×

iOS 浅谈:深.浅拷贝与copy.strong

96
汉斯哈哈哈
2015.08.11 13:38* 字数 285

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()<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);

文章同步到微信公众号:hans_iOS 有疑问可以在公众号里直接发

iOS开发
Web note ad 1