What is an unsigned char?

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' (in C++ only, in C their type is int)
• 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.