check bitfield order of struct

check_bitfield_order.cpp

#include <cstdint>
#include <cstdio>

enum class StructBitfieldOrder {
	/// the most significant bit as the first appearing field in a struct
	MSB_AS_FIRST_FIELD,
	/// the least significant bit as the first appearing field in a struct
	LSB_AS_FIRST_FIELD,
	/// should not happen
	OTHER,
};

/**
 * @brief detect the bitfield order of the struct
 * @note Assuming the compiler does not reorder the bitfields
 */
StructBitfieldOrder struct_bitfield_order() {
	struct t {
		uint8_t zeros : 4 = 0b0000;
		uint8_t ones : 4  = 0b1111;
	};

	static_assert(sizeof(t) == 1);
	static bool has_value             = false;
	static StructBitfieldOrder result = StructBitfieldOrder::OTHER;
	if (has_value) {
		return result;
	}
	constexpr auto test = t{
		.zeros = 0x0000,
		.ones  = 0b1111};
	if (const uint8_t test_raw = *reinterpret_cast<const uint8_t *>(&test);
		test_raw == 0b00001111) {
		result = StructBitfieldOrder::MSB_AS_FIRST_FIELD;
	} else if (test_raw == 0b11110000) {
		result = StructBitfieldOrder::LSB_AS_FIRST_FIELD;
	} else {
		result = StructBitfieldOrder::OTHER;
	}
	has_value = true;
	return result;
}


const char *to_string(const StructBitfieldOrder order) {
	switch (order) {
	case StructBitfieldOrder::MSB_AS_FIRST_FIELD:
		return "MSB_AS_FIRST_FIELD";
	case StructBitfieldOrder::LSB_AS_FIRST_FIELD:
		return "LSB_AS_FIRST_FIELD";
	default:
		return "OTHER";
	}
}

int main(){
    printf("%s", to_string(struct_bitfield_order()));
}