【转】iOS 浅谈:深.浅拷贝与copy.strong
深.浅拷贝
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copy mutableCopy NSString
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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); |
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copy mutableCopy NSMutableString
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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
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copy NSObject
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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:
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- (id)copyWithZone:(NSZone *)zone; |
测试:
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#import "HSPerson.h" @interface HSPerson()@end @implementation HSPerson - (id)copyWithZone:(NSZone *)zone { return @ "汉斯哈哈哈" ; } @end |
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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:
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- (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]; } |
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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:
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- (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:
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- (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!
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copy
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#import @interface HSPerson : NSObject @property (nonatomic, copy) NSString *name; @end |
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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:
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- ( void )setName:(NSString *)name { _name = [name copy]; } |
假设name为NSMutableString,会发生什么事?
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@property (nonatomic, copy) NSMutableString *name; |
这样会挨骂哦,实际上内部还是:
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- ( 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:
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strong
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@property (nonatomic, strong) NSString *name; |
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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
http://www.cocoachina.com/ios/20150908/13240.html