介紹 :

NSCFConstantString (retain/release不起作用,系統(tǒng)進(jìn)行維護(hù),程序中內(nèi)容相同的常量字符串只有一個)
NSCFString (retain/release有作用)
NSTaggedPointerString (retain/release不起作用) 

驗(yàn)證代碼

NSString *str = @"test";
NSLog(@"str--->%ld",[str retainCount]);   // NSCFConstanString 類型

NSString *str1 = [@"test" copy];
NSLog(@"str1--->%ld",[str1 retainCount]);  // NSCFConstanString

// 拷貝后是可變類型
NSString *str2 = [@"test" mutableCopy];
NSLog(@"str2--->%ld",[str2 retainCount]); // NSCFString

NSString *str3 = [NSString stringWithString:@"test"];
NSLog(@"str3--->%ld",[str3 retainCount]);  // NSCFConstanString

NSString *str4 = [NSString stringWithFormat:@"%@",@"test"];
NSLog(@"str4--->%ld",[str4 retainCount]);  // NSTaggedPointerString

NSString  *str5 = [[NSString alloc] initWithString:@"Hello World"];
NSLog(@"str5--->%ld",[str5 retainCount]);  // NSCFConstanString

NSString* str6 = [[NSString alloc] initWithFormat:@"%@",@"Hello World"];
NSLog(@"str6--->%ld",[str6 retainCount]);   // NSCFString


// 本質(zhì)上還是不可變類型
NSMutableString *mutableStr = [@"test" copy];
NSLog(@"mutableStr--->%ld",[mutableStr retainCount]);  // NSCFConstanString

NSMutableString *mutableStr1 = [@"test" mutableCopy];
NSLog(@"mutableStr1--->%ld",[mutableStr1 retainCount]);  // NSCFString

NSMutableString *mutableStr2 = [NSMutableString stringWithString:@"test"];
NSLog(@"mutableStr2--->%ld",[mutableStr2 retainCount]);  // NSCFString

NSMutableString *mutableStr3 = [NSMutableString stringWithFormat:@"%@",@"test"];
NSLog(@"mutableStr3--->%ld",[mutableStr3 retainCount]);  // NSCFString

NSMutableString *mutableStr4 = [[NSMutableString alloc]initWithString:@"test"];
NSLog(@"mutableStr4--->%ld",[mutableStr4 retainCount]);  // NSCFString

NSMutableString *mutableStr5 = [[NSMutableString alloc]initWithFormat:@"%@",@"test"];
NSLog(@"mutableStr5--->%ld",[mutableStr5 retainCount]);   // NSCFString

輸出 :

 2018-09-23 10:38:08.187 LZX_Test001[2713:191391] str--->-1
 2018-09-23 10:38:08.188 LZX_Test001[2713:191391] str1--->-1
 2018-09-23 10:38:08.188 LZX_Test001[2713:191391] str2--->1
 2018-09-23 10:38:08.188 LZX_Test001[2713:191391] str3--->-1
 2018-09-23 10:38:08.188 LZX_Test001[2713:191391] str4--->-1
 2018-09-23 10:38:08.189 LZX_Test001[2713:191391] str5--->-1
 2018-09-23 10:38:08.189 LZX_Test001[2713:191391] str6--->1


  2018-09-23 10:53:03.549 LZX_Test001[2757:198382] mutableStr--->-1
  2018-09-23 10:53:03.549 LZX_Test001[2757:198382] mutableStr1--->1
  2018-09-23 10:53:03.550 LZX_Test001[2757:198382] mutableStr2--->1
  2018-09-23 10:53:03.550 LZX_Test001[2757:198382] mutableStr3--->1
  2018-09-23 10:53:03.550 LZX_Test001[2757:198382] mutableStr4--->1

結(jié)論 :
1. 只有NSCFString 類型指針才會參與引用計數(shù)
2. NSString使用initWithFormat創(chuàng)建是NSCFString類型指針