How to write constexpr swap function to change end

Question

How to write a constexpr function to swap endianess of an integer, without relying on compiler extensions and can you give an example on how to do it?

Answer 1

Yes, it's pretty easy; here's a recursive (C++11-compatible) implementation (unsigned integral types only):

#include <climits>
#include <cstdint>
#include <type_traits>

template<class T>
constexpr typename std::enable_if<std::is_unsigned<T>::value, T>::type
bswap(T i, T j = 0u, std::size_t n = 0u) {
  return n == sizeof(T) ? j :
    bswap<T>(i >> CHAR_BIT, (j << CHAR_BIT) | (i & (T)(unsigned char)(-1)), n + 1);
}

Example.

Here I'm using j as the accumulator and n as the loop counter (indexing bytes).

If you have a compiler supporting C++17 fold expressions, it's possible to write something that expands out into exactly what you'd write by hand:

template<class T, std::size_t... N>
constexpr T bswap_impl(T i, std::index_sequence<N...>) {
  return ((((i >> (N * CHAR_BIT)) & (T)(unsigned char)(-1)) <<
           ((sizeof(T) - 1 - N) * CHAR_BIT)) | ...);
}; //                                        ^~~~~ fold expression
template<class T, class U = typename std::make_unsigned<T>::type>
constexpr U bswap(T i) {
  return bswap_impl<U>(i, std::make_index_sequence<sizeof(T)>{});
}

The advantage of this form is that because it doesn't use loops or recursion, you're pretty much guaranteed to get optimal assembly output - on x86-64, clang even manages to work out to use the bswap instruction.

Answer 2

Inspired by ecatmur I suggest the following solution, that has potentially better performance when bswap is not detected by the compiler (O(log(n)) vs O(N)). Given that N is usually <=8 this is probably irrelevant, still:

template <typename T>
typename std::enable_if<std::is_unsigned<T>::value,T>::type
constexpr alternating_bitmask(const size_t step){
  T mask(0);
  for (size_t i=0;i<digits<T>();i+=2*step){
    mask|=(~T(0)>>(digits<T>()-step))<<i;
  }
  return mask;
}

template <typename T>
typename std::enable_if<std::is_unsigned<T>::value,T>::type
constexpr bswap(T n){
  for (size_t i=digits<unsigned char>();i<digits<T>();i*=2){
    n = ((n&(~(alternating_bitmask<T>(i))))>>i)|
        ((n&( (alternating_bitmask<T>(i))))<<i);
  }
  return n;
}

As this form is more complex than ecatmur's solution the compiler has a harder job optimizing, but clang still finds out that we mean bswap.