compile-time calculate the magnitude (the number of bits needed to store a number) of a given number

mag.cpp

template <unsigned int n, unsigned int bit = 31>
struct mag
{
	enum {
		lval = (n & (1UL << bit)) != 0 ? bit : 0,
		rval = mag<n, bit - 1>::value,
		value = lval > rval ? lval : rval // max
	};
};

template <unsigned int n>
struct mag<n, 0>
{
	enum { value = (n & 1) != 0 ? 1 : 0 };
};

int main(int argc, char *argv[])
{
	printf("%d\n", mag<63>::value);
	printf("%d\n", mag<64>::value);
	printf("%d\n", mag<256>::value);
	printf("%d\n", mag<0>::value);
	printf("%d\n", mag<1 << 16>::value);
	printf("%d\n", mag<(1 << 16) - 1>::value);
	return 0;
}