copy和mutableCopy实践

首先说下两个基本定义：
浅复制：只copy指针而不是copy实际的资源，两个实例指向内存中统一资源。
深复制：不仅复制指针还复制指针，还复制指针所指向的资源，副本对象的指针指向内存中不同位置的同一资源的副本。

不可变对象的copy和mutableCopy操作：

不可变数组
NSArray *arr = [NSArray array];
NSMutableArray  *arr1 = arr.copy;
NSMutableArray *arr2 = arr.mutableCopy;
NSLog(@"%p--%p--%p",arr,arr1,arr2);//0x60800001a1b0--0x60800001a1b0--0x608000047290 
[arr2 insertObject:@"1" atIndex:0];//插入元素成功
[arr1 insertObject:@"1" atIndex:0];//'-[__NSArray0 insertObject:atIndex:]: unrecognized selector sent to instance

 不可变字典
 NSDictionary *dict1 = [NSDictionary dictionaryWithObjectsAndKeys:@"value1",@"key1",@"value2",@"key2", nil];
 NSMutableDictionary *dicM = dict1.copy;
 NSMutableDictionary *dicM1 = dict1.mutableCopy;
 NSLog(@"%p--%p--%p",dict1,dicM,dicM1);//0x6000002641c0--0x6000002641c0--0x600000045280
 [dicM1 removeObjectForKey:@"key1"];
    Printing description of dicM1:
   {
      key2 = value2;
   }

 [dicM removeObjectForKey:@"key1"];//-[__NSDictionaryI removeObjectForKey:]: unrecognized selector sent to instance

 不可变字符串
 NSString *str = @"123";
 NSMutableString *strM1 = str.copy;
 NSMutableString *strM2 = str.mutableCopy;
 NSLog(@"%p--%p--%p",str,strM1,strM2);// 0x1047ee068--0x1047ee068--0x608000079800
 [strM2 appendString:@"4"];//Printing description of strM2:
1234
 [strM1 appendString:@"5"]; //Attempt to mutate immutable object with appendString:

结论：对不可变对象进行copy操作，不会开辟新的内存，copy的只是内存地址；对不可变对象进行mutableCopy操作，会开辟新的内存，拷贝出的对象是可变的；

可变对象的copy和mutableCopy操作：

可变数组
NSMutableArray *arrM = [NSMutableArray arrayWithCapacity:0];
NSMutableArray *arrM1 = arrM.copy;
NSMutableArray *arrM2 = arrM.mutableCopy;
NSLog(@"%p--%p--%p",arrM,arrM1,arrM2);//arrM:0x608000047e60--arrM1:0x60800000f160--arrM2:0x608000047ce0
[arrM insertObject:@"2" atIndex:0];
[arrM2 insertObject:@"3" atIndex:0];
[arrM1 addObject:@"1"];//-[__NSArray0 addObject:]: unrecognized selector sent to instance


可变字典
NSMutableDictionary *dict1 = [NSMutableDictionary dictionaryWithObjectsAndKeys:@"value1",@"key1",@"value2",@"key2", nil];
NSMutableDictionary *dicM = dict1.copy;
NSMutableDictionary *dicM1 = dict1.mutableCopy;
NSLog(@"%p--%p--%p",dict1,dicM,dicM1);//0x60000005d7f0--0x600000074780--0x60000005b810
[dicM1 removeObjectForKey:@"key1"];
 Printing description of dicM1:
   {
      key2 = value2;
   }

[dicM removeObjectForKey:@"key1"];//-[__NSDictionaryI removeObjectForKey:]: unrecognized selector sent to instance

可变字符串
NSMutableString *str = [NSMutableString stringWithCapacity:0]; 
NSMutableString *strM1 = str.copy;
NSMutableString *strM2 = str.mutableCopy;
NSLog(@"%p--%p--%p",str,strM1,strM2);//0x6080000716c0--0x103d94570--0x608000071680
[strM2 appendString:@"4"];//Printing description of strM2:
4
[strM1 appendString:@"5"];//Attempt to mutate immutable object with appendString:

结论：对可变对象进行copy操作，会开辟新的内存，拷贝出的对象是不可变的；对可变对象进行mutableCopy操作，会开辟新的内存，拷贝出的对象是可变的；

关于有可变类型的不可变类（如NSDictionary，NSArray，NSString），声明属性时内存管理关键字为什么要用copy？

用strong声明结果如下：