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刚学习oc,发现一个很难理解的问题
假设我在某一个类里面定义如下几个实例属性

@property (nonatomic, strong) NSString *strongString;
@property (nonatomic, strong) NSMutableString *strongMutableString;
@property (nonatomic, copy) NSString *copyedString;
@property (nonatomic, copy) NSMutableString *copyedMutableString;

调用实例方法,实现的功能如下,就是简单的赋值

  1. 将不可变字符串赋值给属性

        NSString *string = [NSString stringWithFormat:@"abc"];
        self.strongString = string;
        self.strongMutableString = string;
        self.copyedMutableString = string;
        self.copyedString = string;
        
        string = [string stringByAppendingString:@"123"];
        // 打印输出实例属性的地址
        NSLog(@"\n origin string: %p, %p  %@", string, &string, string);
        NSLog(@"\n strong string: %p, %p  %@", _strongString, &_strongString, _strongString);
        NSLog(@"\n strongMutable string: %p, %p  %@", _strongMutableString, &_strongMutableString, _strongMutableString);
        NSLog(@"\n copy string: %p, %p  %@", _copyedString, &_copyedString, _copyedString);
        NSLog(@"\n copyMutable: %p, %p  %@", _copyedMutableString, &_copyedMutableString, _copyedMutableString);

    输出如下

     origin string: 0x100534da0, 0x7ffeefbff4c8  abc123
     strong string: 0x21bd4156d4fa7111, 0x100534c88  abc
     strongMutable string: 0x21bd4156d4fa7111, 0x100534c90  abc
     copy string: 0x21bd4156d4fa7111, 0x100534c98  abc
     copyMutable: 0x21bd4156d4fa7111, 0x100534ca0  abc

    这个大概能理解 都是浅拷贝 地址没有变化

  2. 然而将可变字符串赋值给属性时,我就搞不懂了

        NSMutableString *multableString = [NSMutableString stringWithFormat:@"def"];
        self.strongString = multableString;
        self.strongMutableString = multableString;
        self.copyedMutableString = multableString;
        self.copyedString = multableString;
        
        [multableString appendString:@"123"];
        // 打印输出实例属性的地址
        NSLog(@"\n origin string: %p, %p  %@", multableString, &multableString, multableString);
        NSLog(@"\n strong string: %p, %p  %@", _strongString, &_strongString, _strongString);
        NSLog(@"\n strongMutable string: %p, %p  %@", _strongMutableString, &_strongMutableString, _strongMutableString);
        NSLog(@"\n copy string: %p, %p  %@", _copyedString, &_copyedString, _copyedString);
        NSLog(@"\n copyMutable: %p, %p  %@", _copyedMutableString, &_copyedMutableString, _copyedMutableString);

    输出结果

     origin string: 0x100651af0, 0x7ffeefbff4c8  def123
     strong string: 0x100651af0, 0x100651a28  def123
     strongMutable string: 0x100651af0, 0x100651a30  def123
     copy string: 0x64fd82cbad71056f, 0x100651a38  def
     copyMutable: 0x64fd82cbad71056f, 0x100651a40  def

    strong修饰的属性是浅拷贝,地址一样我能理解但是!!
    为什么copy的属性,地址是一样的????

4 天前提问
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