Round an integer up to the closest 2's power.

pow2_roundup.cc

#include <limits>
#include <type_traits>

template <int bit>
struct or_shift
{
	template <typename Int>
	static constexpr auto apply(Int n) -> Int
	{
		return or_shift<bit / 2>::apply(n | (n >> bit));
	}
};

template <>
struct or_shift<1>
{
	template <typename Int>
	static constexpr auto apply(Int n) -> Int
	{
		return n | (n >> 1);
	}
};

template <typename Int, typename R = typename std::make_unsigned<Int>::type>
constexpr auto pow2_roundup(Int n) -> R
{
	return or_shift<
	    std::numeric_limits<R>::digits / 2>::apply(R(n) - 1) + 1;
}

#include <iostream>
#include <iomanip>

int main()
{
    using namespace std;

	cout
	    << "0\t" << pow2_roundup(0) << endl
	    << "1\t" << pow2_roundup(1) << endl
	    << "2\t" << pow2_roundup(2) << endl
	    << "3\t" << pow2_roundup(3) << endl
	    << "143\t" << pow2_roundup(143) << endl
	    << "max\t" << pow2_roundup(numeric_limits<int>::max()) << endl;
}