C++11重写C++17 std::size std::empty std::data

template <typename C>
constexpr auto size(C const& c) -> decltype(c.size()) {
  return c.size();
}
template <typename T, std::size_t N>
constexpr std::size_t size(T const (&)[N]) noexcept {
  return N;
}

//  mimic: std::empty, C++17
template <typename C>
constexpr auto empty(C const& c) -> decltype(c.empty()) {
  return c.empty();
}
template <typename T, std::size_t N>
constexpr bool empty(T const (&)[N]) noexcept {
  //  while zero-length arrays are not allowed in the language, some compilers
  //  may permit them in some cases
  return N == 0;
}
template <typename E>
constexpr bool empty(std::initializer_list<E> il) noexcept {
  return il.size() == 0;
}

//  mimic: std::data, C++17
template <typename C>
constexpr auto data(C& c) -> decltype(c.data()) {
  return c.data();
}
template <typename C>
constexpr auto data(C const& c) -> decltype(c.data()) {
  return c.data();
}
template <typename T, std::size_t N>
constexpr T* data(T (&a)[N]) noexcept {
  return a;
}
template <typename E>
constexpr E const* data(std::initializer_list<E> il) noexcept {
  return il.begin();
}