Better is_default_constructible in C++98

[Kim-J-Smith]

• Lines 84 to 92 in boost/type_traits/is_default_constructible.hpp are the compromise solution without std::decltype. In fact, there are better solutions.

---

• The original code is as follows(line 84 ~ 92 in boost/type_traits/is_default_constructible.hpp)

   // We don't know how to implement this, note we can not use has_trivial_constructor here
   // because the correct implementation of that trait requires this one:
   template <class T> struct is_default_constructible : public is_pod<T>{};
   template <> struct is_default_constructible<void> : public integral_constant<bool, false>{};
   template <> struct is_default_constructible<void const> : public integral_constant<bool, false>{};
   template <> struct is_default_constructible<void volatile> : public integral_constant<bool, false>{};
   template <> struct is_default_constructible<void const volatile> : public integral_constant<bool, false>{};

• Better solution (without std::decltype or other C++11 feature)

  // __is_default_constructible_helper
  template <typename T, bool not_void>
  struct __is_default_constructible_helper
  {
    typedef char        yes[1];
    typedef char        no[2];

    template <typename U>
    static yes& check(int (*) [sizeof((U()))]);

    template <typename U>
    static no& check(...);

    static const bool value = sizeof(check<T>(0)) == sizeof(yes);
  };

  // void : false
  template <typename T>
  struct __is_default_constructible_helper<T, false>
  : public false_type { };

  // T[S] -> T
  template <typename T, size_t S>
  struct __is_default_constructible_helper<T[S], true>
  : public __is_default_constructible_helper<T, true> { };

  // T[] : false
  template <typename T>
  struct __is_default_constructible_helper<T[], true>
  : public false_type { };

  // T&& and T& : false
  template <typename T>
  struct __is_default_constructible_helper<T&, true>
  : public false_type { };

#if __cplusplus >= 201103L
  template <typename T>
  struct __is_default_constructible_helper<T&&, true>
  : public false_type { };
#endif // __cplusplus >= 201103L

  /// is_default_constructible
  template <typename T>
  struct is_default_constructible
  : public kim::bool_constant<
    __is_default_constructible_helper<
      typename kim::__remove_cv<T>::type, !is_void<T>::value
    >::value
  >::type
  { };