NSString与奇怪的retainCount

话题从sunnyxx的《黑幕背后的Autorelease》开始

文章开头有个小例子

__weak id reference = nil;
- (void)viewDidLoad {
    [super viewDidLoad];
    NSString *str = [NSString stringWithFormat:@"sunnyxx"];
    // str是一个autorelease对象，设置一个weak的引用来观察它
    reference = str;
}
- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    NSLog(@"%@", reference); // Console: sunnyxx
}
- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    NSLog(@"%@", reference); // Console: (null)
}

测试以后发现三个方法中都输出了字符串”sunnyxx“，实际上sunnyxx对autorelease的分析还是很准确深刻的，输出结果不符合预期，是NSString在搞怪。

NSString是一个类簇（Class Clusters），最后生成的对象类型，取决于我们调用的初始化方法。不同的对象类型的管理方式不一样（在retainCount上有所体现）

类型	初始化后的retaincount
__NSCFConstantString	-1
__NSCFString	1
__NSTaggedPointerString	-1

• 对于__NSCFConstantString，系统进行维护，retain和release不起作用（程序中内容相同的常量字符串只有一个）
• __NSCFString与其他oc对象一样维护retainCount
• __NSTaggedPointerString，retain/release不起作用

NSString *a = @"a";
NSString *c = [a copy];
NSLog(@"%p %p", a,c);//输出两个相同的地址，并没有真正的复制

example

__NSCFConstantString

@""格式的字符串

    NSString *str1 = @"rainySue";//__NSCFConstantString
    NSLog(@"str1:%d",[str1 retainCount]);// -1
    

stringWithString+__NSCFConstantString

 NSString *str3 = [NSString stringWithString:@"str3"];//__NSCFConstantString
NSLog(@"str3:%d",[str3 retainCount]); // －1

__NSTaggedPointerString

case1

NSString *str2 = [NSString stringWithFormat:@"%s", "str2"];//__NSTaggedPointerString
NSLog(@"str2:%d",[str2 retainCount]); // -1  

case2

 NSString *a = @"a";
NSString *b = [[a mutableCopy] copy];//__NSTaggedPointerString
NSLog(@"b:%d",[b retainCount]); // -1

__NSCFString

通过stringWithFormat构造

NSString *str2_2 = [NSString stringWithFormat:@"%s,%d", "str2_2",22];//__NSCFString
NSLog(@"str2_2:%d",[str2_2 retainCount]); //1

stringWithString + stringWithFormat

NSString *str4_2 = [NSString stringWithString:[NSString stringWithFormat:@"hahah"]];//__NSCFString
NSLog(@"str4_2:%d",[str4_2 retainCount]); // 1

NSString *str4_1_1 = [NSString stringWithString:[NSString stringWithFormat:@"%s", "str2"]];
NSLog(@"str4_1_1:%d",[str4_1_1 retainCount]); //1

NSString *str4 = [NSString stringWithString:[NSString stringWithFormat:@"%s,%d", "str4",22]];//__NSCFString
    　　NSLog(@"str4:%d",[str4 retainCount]); // 1

stringWithString+__NSTaggedPointerString对象

NSString *str2 = [NSString stringWithFormat:@"%s", "str2"];//__NSTaggedPointerString
NSString *str4_1_1 = [NSString stringWithString:[NSString stringWithFormat:@"%s", "str2"]];
NSLog(@"str4_1_1:%d",[str4_1_1 retainCount]); //1

stringWithString+__NSCFString对象

NSString *str2_2 = [NSString stringWithFormat:@"%s,%d", "str2_2",22];//__NSCFString
SString *str4_3 = [NSString stringWithString:str2_2];
NSLog(@"str4_3:%d",[str4_3 retainCount]); //2

NSMutableString对象

NSMutableString* str5 = [NSMutableString stringWithString:@"str5"];//__NSCFString
NSLog(@"str5:%d",[str5 retainCount]); //1

总结

• retainCount的不同本质上是因为NSString类簇返回的子类的不同，__NSCFConstantString 和__NSTaggedPointerString初始值为-1，__NSCFString为1

• 亦可通过方法来区分得到的字符串的类型

  • @”“格式得到的为常量字符串
  • stringWithFormat得到的可能为__NSCFConstantString或者__NSTaggedPointerString

  • stringWithString

  1. stringWithString+__NSCFConstantString得到__NSCFConstantString
  2. stringWithString+stringWithFormat得到__NSCFString，初始计数值为1
  3. stringWithString+__NSTaggedPointerString对象得到__NSCFString，初始计数值为1
  4. stringWithString+__NSCFString对象得到__NSCFString，初始计数值为2

PS：
当开成程序中viewDidLoad里的str指向__NSCFString时，在viewWillAppear和viewDidAppear中就能看到预期的NULL了。