iOS 浅谈：深.浅拷贝与copy.strong

http://help.3g.163.com/15/0908/11/B305R99R00964KAD.html

深.浅拷贝

copy mutableCopy NSString

NSString *string = @"汉斯哈哈哈";// 没有产生新对象NSString *copyString = [string copy];// 产生新对象NSMutableString *mutableCopyString = [string mutableCopy];NSLog(@"string = %p copyString = %p mutableCopyString = %p", string, copyString, mutableCopyString);

copy mutableCopy NSMutableString

NSMutableString *string = [NSMutableString stringWithString:@"汉斯哈哈哈"];// 产生新对象NSString *copyString = [string copy];// 产生新对象NSMutableString *mutableCopyString = [string mutableCopy];NSLog(@"string = %p copyString = %p mutableCopyString = %p", string, copyString, mutableCopyString);

结论：



注意：其他对象NSArray、NSMutableArray 、NSDictionary、NSMutableDictionary一样适用



copy NSObject

HSPerson *p = [[HSPerson alloc] init];p.age = 20;p.height = 170.0;HSPerson *copyP = [p copy]; // 这里崩溃

崩溃：

看崩溃信息HSPerson应该先实现：
- (id)copyWithZone:(NSZone *)zone;

测试：
#import "HSPerson.h"@interface HSPerson()@end@implementation HSPerson- (id)copyWithZone:(NSZone *)zone{ return @"汉斯哈哈哈";}@end HSPerson *p = [[HSPerson alloc] init];p.age = 20;p.height = 170.0;HSPerson *copyP = [p copy];NSLog(@"copyP: %@", copyP);

可以看出copyWithZone重新分配新的内存空间，则：
- (id)copyWithZone:(NSZone *)zone{ HSPerson *person = [[HSPerson allocWithZone:zone] init]; return person;// 有些人可能下面alloc,重新初始化空间，但这方法已给你分配了zone，自己就无需再次alloc内存空间了// HSPerson *person = [[HSPerson alloc] init];}
HSPerson *p = [[HSPerson alloc] init];p.age = 20;p.height = 170.0;HSPerson *copyP = [p copy];NSLog(@"p = %p copyP = %p", p, copyP);NSLog(@"age = %d height = %f", copyP.age, copyP.height);



虽然copy了份新的对象，然而age,height值并未copy，那么：
- (id)copyWithZone:(NSZone *)zone{ HSPerson *person = [[HSPerson allocWithZone:zone] init]; person.age = self.age; person.height = self.height; // 这里self其实就要被copy的那个对象，很显然要自己赋值给新对象，所以这里可以控制copy的属性 return person;}



这时你会想，有NSMutableCopying？没错，是有这货：
- (id)mutableCopyWithZone:(NSZone *)zone{ HSPerson *person = [[HSPerson allocWithZone:zone] init]; person.age = self.age; person.height = self.height; return person;}

NSCopying、NSMutableCopying有啥区别？

其实感觉没必要有NSMutableCopying，因为压根就没可变的HSPerson，但如果该对象有其他行为，可以借用NSMutableCopying实现，哈哈哈

copy.strong

说完深浅拷贝，理解copy.strong就轻松多了！

copy

#import @interface HSPerson : NSObject@property (nonatomic, copy) NSString *name;@end NSMutableString *string = [NSMutableString stringWithFormat:@"汉斯哈哈哈"];HSPerson *person = [[HSPerson alloc] init];person.name = string;// 不能改变person.name的值，因为其内部copy新的对象[string appendString:@" hans"]; NSLog(@"name = %@", person.name);

property copy 实际上就对name干了这个：
- (void)setName:(NSString *)name{ _name = [name copy];}

假设name为NSMutableString，会发生什么事？
@property (nonatomic, copy) NSMutableString *name;

这样会挨骂哦，实际上内部还是：
- (void)setName:(NSMutableString *)name{ _name = [name copy];}

copy出来的仍然是不可变字符！如果有人用NSMutableString的方法，就会崩溃:



strong

@property (nonatomic, strong) NSString *name; NSMutableString *string = [NSMutableString stringWithFormat:@"汉斯哈哈哈"];HSPerson *person = [[HSPerson alloc] init];person.name = string;// 可以改变person.name的值，因为其内部没有生成新的对象[string appendString:@" hans"];NSLog(@"name = %@", person.name);

总结：用copy与strong取决于需求，如果不希望被外界更改用copy，反之用strong