對象的copy&mutableCopy

參考：

[1]參考庫：記憶體管理編程指南

將一個指標p1直接賦值給另外一個指標p2，即：p2=p1，其實p1,p2指向的同一個執行個體對象，任何對p1進行的操作都會影響p2，因為他們指向同個地址的執行個體對象。具體的就不說了，所以如果需要拷貝一個對象，而不只是指標賦值，在C++中有“拷貝建構函式”來實現此功能。objetive-c則通過copy和mutableCopy方法來實現。

首先，objetive-c中有NSCopying 和NSMutableCopying 兩種協議，分別聲明了copyWithZone和mutableCopyWithZone方法。協議：就是把若干類中具有相同功能的方法都抽象出來，定義在一起。因此任何需要copy操作的類，只需要遵從NSCopying 和NSMutableCopying協議，並實現copyWithZone和mutableCopyWithZone方法，實現對象的拷貝。

其次，方法copy產生不可修改的對象，方法mutableCopy產生可修改的對象。產生是否可修改的對象跟之前拷貝的對象是否可修改沒有關係，跟調用copy還是mutablecopy有關係。

測試代碼如下，首先LOCBird為包含NSCopying 或NSMutableCopying 協議，如下所示。

@interface LOCBird : NSObject{NSString* name_;    }@end

@implementation LOCBird- (id)init{self = [super init];if (self) {name_ = [[NSString alloc] initWithString:@"I am a Bird!!"];}return self;}- (void)dealloc{[name_release];[super dealloc];}- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector{NSMethodSignature* signature = [super methodSignatureForSelector:aSelector];if (signature==nil) {signature = [name_ methodSignatureForSelector:aSelector];}NSUInteger argCount = [signature numberOfArguments];for (NSInteger i=0 ; i<argCount ; i++) {NSLog(@"%s" , [signature getArgumentTypeAtIndex:i]);}NSLog(@"returnType:%s ,returnLen:%d" , [signature methodReturnType] , [signature methodReturnLength]);NSLog(@"signature:%@" , signature);return signature;}- (void)forwardInvocation:(NSInvocation *)anInvocation{NSLog(@"forwardInvocation:%@" , anInvocation);SEL seletor = [anInvocation selector];if ([name_ respondsToSelector:seletor]) {[anInvocation invokeWithTarget:name_];}}@end

  建立對象並測試

LOCBird* bird1 =[[LOCBird alloc] init];LOCBird* bird2 = [bird1 copy];LOCBird* bird3 = [bird1 mutableCopy];NSLog(@"%@ %@",bird2 , NSStringFromClass([bird2 class]));NSLog(@"%@ %@",bird3 , NSStringFromClass([bird3 class]));

輸出結果如下：

2012-03-22 22:17:28.384 LOCMessageForward[1082:f803] @

2012-03-22 22:17:28.385 LOCMessageForward[1082:f803] :

2012-03-22 22:17:28.385 LOCMessageForward[1082:f803] ^{_NSZone=}

2012-03-22 22:17:28.386 LOCMessageForward[1082:f803] returnType:@ ,returnLen:4

2012-03-22 22:17:28.387 LOCMessageForward[1082:f803] signature:<NSMethodSignature: 0x685ae90>

2012-03-22 22:17:28.388 LOCMessageForward[1082:f803] forwardInvocation:<NSInvocation: 0x6a45670>

2012-03-22 22:17:28.389 LOCMessageForward[1082:f803] @

2012-03-22 22:17:28.390 LOCMessageForward[1082:f803] :

2012-03-22 22:17:28.391 LOCMessageForward[1082:f803] ^{_NSZone=}

2012-03-22 22:17:28.392 LOCMessageForward[1082:f803] returnType:@ ,returnLen:4

2012-03-22 22:17:28.393 LOCMessageForward[1082:f803] signature:<NSMethodSignature: 0x685ae90>

2012-03-22 22:17:28.393 LOCMessageForward[1082:f803] forwardInvocation:<NSInvocation: 0x68438b0>

2012-03-22 22:17:28.394 LOCMessageForward[1082:f803] I am a Bird!! __NSCFConstantString

2012-03-22 22:17:28.395 LOCMessageForward[1082:f803] I am a Bird!! __NSCFString

因為LOCBird為遵從NSCopying 或NSMutableCopying 協議，所以不能響應copy訊息，但由於重載了methodSignatureForSelector和forwardInvocation，實現將訊息轉寄給NSString的name_成員，因此最後拷貝產生的NSString對象。

修改後遵從NSCopying 或NSMutableCopying 協議，輸出如下：

2012-03-22 22:41:09.438 LOCMessageForward[1196:f803] <LOCBird: 0x6a6c760> LOCBird

2012-03-22 22:41:09.439 LOCMessageForward[1196:f803] <LOCBird: 0x687c880> LOCBird

最後測試NSString和NSarray的拷貝方法

NSString* str1 = @"hello";NSString* str2 = @"hello";NSString* str3 = [NSString stringWithString:@"hello"];NSString* str4 = [NSString stringWithFormat:@"hello"];NSString* str5 = [[NSString alloc]initWithString:@"hello"];NSString* str6 = [[NSString alloc]initWithFormat:@"hello"];NSString* str7 = [NSString stringWithCString:"hello" encoding:NSUTF8StringEncoding];NSString* str8 = [NSString stringWithCString:"hello" encoding:NSUTF8StringEncoding];NSLog(@"%p,%p,%p,%p,%p,%p,%p,%p",str1,str2,str3,str4,str5,str6,str7,str8);NSString* cpStr1 = [str1 copy];NSMutableString* cpStr2 = [str1 mutableCopy];NSLog(@"cpStr1=%p mutCpStr1=%p",cpStr1 , cpStr2);NSArray* array = [NSArray arrayWithObjects:str1,str2,str3,str4,str5,str6,str7,str8,nil];NSArray* imArray = [array copy];for (NSString* str in imArray) {NSLog(@"%p %@",str,str);}NSLog(@"-----------------");NSMutableArray* mutArray = [array mutableCopy];[mutArray addObject:@"hello"];for (NSString* str in mutArray) {NSLog(@"%p %@",str,str);}

輸出結果如下：

0x4640,0x4640,0x4640,0x6a69860,0x4640,0x6a65a50,0x6a6b7a0,0x6a6aec0

2012-03-22 22:41:06.906 LOCMessageForward[1196:f803] cpStr1=0x4640 mutCpStr1=0x68695f0

2012-03-22 22:41:06.906 LOCMessageForward[1196:f803] 0x4640 hello

2012-03-22 22:41:06.907 LOCMessageForward[1196:f803] 0x4640 hello

2012-03-22 22:41:06.907 LOCMessageForward[1196:f803] 0x4640 hello

2012-03-22 22:41:06.908 LOCMessageForward[1196:f803] 0x6a69860 hello

2012-03-22 22:41:06.908 LOCMessageForward[1196:f803] 0x4640 hello

2012-03-22 22:41:06.909 LOCMessageForward[1196:f803] 0x6a65a50 hello

2012-03-22 22:41:06.909 LOCMessageForward[1196:f803] 0x6a6b7a0 hello

2012-03-22 22:41:06.910 LOCMessageForward[1196:f803] 0x6a6aec0 hello

2012-03-22 22:41:06.910 LOCMessageForward[1196:f803] -----------------

2012-03-22 22:41:06.911 LOCMessageForward[1196:f803] 0x4640 hello

2012-03-22 22:41:06.911 LOCMessageForward[1196:f803] 0x4640 hello

2012-03-22 22:41:06.912 LOCMessageForward[1196:f803] 0x4640 hello

2012-03-22 22:41:06.912 LOCMessageForward[1196:f803] 0x6a69860 hello

2012-03-22 22:41:06.912 LOCMessageForward[1196:f803] 0x4640 hello

2012-03-22 22:41:06.913 LOCMessageForward[1196:f803] 0x6a65a50 hello

2012-03-22 22:41:06.913 LOCMessageForward[1196:f803] 0x6a6b7a0 hello

2012-03-22 22:41:06.914 LOCMessageForward[1196:f803] 0x6a6aec0 hello

2012-03-22 22:41:06.914 LOCMessageForward[1196:f803] 0x4640 hello

注意，由於“hello”為常量字串，編譯後被放在常量資料區，因此str1和str2具有相同的指標地址，但是stringWithString和stringWithFormat為啥表現不同就不明白了，不知道但是蘋果那班人怎麼想的？

可以肯定的是NSString和NSArray都是進行的淺拷貝，只拷貝指標，並增加指標所指執行個體的retainCount，並未複製該對象。