朝花夕拾, 隨著代碼量的增加, 自己學(xué)習(xí)的重心逐漸從熟練度,轉(zhuǎn)向?qū)Υa的理解和使用之上來(lái).因此決定整理細(xì)嚼慢咽系列文章,以便自己學(xué)習(xí)鞏固..
一般我們聲明一個(gè)全局字符串變量都是這樣的
@property (nonatomic, copy) NSString *copyedString;
但是為什么字符串一般都是copy呢?
我們需要先弄清楚copy的作用:
copy語(yǔ)法的目的：改變副本的時(shí)候右蕊，不會(huì)影響到源對(duì)象；
深拷貝：內(nèi)容拷貝，會(huì)產(chǎn)生新的對(duì)象。新對(duì)象計(jì)數(shù)器置為1曲楚，源對(duì)象計(jì)數(shù)器不變。
淺拷貝：指針拷貝褥符，不會(huì)產(chǎn)生新的對(duì)象龙誊。源對(duì)象計(jì)數(shù)器+1。
知道這些后我們看以下測(cè)試代碼:
@property (nonatomic, strong) NSString *strongString;
@property (nonatomic, copy) NSString *copyedString;
@property (nonatomic, retain) NSString *retainString;
定義了三種聲明不同的String;
接下來(lái)繼續(xù), 進(jìn)行測(cè)試
NSString *string = [NSString stringWithFormat:@"原始值"];
self.strongString = string;
self.copyedString = string;
self.retainString = string;
NSLog(@"origin string: %p, %p", string, &string);
NSLog(@"strong string: %p, %p", _strongString, &_strongString);
NSLog(@"copy string: %p, %p", _copyedString, &_copyedString);
string = @"新值";
NSLog(@"\n originString -- %@, strongString -- %@, copyString ---%@, retainString --- %@", string, self.strongString, self.copyedString, self.retainString);
NSLog(@"\n 改變后的 origin string: %p, %p", string, &string);
NSLog(@"\n 改變后的 strong string: %p, %p", _strongString, &_strongString);
NSLog(@"\n 改變后的 copy string: %p, %p", _copyedString, &_copyedString);
打印結(jié)果:
2016-09-07 17:14:02.483 AFNetWorkingTest[2412:1184257] origin string: 0x7f80a9c87720, 0x7fff531f29e8**
2016-09-07 17:14:02.484 AFNetWorkingTest[2412:1184257] strong string: 0x7f80a9c87720, 0x7f80a9c81410**
2016-09-07 17:14:02.484 AFNetWorkingTest[2412:1184257] copy string: 0x7f80a9c87720, 0x7f80a9c81418**
2016-09-07 17:14:02.484 AFNetWorkingTest[2412:1184257] originString -- 新值, strongString -- 原始值, copyString ---原始值, retainString --- 原始值
2016-09-07 17:14:02.484 AFNetWorkingTest[2412:1184257] 改變后的**** origin string: 0x10ca53680, 0x7fff531f29e8**
2016-09-07 17:14:02.484 AFNetWorkingTest[2412:1184257] 改變后的**** strong string: 0x7f80a9c87720, 0x7f80a9c81410**
2016-09-07 17:14:02.484 AFNetWorkingTest[2412:1184257] 改變后的**** copy string: 0x7f80a9c87720, 0x7f80a9c81418**
由此看出
轉(zhuǎn)載自http://blog.csdn.net/itianyi/article/details/9018567
對(duì)源頭是NSString的字符串喷楣，無(wú)論是retain聲明的變量還是copy聲明的變量趟大，當(dāng)?shù)诙卧搭^的字符串重新指向其它的地方的時(shí)候，
它還是指向原來(lái)的最初的那個(gè)位置铣焊，也就是說(shuō)其實(shí)二者都是指針引用逊朽，也就是淺拷貝。
之后我們替換為NSMutableString:
NSMutableString *string = [NSMutableString stringWithFormat:@"原始值"];
self.strongString = string;
self.copyedString = string;
self.retainString = string;
NSLog(@"origin string: %p, %p", string, &string);
NSLog(@"strong string: %p, %p", _strongString, &_strongString);
NSLog(@"copy string: %p, %p", _copyedString, &_copyedString);
[string appendString:@"de"];
NSLog(@"\n originString -- %@, strongString -- %@, copyString ---%@, retainString --- %@", string, self.strongString, self.copyedString, self.retainString);
NSLog(@"\n 改變后的 origin string: %p, %p", string, &string);
NSLog(@"\n 改變后的 strong string: %p, %p", _strongString, &_strongString);
NSLog(@"\n 改變后的 copy string: %p, %p", _copyedString, &_copyedString);
看打印結(jié)果:
**2016-09-07 18:28:49.491 AFNetWorkingTest[2759:1441018] origin string: 0x7fb219627df0, 0x7fff516359e8**
**2016-09-07 18:28:49.492 AFNetWorkingTest[2759:1441018] strong string: 0x7fb219627df0, 0x7fb219622540**
**2016-09-07 18:28:49.492 AFNetWorkingTest[2759:1441018] copy string: 0x7fb219628230, 0x7fb219622548**
**2016-09-07 18:28:49.492 AFNetWorkingTest[2759:1441018] **** originString--****原始值****de, strongString--****原始值****de, copyString---****原始值****, retainString---****原始值****de**
**2016-09-07 18:28:49.492 AFNetWorkingTest[2759:1441018] **** ****改變后的**** origin string: 0x7fb219627df0, 0x7fff516359e8**
**2016-09-07 18:28:49.492 AFNetWorkingTest[2759:1441018] **** ****改變后的**** strong string: 0x7fb219627df0, 0x7fb219622540**
**2016-09-07 18:28:49.492 AFNetWorkingTest[2759:1441018] **** ****改變后的**** copy string: 0x7fb219628230, 0x7fb219622548**
可以看到, copy的地址已經(jīng)出現(xiàn)了新的內(nèi)存地址.
結(jié)論:
轉(zhuǎn)載自http://blog.csdn.net/itianyi/article/details/9018567
對(duì)源頭是NSMutableString的字符串曲伊，retain僅僅是指針引用叽讳，增加了引用計(jì)數(shù)器，這樣源頭改變的時(shí)候坟募，用這種retain方式聲明的變量
（無(wú)論被賦值的變量是可變的還是不可變的）岛蚤，它也會(huì)跟著改變;而copy聲明的變量，它不會(huì)跟著源頭改變婿屹，它實(shí)際上是深拷貝。
之后, 了解到NSString 編譯后是一個(gè)
__NSCFConstantString
對(duì)象,
而NSMutableString 是一個(gè)
__NSCFString
對(duì)象
具體請(qǐng)看
https://blog.cnbluebox.com/blog/2014/04/16/nsstringte-xing-fen-xi-xue-xi/
http://www.cocoachina.com/ios/20150512/11805.html
感謝
參考,轉(zhuǎn)載博客:
http://blog.csdn.net/itianyi/article/details/9018567
http://stackoverflow.com/questions/2521468/nsstring-copy-not-copying
https://blog.cnbluebox.com/blog/2014/04/16/nsstringte-xing-fen-xi-xue-xi/
