创建了Person类,里面声明个name属性,关键字用copy
@property (nonatomic, copy)NSString *name;
在ViewController里给name赋值
NSMutableString *str = [[NSMutableString alloc] initWithString:@"iPhone"];
Person *person = [[Person alloc]init];
person.name = str;
[str appendString:@"6"];
NSLog(@"\n%@\n%@", str, person.name);
NSLog(@"\n%p\n%p", str, person.name);
打印结果
//iphone6 iphone6 strong
//0x600001f8c030 //0xd5a5b31a6478d3f4 copy
//0x60000310ea90 //0x60000310ea90 strong
如果strong来修饰,如果NSMutableString的值赋值给NSString,那么只是将NSString指向了NSMutableString的内存地址,并对NSMUtbaleString计数器加一,此时,如果对NSMutableString进行修改,也会导致NSString的值修改,person的name属性会随着str的改变而改变,(试想一下一个人的名字怎么能在不知情的情况下被被改变那),破坏了封装性,原则上这是不允许的.如果是copy修饰的NSString对象,在用NSMutableString给他赋值时,会进行深拷贝,及把内容也给拷贝了一份,两者指向不同的位置,即使改变了NSMutableString的值,NSString的值也不会改变.所以用copy是为了安全,防止NSMutableString赋值给NSString时,前者修改引起后者值变化而用的.
以上规则不止适用于NSString，NSArray，NSDictionary等同理
//NSArray
NSMutableArray *arr = [[NSMutableArray alloc] initWithArray:@[@"iphone6",@"iphone8"]];
Person *p = [[Person alloc] init];
p.arr = arr;
[arr addObject:@"iphone x"];
DLog(@"\n%@\n%@",arr,p.arr);
/*
(
iphone6,
iphone8, copy (arr)
"iphone x"
)
(
iphone6, copy(p.arr)
iphone8
)
strong
(
iphone6,
iphone8,
"iphone x"
)
(
iphone6,
iphone8,
"iphone x"
)
*/
DLog(@"\n%p\n%p",arr,p.arr);
//copy 0x60000085d1d0 0x60000066d500
//strong 0x600001ba84b0 0x600001ba84b0
Person *p = [[Person alloc] init];
p.arr = arr;
[arr addObject:@"iphone x"];
DLog(@"\n%@\n%@",arr,p.arr);
/*
(
iphone6,
iphone8, copy (arr)
"iphone x"
)
(
iphone6, copy(p.arr)
iphone8
)
strong
(
iphone6,
iphone8,
"iphone x"
)
(
iphone6,
iphone8,
"iphone x"
)
*/
DLog(@"\n%p\n%p",arr,p.arr);
//copy 0x60000085d1d0 0x60000066d500
//strong 0x600001ba84b0 0x600001ba84b0
NSDictionary
NSMutableDictionary *dict = [[NSMutableDictionary alloc] initWithObjectsAndKeys:@"iphone6",@"key1",@"iphone7",@"key2", nil];
Person *p = [[Person alloc] init];
p.dict = dict;
[dict addEntriesFromDictionary:@{@"key3":@"iphone8"}];
DLog(@"\n%@\n%@",dict,p.dict);
DLog(@"\n%p\n%p",dict,p.dict);
/*
copy
{
key1 = iphone6;
key2 = iphone7;
key3 = iphone8;
}
{
key1 = iphone6;
key2 = iphone7;
}
0x600003436800
0x600003437700
*/
/*
strong
{
key1 = iphone6;
key2 = iphone7;
key3 = iphone8;
}
{
key1 = iphone6;
key2 = iphone7;
key3 = iphone8;
}
0x60000276ee60
0x60000276ee60
*/
Person *p = [[Person alloc] init];
p.dict = dict;
[dict addEntriesFromDictionary:@{@"key3":@"iphone8"}];
DLog(@"\n%@\n%@",dict,p.dict);
DLog(@"\n%p\n%p",dict,p.dict);
/*
copy
{
key1 = iphone6;
key2 = iphone7;
key3 = iphone8;
}
{
key1 = iphone6;
key2 = iphone7;
}
0x600003436800
0x600003437700
*/
/*
strong
{
key1 = iphone6;
key2 = iphone7;
key3 = iphone8;
}
{
key1 = iphone6;
key2 = iphone7;
key3 = iphone8;
}
0x60000276ee60
0x60000276ee60
*/