Check the system endianness at compile time with C++

endianess.cpp

#include <cstdint>
#include <iostream>

struct Endian {
	constexpr static bool is_big() {
		union {
			uint32_t i;
			uint8_t c[4];
		} check = {
			.i = 0x01000000
		};

		return check.c[0];
	}

	constexpr static bool is_little() {
		return !is_big();
	}
};

int main() {
	std::cout << "Is big endian: " << Endian::is_big() << std::endl;
	std::cout << "Is little endian: " << Endian::is_little() << std::endl;

	return 0;
}

And it has undefined behavior: If you intialize member .i of a union, then you can only read .i, but not .c. So, if the program prints what you expect, then it is pure luck.