ios - 为什么用 %c 指定的 NSString 字符被改变了？

Question

我试图测试一些涉及字符串编码之间转换的代码，并在尝试创建具有无效 UTF-8 序列的 NSString 时发现了这种现象:

char before = 0xa1;
NSString *s = [NSString stringWithFormat:@"%c",before];
char after = [s characterAtIndex:0]; // = 0xb0

对于 0x80-0xFF 范围内的大多数(但不是全部)字符，NSString 中的字符与我指定的字符不同。

有人知道这是为什么吗？

以下是所有可能的 char 值的前后值:

1 -> 1

2 -> 2

3 -> 3

4 -> 4

5 -> 5

6 -> 6

7 -> 7

8 -> 8

9 -> 9

a -> a

b -> b

c -> c

d -> d

e -> e

f -> f

10 -> 10

11 -> 11

12 -> 12

13 -> 13

14 -> 14

15 -> 15

16 -> 16

17 -> 17

18 -> 18

19 -> 19

1a -> 1a

1b -> 1b

1c -> 1c

1d -> 1d

1e -> 1e

1f -> 1f

20 -> 20

21 -> 21

22 -> 22

23 -> 23

24 -> 24

25 -> 25

26 -> 26

27 -> 27

28 -> 28

29 -> 29

2a -> 2a

2b -> 2b

2c -> 2c

2d -> 2d

2e -> 2e

2f -> 2f

30 -> 30

31 -> 31

32 -> 32

33 -> 33

34 -> 34

35 -> 35

36 -> 36

37 -> 37

38 -> 38

39 -> 39

3a -> 3a

3b -> 3b

3c -> 3c

3d -> 3d

3e -> 3e

3f -> 3f

40 -> 40

41 -> 41

42 -> 42

43 -> 43

44 -> 44

45 -> 45

46 -> 46

47 -> 47

48 -> 48

49 -> 49

4a -> 4a

4b -> 4b

4c -> 4c

4d -> 4d

4e -> 4e

4f -> 4f

50 -> 50

51 -> 51

52 -> 52

53 -> 53

54 -> 54

55 -> 55

56 -> 56

57 -> 57

58 -> 58

59 -> 59

5a -> 5a

5b -> 5b

5c -> 5c

5d -> 5d

5e -> 5e

5f -> 5f

60 -> 60

61 -> 61

62 -> 62

63 -> 63

64 -> 64

65 -> 65

66 -> 66

67 -> 67

68 -> 68

69 -> 69

6a -> 6a

6b -> 6b

6c -> 6c

6d -> 6d

6e -> 6e

6f -> 6f

70 -> 70

71 -> 71

72 -> 72

73 -> 73

74 -> 74

75 -> 75

76 -> 76

77 -> 77

78 -> 78

79 -> 79

7a -> 7a

7b -> 7b

7c -> 7c

7d -> 7d

7e -> 7e

7f -> 7f

80 -> c4 [CHANGED]

81 -> c5 [CHANGED]

82 -> c7 [CHANGED]

83 -> c9 [CHANGED]

84 -> d1 [CHANGED]

85 -> d6 [CHANGED]

86 -> dc [CHANGED]

87 -> e1 [CHANGED]

88 -> e0 [CHANGED]

89 -> e2 [CHANGED]

8a -> e4 [CHANGED]

8b -> e3 [CHANGED]

8c -> e5 [CHANGED]

8d -> e7 [CHANGED]

8e -> e9 [CHANGED]

8f -> e8 [CHANGED]

90 -> ea [CHANGED]

91 -> eb [CHANGED]

92 -> ed [CHANGED]

93 -> ec [CHANGED]

94 -> ee [CHANGED]

95 -> ef [CHANGED]

96 -> f1 [CHANGED]

97 -> f3 [CHANGED]

98 -> f2 [CHANGED]

99 -> f4 [CHANGED]

9a -> f6 [CHANGED]

9b -> f5 [CHANGED]

9c -> fa [CHANGED]

9d -> f9 [CHANGED]

9e -> fb [CHANGED]

9f -> fc [CHANGED]

a0 -> 2020 [CHANGED]

a1 -> b0 [CHANGED]

a2 -> a2

a3 -> a3

a4 -> a7 [CHANGED]

a5 -> 2022 [CHANGED]

a6 -> b6 [CHANGED]

a7 -> df [CHANGED]

a8 -> ae [CHANGED]

a9 -> a9

aa -> 2122 [CHANGED]

ab -> b4 [CHANGED]

ac -> a8 [CHANGED]

ad -> 2260 [CHANGED]

ae -> c6 [CHANGED]

af -> d8 [CHANGED]

b0 -> 221e [CHANGED]

b1 -> b1

b2 -> 2264 [CHANGED]

b3 -> 2265 [CHANGED]

b4 -> a5 [CHANGED]

b5 -> b5

b6 -> 2202 [CHANGED]

b7 -> 2211 [CHANGED]

b8 -> 220f [CHANGED]

b9 -> 3c0 [CHANGED]

ba -> 222b [CHANGED]

bb -> aa [CHANGED]

bc -> ba [CHANGED]

bd -> 3a9 [CHANGED]

be -> e6 [CHANGED]

bf -> f8 [CHANGED]

c0 -> bf [CHANGED]

c1 -> a1 [CHANGED]

c2 -> ac [CHANGED]

c3 -> 221a [CHANGED]

c4 -> 192 [CHANGED]

c5 -> 2248 [CHANGED]

c6 -> 2206 [CHANGED]

c7 -> ab [CHANGED]

c8 -> bb [CHANGED]

c9 -> 2026 [CHANGED]

ca -> a0 [CHANGED]

cb -> c0 [CHANGED]

cc -> c3 [CHANGED]

cd -> d5 [CHANGED]

ce -> 152 [CHANGED]

cf -> 153 [CHANGED]

d0 -> 2013 [CHANGED]

d1 -> 2014 [CHANGED]

d2 -> 201c [CHANGED]

d3 -> 201d [CHANGED]

d4 -> 2018 [CHANGED]

d5 -> 2019 [CHANGED]

d6 -> f7 [CHANGED]

d7 -> 25ca [CHANGED]

d8 -> ff [CHANGED]

d9 -> 178 [CHANGED]

da -> 2044 [CHANGED]

db -> 20ac [CHANGED]

dc -> 2039 [CHANGED]

dd -> 203a [CHANGED]

de -> fb01 [CHANGED]

df -> fb02 [CHANGED]

e0 -> 2021 [CHANGED]

e1 -> b7 [CHANGED]

e2 -> 201a [CHANGED]

e3 -> 201e [CHANGED]

e4 -> 2030 [CHANGED]

e5 -> c2 [CHANGED]

e6 -> ca [CHANGED]

e7 -> c1 [CHANGED]

e8 -> cb [CHANGED]

e9 -> c8 [CHANGED]

ea -> cd [CHANGED]

eb -> ce [CHANGED]

ec -> cf [CHANGED]

ed -> cc [CHANGED]

ee -> d3 [CHANGED]

ef -> d4 [CHANGED]

f0 -> f8ff [CHANGED]

f1 -> d2 [CHANGED]

f2 -> da [CHANGED]

f3 -> db [CHANGED]

f4 -> d9 [CHANGED]

f5 -> 131 [CHANGED]

f6 -> 2c6 [CHANGED]

f7 -> 2dc [CHANGED]

f8 -> af [CHANGED]

f9 -> 2d8 [CHANGED]

fa -> 2d9 [CHANGED]

fb -> 2da [CHANGED]

fc -> b8 [CHANGED]

fd -> 2dd [CHANGED]

fe -> 2db [CHANGED]

ff -> 2c7 [CHANGED]

Answer 1

尝试使用:

unichar before = 0xa1;
NSString *s = [NSString stringWithFormat:@"%C",before];

unichar after = [s characterAtIndex:0];
NSLog(@"Read back char was %C", after);

从技术上讲，“char”tpe 应该是 0-127。 UTF8 使用两个较高的位，因此它有点不确定单个“0xFF”会生成什么。当您使用 stringWithFormat 时，chars 会被提升为 ints，因此您的 0xA0 会变成 0xFFFFFFA0，系统可能会查找负值并执行谁知道的操作。