you see,the string would never be deallocated.

Our next attempt might look something like this:

This code would not work at all. When sent the message release

The problem,then,is that you need to return a string,but you do not want to retain it. This is a common problem throughout the frameworks,which leads us to how the NSautoreleasePool is used in a non-GC application.

Objects are added to the current autorelease pool when they are sent the message autorelease. When the autorelease pool is drained,it sends the message release to all objects in the pool.

In other words,when an object is autoreleased,it is marked to be sent release sometime in the future. In particular,in a Cocoa application,an autorelease pool is created before every event is handled and is drained after the event has been handled. Thus,unless the objects in the autorelease pools are being retained,they will be destroyed as soon as the event has been handled.

A correct solution,is

Rules Concerning Release

Objects created by alloc, new,monospace">copy,or mutablecopy have a retain count of 1 and are not in the autorelease pool.

If you get an object by any other method,assume that it has a retain count of 1 and is in the autorelease pool. If you do not wish it to be deallocated with the current autorelease pool,you must retain it.

Because you will frequently need objects that you are not retaining,many classes have class methods that return autoreleased objects. Nsstring,has stringWithFormat:. The simplest correct solution then would be

Note that the autoreleased object won't be released until the event loop ends. This behavior makes it perfect for provIDing an intermediate result. For example,if you had an array of Nsstring objects,you Could create a string with all the elements in uppercase and concatenated together,like this:

With this method,if you have 13 strings in the array,26 autoreleased strings will be created: 13 by uppercaseString and 13 bystringWithFormat:; the initial constant string is a special case and doesn't count. One of the resulting strings is returned and may be retained by the object that asked for it. The other 25 strings are deallocated automatically at the end of the event loop. (Note that you would probably get better performance in this example by appending the uppercased string to anNSMutableString instead of creating a new string and adding it to the autorelease pool each time through the loop.)

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