什么是无符号字符？

Question

In C/C++, what an unsigned char is used for? How is it different from a regular char ?

Answer 1

In C++, there are three distinct character types:

• char
• signed char
• unsigned char

If you are using character types for text , use the unqualified char :

• it is the type of character literals like 'a' or '0' .
• it is the type that makes up C strings like "abcde"

It also works out as a number value, but it is unspecified whether that value is treated as signed or unsigned. Beware character comparisons through inequalities - although if you limit yourself to ASCII (0-127) you're just about safe.

If you are using character types as numbers , use:

• signed char , which gives you at least the -127 to 127 range. (-128 to 127 is common)
• unsigned char , which gives you at least the 0 to 255 range.

"At least", because the C++ standard only gives the minimum range of values that each numeric type is required to cover. sizeof (char) is required to be 1 (ie one byte), but a byte could in theory be for example 32 bits. sizeof would still be report its size as 1 - meaning that you could have sizeof (char) == sizeof (long) == 1 .

Answer 2

This is implementation dependent, as the C standard does NOT define the signed-ness of char . Depending on the platform, char may be signed or unsigned , so you need to explicitly ask for signed char or unsigned char if your implementation depends on it. Just use char if you intend to represent characters from strings, as this will match what your platform puts in the string.

The difference between signed char and unsigned char is as you'd expect. On most platforms, signed char will be an 8-bit two's complement number ranging from -128 to 127 , and unsigned char will be an 8-bit unsigned integer ( 0 to 255 ). Note the standard does NOT require that char types have 8 bits, only that sizeof(char) return 1 . You can get at the number of bits in a char with CHAR_BIT in limits.h . There are few if any platforms today where this will be something other than 8 , though.

There is a nice summary of this issue here .

As others have mentioned since I posted this, you're better off using int8_t and uint8_t if you really want to represent small integers.

Answer 3

Because i feel it's really called for, i just want to state some rules of C and C++ (they are the same in this regard). First, all bits of unsigned char participate in determining the value if any unsigned char object. Second, unsigned char is explicitly stated unsigned.

Now, i had a discussion with someone about what happens when you convert the value -1 of type int to unsigned char . He refused the idea that the resulting unsigned char has all its bits set to 1, because he was worried about sign representation. But he don't have to. It's immediately following out of this rule that the conversion does what is intended:

If the new type is unsigned, the value is converted by repeatedly adding or subtracting one more than the maximum value that can be represented in the new type until the value is in the range of the new type. ( 6.3.1.3p2 in a C99 draft)

That's a mathematical description. C++ describes it in terms of modulo calculus, which yields to the same rule. Anyway, what is not guaranteed is that all bits in the integer -1 are one before the conversion. So, what do we have so we can claim that the resulting unsigned char has all its CHAR_BIT bits turned to 1?

1. All bits participate in determining its value - that is, no padding bits occur in the object.
2. Adding only one time UCHAR_MAX+1 to -1 will yield a value in range, namely UCHAR_MAX

That's enough, actually! So whenever you want to have an unsigned char having all its bits one, you do

unsigned char c = (unsigned char)-1;

It also follows that a conversion is not just truncating higher order bits. The fortunate event for two's complement is that it is just a truncation there, but the same isn't necessarily true for other sign representations.

Answer 4

As for example usages of unsigned char :

unsigned char is often used in computer graphics, which very often (though not always) assigns a single byte to each colour component. It is common to see an RGB (or RGBA) colour represented as 24 (or 32) bits, each an unsigned char . Since unsigned char values fall in the range [0,255], the values are typically interpreted as:

• 0 meaning a total lack of a given colour component.
• 255 meaning 100% of a given colour pigment.

So you would end up with RGB red as (255,0,0) -> (100% red, 0% green, 0% blue).

Why not use a signed char ? Arithmetic and bit shifting becomes problematic. As explained already, a signed char 's range is essentially shifted by -128. A very simple and naive (mostly unused) method for converting RGB to grayscale is to average all three colour components, but this runs into problems when the values of the colour components are negative. Red (255, 0, 0) averages to (85, 85, 85) when using unsigned char arithmetic. However, if the values were signed char s (127,-128,-128), we would end up with (-99, -99, -99), which would be (29, 29, 29) in our unsigned char space, which is incorrect.

Answer 5

如果要将字符用作小整数，最安全的方法是使用int8_t和uint8_t类型。

Answer 6

unsigned char takes only positive values....like 0 to 255

where as

signed char takes both positive and negative values....like -128 to +127

Answer 7

signed char has range -128 to 127; unsigned char has range 0 to 255.

char will be equivalent to either signed char or unsigned char, depending on the compiler, but is a distinct type.

If you're using C-style strings, just use char . If you need to use chars for arithmetic (pretty rare), specify signed or unsigned explicitly for portability.

Answer 8

char and unsigned char aren't guaranteed to be 8-bit types on all platforms—they are guaranteed to be 8-bit or larger. Some platforms have 9-bit, 32-bit, or 64-bit bytes . However, the most common platforms today (Windows, Mac, Linux x86, etc.) have 8-bit bytes.

Answer 9

An unsigned char is an unsigned byte value (0 to 255). You may be thinking of char in terms of being a "character" but it is really a numerical value. The regular char is signed, so you have 128 values, and these values map to characters using ASCII encoding. But in either case, what you are storing in memory is a byte value.

Answer 10

In terms of direct values a regular char is used when the values are known to be between CHAR_MIN and CHAR_MAX while an unsigned char provides double the range on the positive end. For example, if CHAR_BIT is 8, the range of regular char is only guaranteed to be [0, 127] (because it can be signed or unsigned) while unsigned char will be [0, 255] and signed char will be [-127, 127].

In terms of what it's used for, the standards allow objects of POD (plain old data) to be directly converted to an array of unsigned char. This allows you to examine the representation and bit patterns of the object. The same guarantee of safe type punning doesn't exist for char or signed char.

Answer 11

如果您喜欢使用各种类型的特定长度和符号，那么使用uint8_t 、 int8_t 、 uint16_t等可能会更好，因为它们完全按照他们所说的去做。

Answer 12

unsigned char is the heart of all bit trickery. In almost ALL compiler for ALL platform an unsigned char is simply a byte and an unsigned integer of (usually) 8 bits that can be treated as a small integer or a pack of bits.

In addiction, as someone else has said, the standard doesn't define the sign of a char. so you have 3 distinct char types: char , signed char , unsigned char .

Answer 13

unsigned char只取正值：0 到 255，而signed char取正值和负值：-128 到 +127。

Answer 14

Some googling found this , where people had a discussion about this.

An unsigned char is basically a single byte. So, you would use this if you need one byte of data (for example, maybe you want to use it to set flags on and off to be passed to a function, as is often done in the Windows API).

Answer 15

An unsigned char uses the bit that is reserved for the sign of a regular char as another number. This changes the range to [0 - 255] as opposed to [-128 - 127].

Generally unsigned chars are used when you don't want a sign. This will make a difference when doing things like shifting bits (shift extends the sign) and other things when dealing with a char as a byte rather than using it as a number.

Answer 16

quoted frome "the c programming laugage" book:

The qualifier signed or unsigned may be applied to char or any integer. unsigned numbers are always positive or zero, and obey the laws of arithmetic modulo 2^n, where n is the number of bits in the type. So, for instance, if chars are 8 bits, unsigned char variables have values between 0 and 255, while signed chars have values between -128 and 127 (in a two' s complement machine.) Whether plain chars are signed or unsigned is machine-dependent, but printable characters are always positive.

Answer 17

signed char and unsigned char both represent 1byte, but they have different ranges.

   Type        |      range
-------------------------------
signed char    |  -128 to +127
unsigned char  |     0 to 255

In signed char if we consider char letter = 'A' , 'A' is represent binary of 65 in ASCII/Unicode , If 65 can be stored, -65 also can be stored. There are no negative binary values in ASCII/Unicode there for no need to worry about negative values.

Example

#include <stdio.h>

int main()
{
    signed char char1 = 255;
    signed char char2 = -128;
    unsigned char char3 = 255;
    unsigned char char4 = -128;

    printf("Signed char(255) : %d\n",char1);
    printf("Unsigned char(255) : %d\n",char3);

    printf("\nSigned char(-128) : %d\n",char2);
    printf("Unsigned char(-128) : %d\n",char4);

    return 0;
}

Output -:

Signed char(255) : -1
Unsigned char(255) : 255

Signed char(-128) : -128
Unsigned char(-128) : 128