fstream 是一个什么头文件，什么时候需要引用？c++

文件流处理的时候用

<fstream>
namespace std {
template<class E, class T = char_traits<E> >
class basic_filebuf;
typedef basic_filebuf<char> filebuf;
typedef basic_filebuf<wchar_t> wfilebuf;
template<class E, class T = char_traits<E> >
class basic_ifstream;
typedef basic_ifstream<char> ifstream;
typedef basic_ifstream<wchar_t> wifstream;
template<class E, class T = char_traits<E> >
class basic_ofstream;
typedef basic_ofstream<char> ofstream;
typedef basic_ofstream<wchar_t> wofstream;
template<class E, class T = char_traits<E> >
class basic_fstream;
typedef basic_fstream<char> fstream;
typedef basic_fstream<wchar_t> wfstream;
};
Include the iostreams standard header <fstream> to define several template clas