iOS开发中的深拷贝与浅拷贝

今天在一个技术群里看到关于copy和mutableCopy的讨论，感觉那个人说的不全面，今天顺便找个时间弄些例子重新总结下：

一、深拷贝就是拷贝对象本身到新的内存中，生成一个新的对象。浅拷贝就是拷贝指针，并不拷贝对象本身，原有的对象引用计数+1

二、非集合类对象copy与mutableCopy

NSString *str = @"123";
NSLog(@"%p", str);

NSString *copyStr = [str copy];
NSLog(@"%p", copyStr);

NSString *mutableCopyStr = [str mutableCopy];
NSLog(@"%p", mutableCopyStr);

2017-12-12 [2080:42478] 0x1076fa940
2017-12-12 [2080:42478] 0x1076fa940
2017-12-12 [2080:42478] 0x60400004e4f0

非集合类不可变对象的copy是浅拷贝，内存地址不变，mutableCopy是深拷贝

NSMutableString *str = [NSMutableString stringWithFormat:@"123"];
NSLog(@"%p", str);

NSString *copyStr = [str copy];
NSLog(@"%p", copyStr);

NSMutableString *mutableCopyStr = [str mutableCopy];
NSLog(@"%p", mutableCopyStr);

2017-12-12 [2174:48858] 0x6040002463f0
2017-12-12 [2174:48858] 0xa000000003332313
2017-12-12 [2174:48858] 0x60000024cc00

非集合类可变对象进行copy和mutableCopy都是深拷贝，内存地址发生改变。copy操作发生内存地址改变很好理解（从一个可变对象变成了不可变对象），为什么mutableCopy也是深拷贝呢？猜测认为，可变对象后面有可能是要改变的，如果执行的是浅拷贝（指针拷贝），那么原对象或者mutableCopy出来的对象如果其中一个发生了改变，都会影响另外一个，所以应该是新创建一个对象。

三、集合类对象copy与mutableCopy

NSArray *array = @[@"123", @"asf"];
NSLog(@"%p", array);

NSArray *copyArray = [array copy];
NSLog(@"%p", copyArray);

NSMutableArray *mutableCopyArray = [array mutableCopy];
NSLog(@"%p", mutableCopyArray);

2017-12-12 [2340:64439] 0x604000039660
2017-12-12 [2340:64439] 0x604000039660
2017-12-12 [2340:64439] 0x6040000592c0

同非集合类对象，集合类不可变对象的copy也是浅拷贝，mutableCopy是深拷贝

NSMutableArray *array = [@[@"123", @"asf"] mutableCopy];
NSLog(@"%p", array);

NSArray *copyArray = [array copy];
NSLog(@"%p", copyArray);

NSMutableArray *mutableCopyArray = [array mutableCopy];
NSLog(@"%p", mutableCopyArray);

2017-12-12 [2392:70043] 0x60000024d4d0
2017-12-12 [2392:70043] 0x600000032fe0
2017-12-12 [2392:70043] 0x60000024c900

如上，不论是copy还是mutableCopy 都对array进行了深拷贝。类似非集合类对象。

四、集合类的深拷贝

集合类的对象，在执行copy或者mutableCopy方法进行深拷贝的时候，仅仅是拷贝对象本身，对象内的元素仍然是浅拷贝（指针复制）

NSMutableString *string = [NSMutableString stringWithFormat:@"123"];
NSMutableArray *subArray = [@[@"123", @"adsf"] mutableCopy];

NSMutableArray *array = [NSMutableArray arrayWithObjects:string, subArray, nil];

NSArray *copyArray = [array copy];
NSLog(@"copyArray = %@", copyArray);

[string appendFormat:@"456"];
[subArray removeObjectAtIndex:0];
NSLog(@"copyArray = %@", copyArray);

2017-12-12 [3528:131325] copyArray= (
123,
    (
      123,
      adsf
    )
)   
2017-12-12 [3528:131325] copyArray = (
123456,
    (
      adsf
    )
)

如上，如果我们修改了copyArray里面的string和subArray的话，copyArray内的对象也会跟着改变.

对集合内的对象也执行深拷贝可以使用如下两种方法：

• NSArray *copyArray = [[NSArray alloc] initWithArray:array copyItems:YES];

    NSMutableString *string = [NSMutableString stringWithFormat:@"123"];
    NSMutableArray *subArray = [@[@"123", @"adsf"] mutableCopy];

    NSMutableArray *array = [NSMutableArray arrayWithObjects:string, subArray, nil];

    NSArray *copyArray = [[NSArray alloc] initWithArray:array copyItems:YES];
    NSLog(@"copyArray = %@", copyArray);

    [string appendFormat:@"456"];
    [subArray removeObjectAtIndex:0];
    NSLog(@"copyArray = %@", copyArray);
    
    2017-12-12 [3625:137732] copyArray = (
     123,
       (
         123,
         adsf
        )
     )
    
    2017-12-12 [3625:137732] copyArray = (
    123,
        (
          123,
          adsf
        )
    )

如上，发现copyArray内的对象并不会随着改变。但是如果集合内的对象里面的元素也改变了呢？

NSMutableString *string = [NSMutableString stringWithFormat:@"123"];
NSMutableArray *subArray = [@[string, @"adsf"] mutableCopy]; //这里在可变的数组里面加入可变的字符串string

NSMutableArray *array = [NSMutableArray arrayWithObjects:string, subArray, nil];

NSArray *copyArray = [[NSArray alloc] initWithArray:array copyItems:YES];
NSLog(@"copyArray = %@", copyArray);

[string appendFormat:@"456"];
[subArray removeObjectAtIndex:0];
NSLog(@"copyArray = %@", copyArray);

2017-12-12 [3687:144061] copyArray = (
123,
    (
      123,
      adsf
    )
)

2017-12-12 [3687:144061] copyArray = (
123,
    (
      123456,
      adsf
    )
)

这个时候我们发现如果array内部的subArray里面的string发生了改变，是会影响到copyArray的.

initWithXXXX: copyItems:这个方法只能提供一层深拷贝，并不是真正的深拷贝。这个方法是对集合内每个对象都发送copyWithZone:消息，如果集合内的对象遵守了NSCopying:协议那么会深拷贝到新的集合对象内，如果没有遵守，会出错。

那么怎么才能实现完全深拷贝呢:

• NSArray *deepCopyArray = [NSKeyedUnarchiver unarchiveObjectWithData:[NSKeyedArchiver archivedDataWithRootObject:array]];

这种将将集合进行归档(archive)，然后再解档(unarchive)，才可以实现完全深拷贝。

NSMutableString *string = [NSMutableString stringWithFormat:@"123"];
NSMutableArray *subArray = [@[string, @"adsf"] mutableCopy];

NSMutableArray *array = [NSMutableArray arrayWithObjects:string, subArray, nil];

NSArray *copyArray = [NSKeyedUnarchiver unarchiveObjectWithData:[NSKeyedArchiver archivedDataWithRootObject:array]];
NSLog(@"copyArray = %@", copyArray);

[string appendFormat:@"456"];
[subArray removeObjectAtIndex:0];
NSLog(@"copyArray = %@", copyArray);

2017-12-12 [3736:149666] copyArray = (
123,
    (
      123,
      adsf
    )
)

2017-12-12 [3736:149666] copyArray = (
123,
    (
      123,
      adsf
    )
)