复制对象 copy 与mutable copy

 

 

转载 ：  http://blog.csdn.net/u010962810/article/details/18887841

 

通过copy方法可以创建可变对象或不可变对象的不可变副本，对于不可变副本，其对象的值不可以改变。

通过mutableCopy方法可以创建可变对象或不可变对象的可变副本，对于可变副本其对象是可变的。  

复制分为浅复制和深复制两种：浅复制只是复制对象的引用，并没有复制对象的具体内容。深复制则创建了要复制对象的具体内容，并返回对象副本的引用。

对于复制Foundation中的对象，默认并不是深复制，例如copy NSMutableArray对象是浅复制，只是对其引用进行复制；而copy NSMutableString对象是深复制，对原来引用的对象的具体内容也进行了复制。

1.NSMutableArray和NSArray

首先创建1个数组（该数组中有3个对象）和4个数组指针：

NSMutableArray *marray1 = [NSMutableArray arrayWithObjects:
                                   [NSMutableString stringWithString:@"1"],
                                   [NSMutableString stringWithString:@"2"],
                                   [NSMutableString stringWithFormat:@"3"],
                                   nil];
        NSMutableArray *marray2 = [marray1 mutableCopy];
        NSArray *array1 = [marray1 copy];
        NSArray *array2 = [marray1 mutableCopy];

（1）添加字符串到marray1[0]中：

// append string to marray1[0]
        NSMutableString *mstr = marray1[0];
        [mstr appendString:@" append"];
        NSLog(@"append string to marray1[0]");
        NSLog(@"marray1 = %@", marray1);
        NSLog(@"marray2 = %@", marray2);
        NSLog(@"array1 = %@", array1);
        NSLog(@"array2 = %@", array2);

2014-01-31 21:10:39.097 Array[298:303] append string to marray1[0]
2014-01-31 21:10:39.098 Array[298:303] marray1 = (
    "1 append",
    2,
    3
)
2014-01-31 21:10:39.098 Array[298:303] marray2 = (
    "1 append",
    2,
    3
)
2014-01-31 21:10:39.098 Array[298:303] array1 = (
    "1 append",
    2,
    3
)
2014-01-31 21:10:39.099 Array[298:303] array2 = (
    "1 append",
    2,
    3
)

分析：

假设marray1中保存的是obj1，obj2，obj3组成的数组的首地址。将marray1复制给marray2，array1和array2，只是将指向数组第一个对象的内存地址复制到marray2等数组指针中保存。

在marray1[0]指向的对象obj1后面附加字符串append，由于marray1，marray2，array1和array2第一个指针指向的对象都是obj1，所以输出一致。

这里的copy方法和简单的赋值作用是一致的。

（2）为marray1[0]设置新的对象值：（注意（2）、（3）和（1）是相互独立的，下同）

// set new object to marray1[0]
        marray1[0] = [NSMutableString stringWithString:@"new 1"];
        NSLog(@"set new object to marray1[0]");
        NSLog(@"marray1 = %@", marray1);
        NSLog(@"marray2 = %@", marray2);
        NSLog(@"array1 = %@", array1);
        NSLog(@"array2 = %@", array2);

2014-01-31 20:50:52.003 Array[4151:303] set new object to marray1[0]
2014-01-31 20:50:52.004 Array[4151:303] marray1 = (
    "new 1",
    2,
    3
)
2014-01-31 20:50:52.005 Array[4151:303] marray2 = (
    1,
    2,
    3
)
2014-01-31 20:50:52.005 Array[4151:303] array1 = (
    1,
    2,
    3
)
2014-01-31 20:50:52.005 Array[4151:303] array2 = (
    1,
    2,
    3
)

分析：

这里创建了一个新的NSMutableString对象将其引用赋给marray1[0]，而其他数组指针指向的对象不变，所以输出时其他数组不受影响。

（3）添加新的对象到marray1中：

// add new object to marray1
        [marray1 addObject:[NSMutableString stringWithString:@"4"]];
        NSLog(@"add new object to marray1");
        NSLog(@"marray1 = %@", marray1);
        NSLog(@"marray2 = %@", marray2);
        NSLog(@"array1 = %@", array1);
        NSLog(@"array2 = %@", array2);

2014-01-31 20:51:46.867 Array[4167:303] add new object to marray1
2014-01-31 20:51:46.869 Array[4167:303] marray1 = (
    1,
    2,
    3,
    4
)
2014-01-31 20:51:46.869 Array[4167:303] marray2 = (
    1,
    2,
    3
)
2014-01-31 20:51:46.869 Array[4167:303] array1 = (
    1,
    2,
    3
)
2014-01-31 20:51:46.869 Array[4167:303] array2 = (
    1,
    2,
    3
)

分析：

这里创建了一个新的可变字符串对象，并将其内存地址保存到指针marray1[3]中。而其他几个数组指针的引用不受影响。

（4）操作数组的引用

如果要单独替换marray1[0]指向的对象，应该先创建一个对象，然后再替换marray1[0]的引用。这样就不会对marray2[0]指向的对象造成影响。

NSMutableArray *marray1 = [NSMutableArray arrayWithObjects:
                                   [NSMutableString stringWithString:@"1"],
                                   [NSMutableString stringWithString:@"2"],
                                   [NSMutableString stringWithFormat:@"3"],
                                   nil];
        NSMutableArray *marray2 = [marray1 mutableCopy];
        NSArray *array1 = [marray1 copy];
        NSArray *array2 = [marray1 mutableCopy];
        NSMutableString *mstr = [NSMutableString stringWithString:marray1[0]]; // 创建新的对象
        [mstr appendString:@" append"];
        [marray1 replaceObjectAtIndex:0 withObject:mstr]; // 替换marray1[0]的引用
        
        NSLog(@"marray1 = %@", marray1);
        NSLog(@"marray2 = %@", marray2);
        NSLog(@"array1 = %@", array1);
        NSLog(@"array2 = %@", array2);

控制台输出：

2014-01-31 20:09:33.678 Dictionary[3735:303] marray1 = (
    "1 append",
    2,
    3
)
2014-01-31 20:09:33.679 Dictionary[3735:303] marray2 = (
    1,
    2,
    3
)
2014-01-31 20:09:33.680 Dictionary[3735:303] array1 = (
    1,
    2,
    3
)
2014-01-31 20:09:33.680 Dictionary[3735:303] array2 = (
    1,
    2,
    3
)

再看一个例子：

NSMutableArray *marray1 = [NSMutableArray arrayWithObjects:
                                   [NSMutableString stringWithString:@"1"],
                                   [NSMutableString stringWithString:@"2"],
                                   [NSMutableString stringWithFormat:@"3"],
                                   nil];
        NSMutableArray *marray2 = [marray1 mutableCopy];
        NSArray *array1 = [marray1 copy];
        NSArray *array2 = [marray1 mutableCopy];
        
        [marray1 removeObjectAtIndex:0];
        
        NSLog(@"marray1 = %@", marray1);
        NSLog(@"marray2 = %@", marray2);
        NSLog(@"array1 = %@", array1);
        NSLog(@"array2 = %@", array2);

控制台输出：

2014-01-31 20:10:40.716 Dictionary[3746:303] marray1 = (
    2,
    3
)
2014-01-31 20:10:40.717 Dictionary[3746:303] marray2 = (
    1,
    2,
    3
)
2014-01-31 20:10:40.717 Dictionary[3746:303] array1 = (
    1,
    2,
    3
)
2014-01-31 20:10:40.718 Dictionary[3746:303] array2 = (
    1,
    2,
    3
)

尽管marray1，marray2，array1，array2指向同一个数组对象，但是[marray1 removeObjectAtIndex:0]移除的是marray1第一个元素，即移除的是对对象的引用，而不是移除数组中的第一个对象。所以不会影响到marray2  ，array1，array2 。 

2.NSMutableString和NSString

复制NSMutableString对象测试代码如下：

NSMutableString *mstr1 = [NSMutableString stringWithString:@"1"];
        NSMutableString *mstr2 = [mstr1 mutableCopy];
        NSString *str1 = [mstr1 copy];
        NSString *str2 = [mstr1 mutableCopy];
        
        [mstr1 appendString:@" append"];
        NSLog(@"mstr1 = %@", mstr1);
        NSLog(@"mstr2 = %@", mstr2);
        NSLog(@"str1 = %@", str1);
        NSLog(@"str2 = %@", str2);

2014-01-31 20:54:20.423 String[4221:303] mstr1 = 1 append
2014-01-31 20:54:20.424 String[4221:303] mstr2 = 1
2014-01-31 20:54:20.424 String[4221:303] str1 = 1
2014-01-31 20:54:20.424 String[4221:303] str2 = 1

分析：

对mstr1指向的对象STRING1进行复制，这里复制的是对象  STRING1 本身的内容，也就是mstr2，str1，str2将分别指向对象  STRING2，STRING3，STRING4，对STRING1的修改（在后面附加字符串），将不会影响到其他几个引用指向的对象。

可见这里进行的是深复制。

注意不要改变copy后的对象的值，因为copy生成的对象是不可变的，强行修改会导致程序崩溃。

控制台输出为：

2014-02-01 00:57:24.903 Array[792:303] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to mutate immutable object with appendString:'

3.NSMutableDictionary和NSDictionary

首先创建1个字典（字典中对应3个对象）和4个字典指针：

NSMutableString *obj1 = [NSMutableString stringWithString:@"obj1"];
        NSMutableString *obj2 = [NSMutableString stringWithString:@"obj2"];
        NSMutableString *obj3 = [NSMutableString stringWithString:@"obj3"];
        NSMutableDictionary *mdic1 = [NSMutableDictionary dictionaryWithObjects:@[obj1, obj2, obj3]
                                                                        forKeys:@[@"key1", @"key2", @"key3"]];
        NSMutableDictionary *mdic2 = [mdic1 mutableCopy];
        NSDictionary *dic1 = [mdic1 copy];
        NSDictionary *dic2 = [mdic1 copy];

假设指针mdic1的索引key1，key2，key3分别对应对象obj1，obj2，obj3的内存地址。

对字典进行复制也是浅复制。

对mdic1复制，并赋值给mdic2，dic1，dic2，那么mdic2，dic1，dic2中指向的是同一个字典。

（1）添加字符串到mdic1的key1对应的可变字符串对象后：

// append string to mdic1[@"key1"]
        NSMutableString *mstr = mdic1[@"key1"];
        [mstr appendString:@" append"];
        NSLog(@"append string to midc1[@"key1"]");
        NSLog(@"mdic1 = %@", mdic1);
        NSLog(@"mdic2 = %@", mdic2);
        NSLog(@"dic1 = %@", dic1);
        NSLog(@"dic2 = %@", dic2);

2014-01-31 22:07:29.241 Dictionary[431:303] append string to midc1[@"key1"]
2014-01-31 22:07:29.242 Dictionary[431:303] mdic1 = {
    key1 = "obj1 append";
    key2 = obj2;
    key3 = obj3;
}
2014-01-31 22:07:29.243 Dictionary[431:303] mdic2 = {
    key1 = "obj1 append";
    key2 = obj2;
    key3 = obj3;
}
2014-01-31 22:07:29.243 Dictionary[431:303] dic1 = {
    key1 = "obj1 append";
    key2 = obj2;
    key3 = obj3;
}
2014-01-31 22:07:29.243 Dictionary[431:303] dic2 = {
    key1 = "obj1 append";
    key2 = obj2;
    key3 = obj3;
}

上面代码中对key1对应的对象obj1进行修改（在对象后面附加字符串），由于mdic2等指向同一个字典，所以其输出将会和mdic1的输出一致。

（2）设置新的对象给key1：

// set new object for  key1
        NSMutableString *nobj1 = [NSMutableString stringWithString:@"new obj1"];
        [mdic1 setObject:nobj1 forKey:@"key1"];
        NSLog(@"set new object for key");
        NSLog(@"mdic1 = %@", mdic1);
        NSLog(@"mdic2 = %@", mdic2);
        NSLog(@"dic1 = %@", dic1);
        NSLog(@"dic2 = %@", dic2);

2014-01-31 22:08:01.617 Dictionary[448:303] set new object for key
2014-01-31 22:08:01.618 Dictionary[448:303] mdic1 = {
    key1 = "new obj1";
    key2 = obj2;
    key3 = obj3;
}
2014-01-31 22:08:01.619 Dictionary[448:303] mdic2 = {
    key1 = obj1;
    key2 = obj2;
    key3 = obj3;
}
2014-01-31 22:08:01.619 Dictionary[448:303] dic1 = {
    key1 = obj1;
    key2 = obj2;
    key3 = obj3;
}
2014-01-31 22:08:01.619 Dictionary[448:303] dic2 = {
    key1 = obj1;
    key2 = obj2;
    key3 = obj3;
}

这里只是修改了mdic1的索引key1中保存的内存地址，并不会对其他引用所指向的对象造成影响。

（3）在字典中添加新的key-value对：

// add new key-value
        NSMutableString *obj4 = [NSMutableString stringWithString:@"obj4"];
        [mdic1 setObject:obj4 forKey:@"key4"];
        NSLog(@"add new key-value");
        NSLog(@"mdic1 = %@", mdic1);
        NSLog(@"mdic2 = %@", mdic2);
        NSLog(@"dic1 = %@", dic1);
        NSLog(@"dic2 = %@", dic2);

2014-01-31 22:12:14.678 Dictionary[466:303] add new key-value
2014-01-31 22:12:14.679 Dictionary[466:303] mdic1 = {
    key1 = obj1;
    key2 = obj2;
    key3 = obj3;
    key4 = obj4;
}
2014-01-31 22:12:14.679 Dictionary[466:303] mdic2 = {
    key1 = obj1;
    key2 = obj2;
    key3 = obj3;
}
2014-01-31 22:12:14.679 Dictionary[466:303] dic1 = {
    key1 = obj1;
    key2 = obj2;
    key3 = obj3;
}
2014-01-31 22:12:14.680 Dictionary[466:303] dic2 = {
    key1 = obj1;
    key2 = obj2;
    key3 = obj3;
}

现在为mdic1对应的字典添加新的key-value对，这不会影响到其他字典指针，所以输出不受影响。

小结：

1.浅复制复制的是对象的引用，对指针的修改并不会影响到指向的对象，更不会影响到其他指针。 深复制复制的是对象的内容，对对象的操作修改将影响到所有指向该对象的引用。

2.NSMutableArray和NSMutableDictionary的复制是浅复制，NSMutableString的复制是深复制。由于数组或字典中对象数目或大小可能非常大，所以对对象的复制可能引起大量开销，因此这里只复制引用可以节省开销。