NSString copy strong

从本类及其子类两方面分析：可创建可变对象和不可变对象两种

   NSString *str = @"1234";
    NSString *str2 = [str copy];
    NSString *str3 = str;
    
    NSLog(@"str地址:%p，str:%@",str,str);
    NSLog(@"str2地址:%p，str2:%@",str2,str2);
    NSLog(@"str3地址:%p，str3:%@",str3,str3);
    
//    str2 = @"22222";
    str = @"new 1234";
    NSLog(@"changing\n ...");
    
    NSLog(@"str地址:%p，str:%@",str,str);
    NSLog(@"str2地址:%p，str2:%@",str2,str2);
    NSLog(@"str3地址:%p，str3:%@",str3,str3);

打印结果：
2018-07-04 17:30:25.652461+0800 Test[93406:1493493] str地址:0x104e80aa8，str:1234
2018-07-04 17:30:25.652610+0800 Test[93406:1493493] str2地址:0x104e80aa8，str2:1234
2018-07-04 17:30:25.652762+0800 Test[93406:1493493] str3地址:0x104e80aa8，str3:1234
2018-07-04 17:30:25.652870+0800 Test[93406:1493493] changing
 ...
2018-07-04 17:30:25.653056+0800 Test[93406:1493493] str地址:0x104e80b28，str:new 1234
2018-07-04 17:30:25.653236+0800 Test[93406:1493493] str2地址:0x104e80aa8，str2:1234
2018-07-04 17:30:25.653353+0800 Test[93406:1493493] str3地址:0x104e80aa8，str3:1234

以上结果可见：不可变字符对象，strong copy均不开辟新的内存空间，为浅拷贝

这是其实是事实意义上的深拷贝.因为无论你怎么修改str的值,都不会影响到str3.---bySpring （略有争议）

   NSString  *str = @"a";
    NSString *str2 = @"a";
    
    NSLog(@"str地址:%p，str:%@",str,str);
    NSLog(@"str2地址:%p，str2:%@",str2,str2);

2018-07-04 17:36:22.216211+0800 Test[93558:1499765] str地址:0x10f1a1aa8，str:a
2018-07-04 17:36:22.216344+0800 Test[93558:1499765] str2地址:0x10f1a1aa8，str2:a

相同字符串在内存中开辟的空间是一样的。对于NSString的不可变对象，copy同strong

NSMutableString *str = [NSMutableString stringWithFormat:@"222"];
    NSString *str2 = [str copy];
    NSString *str3 = str;
    
    NSLog(@"str地址:%p，str:%@",str,str);
    NSLog(@"str2地址:%p，str2:%@",str2,str2);
    NSLog(@"str3地址:%p，str3:%@",str3,str3);
    
    [str replaceCharactersInRange:NSMakeRange(0, 1) withString:@"new--"];
    NSLog(@"changing\n ...");
    
    NSLog(@"str地址:%p，str:%@",str,str);
    NSLog(@"str2地址:%p，str2:%@",str2,str2);
    NSLog(@"str3地址:%p，str3:%@",str3,str3);

打印结果
2018-07-04 17:42:02.876714+0800 Test[93703:1505747] str地址:0x604000448700，str:222
2018-07-04 17:42:02.876903+0800 Test[93703:1505747] str2地址:0xa000000003232323，str2:222
2018-07-04 17:42:02.877027+0800 Test[93703:1505747] str3地址:0x604000448700，str3:222
2018-07-04 17:42:02.877156+0800 Test[93703:1505747] changing
 ...
2018-07-04 17:42:02.877332+0800 Test[93703:1505747] str地址:0x604000448700，str:new--22
2018-07-04 17:42:02.877705+0800 Test[93703:1505747] str2地址:0xa000000003232323，str2:222
2018-07-04 17:42:02.878034+0800 Test[93703:1505747] str3地址:0x604000448700，str3:new--22

上述测试说明：NSMutableString copy为深拷贝，strong仅增加引用计数，当原对象内容修改之后，strong指针同步修改

参考链接

https://juejin.im/entry/57b15244a633bd00570955be
https://www.jianshu.com/p/ca7e206a2922