What is an unsigned char?

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 (i.e. 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

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 3

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 4

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 5

An unsigned char is a (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 6

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).