Question

When storing “byte arrays” (blobs…) is it better to use char or unsigned char for the items (unsigned char a.k.a. uint8_t)? (Standard says that sizeof of both is precisely 1 Byte.)

Does it matter at all? Or one is more convenient or prevalent than the other? Maybe, what libraries like Boost do use?

Answer 1

If char is signed, then performing arithmetic on a byte value with the high bit set will result in sign extension when promoting to int; so, for example:

char c = '\xf0';
int res = (c << 24) | (c << 16) | (c << 8) | c;

will give 0xfffffff0 instead of 0xf0f0f0f0. This can be avoided by masking with 0xff.

char may still be preferable if you’re interfacing with libraries that use it instead of unsigned char.

Note that a cast from char * to/from unsigned char * is always safe (3.9p2). A philosophical reason to favour unsigned char is that 3.9p4 in the standard favours it, at least for representing byte arrays that could hold memory representations of objects:

The object representation of an object of type T is the sequence of N unsigned char objects taken up by the object of type T, where N equals sizeof(T).