objective-c - 关于保留和复制

Question

我以为我已经理解了retain和copy之间的区别。但当我遇到下面的代码时，我又困惑了。这些是代码:

ViewController.h:

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController {
    NSMutableString *a;
    NSMutableString *b;
    NSMutableString *c;
}

@property (nonatomic, copy) NSMutableString *a;
@property (nonatomic, copy) NSMutableString *b;
@property (nonatomic, copy) NSMutableString *c;

@end

ViewController.m:

#import "ViewController.h"

@implementation ViewController

@synthesize a, b, c;

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.a = [[NSMutableString alloc] initWithFormat:@"%@", @"aaa"];
    NSLog(@"a:%d", a.retainCount);

    self.b = a;
    NSLog(@"a:%d", a.retainCount);
    NSLog(@"b:%d", b.retainCount);

    self.c = b;
    NSLog(@"a:%d", a.retainCount);
    NSLog(@"b:%d", b.retainCount);
    NSLog(@"b:%d", c.retainCount);
}

@end

谁能解释一下为什么结果是这样的:

2011-12-31 16:54:50.244 RetainCopy[5783:207] a:1
2011-12-31 16:54:50.246 RetainCopy[5783:207] a:2
2011-12-31 16:54:50.246 RetainCopy[5783:207] b:2
2011-12-31 16:54:50.247 RetainCopy[5783:207] a:3
2011-12-31 16:54:50.247 RetainCopy[5783:207] b:3
2011-12-31 16:54:50.248 RetainCopy[5783:207] b:3

？谢谢。

Answer 1

首先，-retainCount 是 useless to you .

第二，一个误解是在复制属性实现的实现上。您不会像您期望的那样在这里获得副本。相反，您得到的结果是 [arg copy] 而不是 [arg mutableCopy]。许多不可变类型会像这样“复制”自身(简化):

- (id)copyWithZone:(NSZone*)zone
{
  return [self retain];
}

所以最终的结果是:

NSMutableString * a = [[NSMutableString alloc] initWithString:@"aaa"];
NSString * b = [a copy]; // << returns an immutable copy!
NSString * c = [b copy]; // << immutable to immutable probably just retains itself!

由于它们是不可变的且纯粹的，因此这在语义上是合理的。这还意味着，如果类型区分不可变副本和可变副本，您将需要自己为可变类型实现副本访问器。否则，当您尝试改变 ivar 时，您的程序可能会崩溃，因为您持有一个不可变的实例。如果将其添加到 -viewDidLoad 的末尾，您应该会看到一个异常:[self.c appendString:@"uh-oh"];

More detail on implementing a mutable property here .