iOS 浅谈:深.浅拷贝与copy.strong
2016-10-20 22:35
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深.浅拷贝
copy/mutableCopy NSStringNSString *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()<NSCopying> @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
property里的copy、strong区别
说完深浅拷贝,理解property里的copy、strong就轻松多了!copy
#import <Foundation/Foundation.h> @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);
原文链接:http://www.jianshu.com/p/e6a7cdcc705d
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