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复制对象 copy 与mutable copy

时间:2015-06-10 20:53:50      阅读:123      评论:0      收藏:0      [点我收藏+]

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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。

技术分享

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。

复制对象 copy 与mutable copy

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原文地址:http://www.cnblogs.com/lizhitai/p/4567156.html

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