c – 为什么在gcc库中声明了相同类型的重复

相关代码粘贴在下面.

// c++/4.2.1/streambuf
template<typename _CharT,typename _Traits>
    class basic_streambuf 
    {
    public:
      //@{
      /**
       *  These are standard types.  They permit a standardized way of
       *  referring to names of (or names dependant on) the template
       *  parameters,which are specific to the implementation.
      */
      typedef _CharT                    char_type;
      typedef _Traits                   traits_type;
      typedef typename traits_type::int_type        int_type;
      typedef typename traits_type::pos_type        pos_type;
      typedef typename traits_type::off_type        off_type;
      //@}

      //@{
      /**
       *  @if maint
       *  This is a non-standard type.
       *  @endif
      */
      typedef basic_streambuf<char_type,traits_type>   __streambuf_type;
      //@}
…
};

// c++/4.2.1/sstream
template<typename _CharT,typename _Traits,typename _Alloc>
    class basic_stringbuf : public basic_streambuf<_CharT,_Traits>
    {
    public:
      // Types:
      typedef _CharT                    char_type;
      typedef _Traits                   traits_type;
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 251. basic_stringbuf missing allocator_type
      typedef _Alloc                        allocator_type;
      typedef typename traits_type::int_type        int_type;
      typedef typename traits_type::pos_type        pos_type;
      typedef typename traits_type::off_type        off_type;

      typedef basic_streambuf<char_type,traits_type>   __streambuf_type;
      typedef basic_string<char_type,_Traits,_Alloc>  __string_type;
      typedef typename __string_type::size_type     __size_type;
…
};

更新：

char_type已在父类中声明为公共成员.子类可以直接使用它.我的问题是为什么gcc不实现basic_stringbuf,如下所示

template<typename _CharT,_Traits>
    {
    public:
      // Types:
      //typedef _CharT                  char_type;
      //typedef _Traits                     traits_type;
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 251. basic_stringbuf missing allocator_type
      typedef _Alloc                        allocator_type;
      //typedef typename traits_type::int_type      int_type;
      //typedef typename traits_type::pos_type      pos_type;
      //typedef typename traits_type::off_type      off_type;

      typedef basic_streambuf<char_type,_Alloc>  __string_type;
      typedef typename __string_type::size_type     __size_type;
…
};

编辑：

谢谢K-ballo.我认为你的答案是有道理的.我试过下面的代码.类型名称char_type不能在子类中使用.

template<typename _CharT>
class Base
{
public:
    typedef _CharT char_type; 
};

template<typename _CharT>
class Child : public Base<_CharT>
{
private:
    char_type _M_Data; // FAIL: Unknown type name 'char_type'
};