template<typename Char>
class String
{
public:
    using char_type = Char;
    using char_ptr = char_type*;
    String();
    char_type get(int i);
};

// 錯誤:
//template<typename Char>
//String<Char>::char_type String<Char>::get(int i)
//{
    ...
//}

// 正確:
template<typename Char>
typename String<Char>::char_type String<Char>::get(int i)
{
    ...
}