Question

I have need to pack four signed bytes into 32-bit integral type.
this is what I came up to:

int32_t byte(int8_t c) { return (unsigned char)c; }

int pack(char c0, char c1, ...) {
  return byte(c0) | byte(c1) << 8 | ...;
}

is this a good solution? Is it portable (not in communication sense)?
is there a ready-made solution, perhaps boost?

issue I am mostly concerned about is bit order when converting of negative bits from char to int. I do not know what the correct behavior should be.

Thanks

Answer 1

I liked Joey Adam’s answer except for the fact that it is written with macros (which cause a real pain in many situations) and the compiler will not give you a warning if ‘char’ isn’t 1 byte wide. This is my solution (based off Joey’s).

inline uint32_t PACK(uint8_t c0, uint8_t c1, uint8_t c2, uint8_t c3) {
    return (c0 << 24) | (c1 << 16) | (c2 << 8) | c3;
}

inline uint32_t PACK(sint8_t c0, sint8_t c1, sint8_t c2, sint8_t c3) {
    return PACK((uint8_t)c0, (uint8_t)c1, (uint8_t)c2, (uint8_t)c3);
}

I’ve omitted casting c0->c3 to a uint32_t as the compiler should handle this for you when shifting and I used c-style casts as they will work for either c or c++ (the OP tagged as both).