Under what circumstances would one use a signed char in C++?

Question

In most situations, one would declare a char object to assign one of the character values on the ascii table ranging from 0 - 127. Even the extended character sets range from 128 - 255 (still positive). So i'm assuming that when dealing with the printing of characters, one only needs to use an unsigned char.

Now, based on some research on SO, people use a signed char when they need to use really small integers, but for that we can utilize the [u]int8 type. So i'm having trouble coming to terms with why one would need to use a signed char ? You can use it if you are dealing with the basic character ascii table (which unsigned char is already capable of doing) or you can use it to represent small integers (which [u]int8 already takes care of).

Can someone please provide a programming example in which a signed char is preferred over the other types ?

Answer 1

The reason is that you don't know, at least portably, if plain char variables are signed or unsigned. Different implementations have different approaches, a plain char may be signed in one platform and unsigned in another.

If you want to store negative values in a variable of type char, you absolutely must declare it as signed char, because only then you can be sure that every platform will be able to store negative values in there. Yes, you can use [u]int8 type, but this was not always the case (it was only introduced in C++11), and in fact, int8 is most likely an alias for signed char.

Moreover, uint8_t and int8_t are defined to be optional types, meaning you can't always rely on its existence (contrary to signed char). In particular, if a machine has a byte unit with more than 8 bits, it is not very likely that uint8_t and int8_t are defined (although they can; a compiler is always free to provide it and do the appropriate calculations). See this related question: What is int8_t if a machine has > 8 bits per byte?