深拷贝与浅拷贝区别

浅拷贝：浅拷贝是指针拷贝，对一个实现NSCopying协议的对象进行浅拷贝，会创建一个新的指针来指向原始对象。两个指针指向同一个内存区域，如果原始对象的内容发生改变，新的对象同样会发生改变；新的对象内容变化，原始对象内容也会发生变化。如图所示：

image.png

深拷贝：深拷贝是内容拷贝，他会把原始对象的内容复制出来，然后让这个内容放到新的对象里面。两个指针指向不同的内存区域，原始对象的内容变化不影响新的对象；新的对象内容变化也不影响原始对象。

image.png

验证上述深拷贝和浅拷贝的代码：

    //深拷贝示例：
    //str内容改变的时候，strCopy内容并没有发生变化
    NSMutableString *str = [NSMutableString stringWithFormat:@"123"];
    NSString *strCopy = [str copy];
    NSLog(@"str:%@, strCopy:%@", str, strCopy);
    [str appendFormat:@"123"];
    NSLog(@"str:%@, strCopy:%@", str, strCopy);
2018-06-26 15:10:21.825600+0800 CopyTest[14774:1661930] str:123, strCopy:123
2018-06-26 15:10:21.825816+0800 CopyTest[14774:1661930] str:123123, strCopy:123

    //想要使用copy方法的条件：必须遵守协议，实现 copyWithZone: 方法
    NSMutableArray *muArr = [NSMutableArray array];
    NSArray *arr = [NSArray array];
    
    NSString *str = [NSString string];
    NSMutableString *muStr = [NSMutableString string];
    
    NSDictionary *dic = [NSDictionary dictionary];
    NSMutableDictionary *muDic = [NSMutableDictionary dictionary];
    
    //拷贝出来对象的类型
    //数组
    NSLog(@"%@", [arr.copy class]);
    NSLog(@"%@", [arr.mutableCopy class]);
    NSLog(@"%@", [muArr.copy class]);
    NSLog(@"%@", [muArr.mutableCopy class]);
    
    //字符串
    NSLog(@"%@", [str.copy class]);
    NSLog(@"%@", [str.mutableCopy class]);
    NSLog(@"%@", [muStr.copy class]);
    NSLog(@"%@", [muStr.mutableCopy class]);
    
    //字典
    NSLog(@"%@", [dic.copy class]);
    NSLog(@"%@", [dic.mutableCopy class]);
    NSLog(@"%@", [muDic.copy class]);
    NSLog(@"%@", [muDic.mutableCopy class]);
    
    //拷贝出来对象的内存地址比较
    //数组
    NSLog(@"数组:%p, %p", arr, arr.copy); //浅拷贝
    NSLog(@"数组:%p, %p", arr, arr.mutableCopy); //深拷贝
    NSLog(@"数组:%p, %p", muArr, muArr.copy); //深拷贝
    NSLog(@"数组:%p, %p", muArr, muArr.mutableCopy); //深拷贝
    
    
    //字符串
    NSLog(@"字符串:%p, %p", str, str.copy); //浅拷贝
    NSLog(@"字符串:%p, %p", str, str.mutableCopy); //深拷贝
    NSLog(@"字符串:%p, %p", muStr, muStr.copy); //深拷贝
    NSLog(@"字符串:%p, %p", muStr, muStr.mutableCopy); //深拷贝
    
    //字典
    NSLog(@"字典:%p, %p", dic, dic.copy); //浅拷贝
    NSLog(@"字典:%p, %p", dic, dic.mutableCopy); //深拷贝
    NSLog(@"字典:%p, %p", muDic, muDic.copy); //深拷贝
    NSLog(@"字典:%p, %p", muDic, muDic.mutableCopy); //深拷贝
2018-06-26 14:51:41.168790+0800 CopyTest[14400:1606517] __NSArray0
2018-06-26 14:51:41.168990+0800 CopyTest[14400:1606517] __NSArrayM
2018-06-26 14:51:41.169081+0800 CopyTest[14400:1606517] __NSArray0
2018-06-26 14:51:41.169164+0800 CopyTest[14400:1606517] __NSArrayM
2018-06-26 14:51:41.169285+0800 CopyTest[14400:1606517] __NSCFConstantString
2018-06-26 14:51:41.169516+0800 CopyTest[14400:1606517] __NSCFString
2018-06-26 14:51:41.169615+0800 CopyTest[14400:1606517] __NSCFConstantString
2018-06-26 14:51:41.169731+0800 CopyTest[14400:1606517] __NSCFString
2018-06-26 14:51:41.169834+0800 CopyTest[14400:1606517] __NSDictionary0
2018-06-26 14:51:41.169940+0800 CopyTest[14400:1606517] __NSDictionaryM
2018-06-26 14:51:41.170034+0800 CopyTest[14400:1606517] __NSFrozenDictionaryM
2018-06-26 14:51:41.170149+0800 CopyTest[14400:1606517] __NSDictionaryM
2018-06-26 14:51:41.170261+0800 CopyTest[14400:1606517] 数组:0x60400001e0a0, 0x60400001e0a0
2018-06-26 14:51:41.170340+0800 CopyTest[14400:1606517] 数组:0x60400001e0a0, 0x604000049270
2018-06-26 14:51:41.170457+0800 CopyTest[14400:1606517] 数组:0x6040000587b0, 0x60400001e0a0
2018-06-26 14:51:41.170564+0800 CopyTest[14400:1606517] 数组:0x6040000587b0, 0x60000044e430
2018-06-26 14:51:41.170790+0800 CopyTest[14400:1606517] 字符串:0x10075c320, 0x10075c320
2018-06-26 14:51:41.170963+0800 CopyTest[14400:1606517] 字符串:0x10075c320, 0x60000044e430
2018-06-26 14:51:41.171171+0800 CopyTest[14400:1606517] 字符串:0x604000058d20, 0x101666030
2018-06-26 14:51:41.171391+0800 CopyTest[14400:1606517] 字符串:0x604000058d20, 0x604000049270
2018-06-26 14:51:41.171873+0800 CopyTest[14400:1606517] 字典:0x60400001e090, 0x60400001e090
2018-06-26 14:51:41.172208+0800 CopyTest[14400:1606517] 字典:0x60400001e090, 0x604000425140
2018-06-26 14:51:41.172415+0800 CopyTest[14400:1606517] 字典:0x60400003dc20, 0x604000425140
2018-06-26 14:51:41.172520+0800 CopyTest[14400:1606517] 字典:0x60400003dc20, 0x604000425140

根据上面代码输出内容可以总结出如下结论：copy拷贝出来的对象类型总是不可变类型

image.png

image.png

延伸：在写代码的时候碰到一个有趣的现象，深拷贝改变了原始数据内容，新的内容依然改变。

    NSMutableArray *arr = [NSMutableArray array];
    NSMutableArray *subArr = [NSMutableArray array];
    [subArr addObject:@"1"];
    [arr addObject:subArr];
    NSMutableArray *muArr = [arr mutableCopy]; //深拷贝
    NSLog(@"arr:%@, muArr:%@", arr, muArr);
    [subArr addObject:@"2"];
    NSLog(@"arr:%@, muArr:%@", arr, muArr);
2018-06-26 15:36:05.813570+0800 CopyTest[15167:1736602] arr:(
        (
        1
    )
), muArr:(
        (
        1
    )
)
2018-06-26 15:36:05.813769+0800 CopyTest[15167:1736602] arr:(
        (
        1,
        2
    )
), muArr:(
        (
        1,
        2
    )
)

猜测：这里的数组相当于一个容器里面套了一个容器。深拷贝的限制只是针对外层的容器而言的，对里面的容器并没有限制。
我们通过代码来验证一下：

    NSMutableArray *arr = [NSMutableArray array];
    NSMutableArray *subArr = [NSMutableArray array];
    [subArr addObject:@"1"];
    [arr addObject:subArr];
    NSMutableArray *muArr = [arr mutableCopy]; //深拷贝
    NSLog(@"arr:%@, muArr:%@", arr, muArr);
    [muArr addObject:@"2"];
    NSLog(@"arr:%@, muArr:%@", arr, muArr);
2018-06-26 15:42:04.404419+0800 CopyTest[15287:1755382] arr:(
        (
        1
    )
), muArr:(
        (
        1
    )
)
2018-06-26 15:42:04.404628+0800 CopyTest[15287:1755382] arr:(
        (
        1
    )
), muArr:(
        (
        1
    ),
    2
)

我们给拷贝出来的新数组外层添加一个数据@“2”，运行出来的结果跟我们预测一样，只是新数组发生了变化。原始数组的数据依然不变。这里可以看做不完全深拷贝，我们平常使用的一般都是不完全深拷贝。想要实现完全深拷贝需要重写copyWithZone：方法来实现。