iOS 浅谈：深.浅拷贝与copy.strong

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