constexpr Verion string generation

version.hpp

//
//  Example of how to use (abuse) constexpr declarations to create program
//  version string at compile-time. This is largely taken from the StackOverflow
//  user Constructor (http://stackoverflow.com/a/23453841), but with one minor
//  fix to null-terminate the string, one to make the code work with MSVC, and
//  and comments added.
//

#include <type_traits>

namespace version {
// Use anonymous namespace to make these helpers only locally accessible
namespace {
template <char... S> struct String {
  static constexpr char value[sizeof...(S)] = { S... };
};
template <char... S> constexpr char String<S...>::value[];

template <typename, typename> struct Concat;
template <char... S1, char... S2> struct Concat<String<S1...>, String<S2...>> {
  using type = String<S1..., S2...>;
};

template <typename...> struct Concatenate;
template <typename S1, typename... S2> struct Concatenate<S1, S2...> {
  using type = typename Concat<S1, typename Concatenate<S2...>::type>::type;
};
template <> struct Concatenate<> {
  using type = String<>;
};

template <std::size_t N> struct NumberToString {
  using type = typename Concat<
    typename std::conditional<(N >= 10), typename NumberToString<N / 10>::type, String<>>::type,
    String<'0' + N % 10>
  >::type;
};
template <> struct NumberToString<0> {
  using type = String<'0'>;
};
}  // anonymous namespace 

constexpr size_t major = 0;
constexpr size_t minor = 1;
constexpr size_t patch = 0;

namespace {
using VersionString = Concatenate<
  NumberToString<major>::type, String<'.'>,
  NumberToString<minor>::type, String<'.'>,
  NumberToString<patch>::type, String<'\0'>
>::type;
}  // anonymous namespace

constexpr const char* string = VersionString::value;

}  // namespace version

//
// ////////
// // Usage
// ////////
//
// #include <iostream>
//
// int main(int argc, char**argv) {
//   std::cout << 'v' << version::string << '\n';
// }
//