iOS 开发刷题系列三：NSString 引用计数

下面的程序会输出什么？

    NSMutableArray *ary = [[NSMutableArray array] retain];
    NSString *str = [NSString stringWithFormat:@"123456789"];
    NSString *longStr = [NSString stringWithFormat:@"1234567890"];
    
    [str retain];
    [longStr retain];
    [ary addObject:str];
    [ary addObject:longStr];
    
    NSLog(@"str = %ld", (unsigned long)[str retainCount]);
    NSLog(@"longStr = %ld", (unsigned long)[longStr retainCount]);
    
    [str retain];
    [str release];
    [str release];
    [longStr retain];
    [longStr release];
    [longStr release];
    
    NSLog(@"str = %ld", (unsigned long)[str retainCount]);
    NSLog(@"longStr = %ld", (unsigned long)[longStr retainCount]);
    [ary removeAllObjects];
    NSLog(@"str = %ld", (unsigned long)[str retainCount]);
    NSLog(@"longStr = %ld", (unsigned long)[longStr retainCount]);

输出结果

2018-07-03 13:54:59.951143+0800 BlockTestDemo[13502:2107264] str = -1
2018-07-03 13:54:59.951374+0800 BlockTestDemo[13502:2107264] longStr = 3
2018-07-03 13:54:59.951613+0800 BlockTestDemo[13502:2107264] str = -1
2018-07-03 13:54:59.951717+0800 BlockTestDemo[13502:2107264] longStr = 2
2018-07-03 13:54:59.951956+0800 BlockTestDemo[13502:2107264] str = -1
2018-07-03 13:54:59.952044+0800 BlockTestDemo[13502:2107264] longStr = 1

在网上搜索了一下，一般人给出的答案是：当字符串长度小于10时，字符串是保存在常量区，没有引用计数。如果长度大于等于10呢，就会被复制到堆去，有引用计数。

后来又出现了一个词：Tagged Pointer 具体了解一下。 尝试着输出字符串的class，发现两者的类名是不同的：

NSString *str = [NSString stringWithFormat:@"123456789"];
NSString *longStr = [NSString stringWithFormat:@"1234567890"];
NSLog(@"str %s %p", object_getClassName(str), str);
NSLog(@"longStr %s %p", object_getClassName(longStr), longStr);

2018-07-03 13:54:59.950804+0800 BlockTestDemo[13502:2107264] str NSTaggedPointerString 0xa1ea1f72bb30ab19
2018-07-03 13:54:59.950979+0800 BlockTestDemo[13502:2107264] longStr __NSCFString 0x60c000224f20

Tagged Pointer专门用来存储小的对象，例如NSNumber和NSDate
Tagged Pointer指针的值不再是地址了，而是真正的值。所以，实际上它不再是一个对象了，它只是一个披着对象皮的普通变量而已。所以，它的内存并不存储在堆中，也不需要malloc和free。

这应该也是上面的NSString在长度小于10的时候，没有引用计数的原因了。

引申

这种情况引申出另外一道题：

@property (nonatomic, strong) NSString *strongStr;

dispatch_queue_t queue = dispatch_queue_create("strongStr", DISPATCH_QUEUE_CONCURRENT);
    for (int i = 0; i < 100000; i++) {
        dispatch_async(queue, ^{
            self.strongStr = [NSString stringWithFormat:@"ab %d", i];
        });
    }

如果将dispatch_async 里面的内容改成：

self.strongStr = [NSString stringWithFormat:@"abcdefghijklmn %d", i];

会如何？
前者不会crash, 而后者会crash。
我们来看一下strongStr的setter方法：

- (void)setStrongStr:(NSString *)strongStr {
    if (strongStr == _strongStr) return;
    id pre = _strongStr;
    [strongStr retain];//1.先保留新值
    _strongStr = strongStr;//2.再进行赋值
    [pre release];//3.释放旧值
}

结合上面的Tagged Pointer的解释，调用retain or release时strongStr的引用计数一直都是-1;
而对于后者，strongStr实际上是一个对象，retain会使引用计数+1，release会使引用计数 -1;
而对于多线程异步并行执行setStrongStr方法，可能会出现这种情况：多个线程拿到同一个旧值，然后给strongStr赋值不同的新值，然后在对旧值的release时候，出现多次release，程序crash;