Deduce the char type of virtually any literal or container (C++11)

char_type_of.cpp

// char_type_of: deduce the char type of virtually any literal or
// container (C++11 SFINAE at its finest)
// Original author: Jean-Charles Lefebvre <[email protected]>

// util: an almighty version of std::decay<>, with super powers
template <typename T>
struct super_decay
{
    typedef
        typename std::remove_cv<
        typename std::remove_pointer<
        typename std::remove_reference<
        typename std::remove_extent<
        typename std::decay<T>::type>::type>::type>::type>::type type;
};

// util: a shortcut to super_decay<T>::type
template <typename T>
using super_decay_t = typename super_decay<T>::type;

// implementation details of char_type_of
namespace detail
{
    template <typename T>
    struct char_type_spec { };

    template <> struct char_type_spec<char> { typedef char type; };
    template <> struct char_type_spec<wchar_t> { typedef wchar_t type; };
    template <> struct char_type_spec<char16_t> { typedef char16_t type; };
    template <> struct char_type_spec<char32_t> { typedef char32_t type; };

    // This specialization should work for "any" container type that exposes the
    // value_type type (typically std::basic_string, std::vector, ...)
    template <template<typename...> typename Container, typename... Args>
    struct char_type_spec<typename Container<Args...>>
    {
        typedef typename Container<Args...>::value_type contvt_;

        // * std::is_integral helps to ensure we're not going to recurse
        // * Re-using char_type_spec<> allows to ensure Container::value_type
        //   type is supported
        typedef std::enable_if_t<
            std::is_integral<contvt_>::value,
            typename char_type_spec<contvt_>::type> type;
    };
}

// Deduce the type of char from the given type T.
// If no char_type_spec<> has been specialized for the specified type,
// msvc will spit error C2039 ('type': is not a member of...)
template <typename T>
struct char_type_of
{
    typedef
        typename detail::char_type_spec<typename super_decay_t<T>>::type type;
};

// A shortcut to char_type_of<T>::type
template <typename T>
using char_type_of_t = typename char_type_of<T>::type;