ios學習路線—Objective-C(深淺拷貝)
在ObjC中,什麼是深淺拷貝? 深淺拷貝分別指深拷貝和淺拷貝,即 mutableCopy 和 copy 方法。 copy複製一個不可變物件,而 mutableCopy 複製一個 mutable 可變物件。
非容器類物件 如NSString,NSNumber等一類物件
示例1:
// 非容器類物件 NSString *str = @"origin string"; NSString *strCopy = [str copy]; NSMutableString *mstrCopy = [str mutableCopy]; [mstrCopy appendString:@"??"]; // NSLog(@"array1 = %p", array1); // NSLog(@"arrayCopy1 = %p", arrayCopy1);
檢視記憶體可以發現,str和strCopy指向的是同一塊記憶體區域,我們稱之為弱引用(weak reference)。而mstrCopy是真正的複製,系統為其分配了新記憶體空間,儲存從str複製過來的字串值。從最後一行程式碼中修改這些值而不影 響str和strCopy中可證明。
示例2:
NSMutableString *mstr = [NSMutableString stringWithString:@"origin"]; NSString *strCopy = [mstr copy]; NSMutableString *mstrCopy = [mstr copy]; NSMutableString *mstrMCopy = [mstr mutableCopy]; //[mstrCopy appendString:@"1111"]; //error [mstr appendString:@"222"]; [mstrMCopy appendString:@"333"];
以上四個物件所分配的記憶體都是不一樣的。而且對於mstrCopy,它所指向的其實是一個imutable物件,是不可改變的,所以會出錯。這點要注意,好好理解。
小結:
1.如果對一個不可變物件複製,copy是指標複製,即淺拷貝;而mutableCopy則是物件複製,即深拷貝。
2.如果是對可變物件複製,都是深拷貝,但是copy複製返回的物件是不可變的。
容器類物件深淺複製
比如NSArray,NSDictionary等。對於容器類本身,上面討論的結論也適用的,下面探討的是複製後容器內物件的變化。
示例3:
/* copy返回不可變物件,mutablecopy返回可變物件 */ NSArray *array1 = [NSArray arrayWithObjects:@"a",@"b",@"c",nil]; NSArray *arrayCopy1 = [array1 copy]; //arrayCopy1是和array同一個NSArray物件(指向相同的物件),包括array裡面的元素也是指向相同的指標 NSLog(@"array1 retain count: %d",[array1 retainCount]); // 2 NSLog(@"array1 retain count: %d",[arrayCopy1 retainCount]); // 2 NSMutableArray *mArrayCopy1 = [array1 mutableCopy]; //mArrayCopy1是array1的可變副本,指向的物件和array1不同,但是其中的元素和array1中的元素指向的還是同一個物件。mArrayCopy1還可以修改自己的物件 [mArrayCopy1 addObject:@"de"]; [mArrayCopy1 removeObjectAtIndex:0];
array1和arrayCopy1是指標複製,而mArrayCopy1是物件複製,符合前面示例1討論的結論。mArrayCopy1可以改變其內的元素:刪除或新增。但容器內的元素內容都是淺拷貝。
示例4
NSArray *mArray1 = [NSArray arrayWithObjects:[NSMutableString stringWithString:@"a"],@"b",@"c",nil]; NSLog(@"mArray1 retain count: %d",[mArray1 retainCount]); NSArray *mArrayCopy2 = [mArray1 copy]; NSLog(@"mArray1 retain count: %d",[mArray1 retainCount]); // mArray1和mArrayCopy2指向同一物件,retain值+1。 NSMutableArray *mArrayMCopy1 = [mArray1 mutableCopy]; NSLog(@"mArray1 retain count: %d",[mArray1 retainCount]); //mArrayCopy2和mArray1指向的是不一樣的物件,但是其中的元素都是一樣的物件——同一個指標 NSMutableString *testString = [mArray1 objectAtIndex:0]; //testString = @"1a1";//這樣會改變testString的指標,其實是將@“1a1”臨時物件賦給了testString [testString appendString:@" tail"];//這樣以上三個陣列的首元素都被改變了
由此可見,對於容器而言,其元素物件始終是指標複製。如果需要元素物件也是物件複製,就需要實現深拷貝。
示例5
NSArray *array = [NSArray arrayWithObjects:[NSMutableString stringWithString:@"first"] ,[NSString stringWithString:@"b"],@"c",nil]; NSArray *deepCopyArray=[[NSArray alloc] initWithArray: array copyItems: YES]; NSLog(@"array[2] = %@, deepCopyArray[2]=%@ ",[array objectAtIndex:2], [deepCopyArray objectAtIndex:2]); //輸出值是一樣的 NSLog(@"array[2] %p", [array objectAtIndex:2]); NSLog(@"deepCopyArray[2] %p", [deepCopyArray objectAtIndex:2]); //最後兩個列印的log記憶體地址值是一樣的 NSArray* trueDeepCopyArray = [NSKeyedUnarchiver unarchiveObjectWithData:[NSKeyedArchiver archivedDataWithRootObject: array]];
trueDeepCopyArray 是完全意義上的深拷貝,而deepCopyArray則不是,對於 deepCopyArray 內的不可變元素其還是指標複製。
自己實現深拷貝的方法 NSDictionaryMutableDeepCopy.h
#import <foundation /Foundation.h> @interface NSDictionary(MutableDeepCopy) - (NSMutableDictionary *)mutableDeepCopy; @end
NSDictionaryMutableDeepCopy.m
#import "NSDictionaryMutableDeepCopy.h" @implementation NSDictionary(MutableDeepCopy) - (NSMutableDictionary *)mutableDeepCopy { NSMutableDictionary *ret = [[NSMutableDictionary alloc] initWithCapacity:[self count]]; NSArray *keys = [self allKeys]; for (id key in keys) { id oneValue = [self valueForKey:key]; id oneCopy = nil; if ([oneValue respondsToSelector:@selector(mutableDeepCopy)]) { oneCopy = [oneValue mutableDeepCopy]; } else if ([oneValue respondsToSelector:@selector(mutableCopy)]) { oneCopy = [oneValue mutableCopy]; } if (oneCopy == nil) { oneCopy = [oneValue copy]; } [ret setValue:oneCopy forKey:key]; } return ret; }
使用類別方法來實現
如果是我們定義的物件,那麼我們自己要實現NSCopying,NSMutableCopying這樣就能呼叫copy和mutablecopy了。舉個例子
@interface MyObj : NSObject<nscopying ,NSMutableCopying> { NSMutableString *name; NSString *imutableStr; int age; } @property (nonatomic, retain) NSMutableString *name; @property (nonatomic, retain) NSString *imutableStr; @property (nonatomic) int age; @end @implementation MyObj @synthesize name; @synthesize age; @synthesize imutableStr; - (id)init { if (self = [super init]) { self.name = [[NSMutableString alloc]init]; self.imutableStr = [[NSString alloc]init]; age = -1; } return self; } - (void)dealloc { [name release]; [imutableStr release]; [super dealloc]; } - (id)copyWithZone:(NSZone *)zone { MyObj *copy = [[[self class] allocWithZone:zone] init]; copy->name = [name copy]; copy->imutableStr = [imutableStr copy]; // copy->name = [name copyWithZone:zone];; // copy->imutableStr = [name copyWithZone:zone];// copy->age = age; return copy; } - (id)mutableCopyWithZone:(NSZone *)zone { MyObj *copy = NSCopyObject(self, 0, zone); copy->name = [self.name mutableCopy]; copy->age = age; return copy; } @end