C++中的I/O stream类

ios_base：顶层基类

basic_ios<>：派生自ios_base的template class 。具体定义：

template<class charT, class traits = char_traits<charT> >
class basic_ios ：public class ios_base {};

basic_istream<> / basic_ostream<>：两者虚拟派生自basic_ios<>的模版类。具体定义：

template<class charT, class traits = char_traits<charT> >
class basic_istream ：virtual public class basic_ios <charT, traits> {};

template<class charT, class traits = char_traits<charT> >
class basic_ostream ：virtual public class basic_ios <charT, traits> {};

basic_iostream<>：派生自basic_istream<>和basic_ostream<>的模版类。为了避免多重继承中由多个父类共同的基类产生的二义性，所以basic_istream<> / basic_ostream<>需要虚拟派生自basic_ios<>。具体定义：

template<class _Elem,class _Traits>
class basic_iostream
	: public basic_istream<_Elem, _Traits>,
	  public basic_ostream<_Elem, _Traits> ｛｝

basic_fstream<>：派生自basic_iostream<>的模版类。具体定义：

template<class _Elem, class _Traits>
class basic_fstream
    : public basic_iostream<_Elem, _Traits> {}

basic_ifstream<>：派生自basic_istream<>的模版类。具体定义：

template<class _Elem, class _Traits>
class basic_ifstream
    : public basic_istream<_Elem, _Traits> {}

basic_ofstream<>：派生自basic_ostream<>的模版类。具体定义：

template<class _Elem, class _Traits>
class basic_ofstream
    : public basic_ostream<_Elem, _Traits> {}

我们熟悉的iostream、istream、ostream、fstream、ifstream、ofstream、ios都是上述模版类的特例型别。具体定义：

typedef basic_ios<char, char_traits<char> > ios;
typedef basic_istream<char, char_traits<char> > istream;
typedef basic_ostream<char, char_traits<char> > ostream;
typedef basic_iostream<char, char_traits<char> > iostream;
typedef basic_ifstream<char, char_traits<char> > ifstream;
typedef basic_ofstream<char, char_traits<char> > ofstream;
typedef basic_fstream<char, char_traits<char> > fstream;

写这些，主要就是为了理清c++中stream classes的整个继承体系。