IO标准库类型和头文件

讲解在《C++ Primer》的第245面有，这里只是为了让自己记忆加深，并让大家好查，所以打上来：

头文件                                                                                            类   型

iostream                                                             istream从流中读取

                                                                           ostream写到流中去

                                                                           iostream对流进行读写；从istream和ostream派生而来

fstream                                                               ifstream从文件中读取；由istream派生而来

                                                                           ofstream写到文件中去；由ostream派生而来

                                                                            fstream读写文件；由iostream派生而来

sstream                                                               istringstream从string对象中读取；由istream派生而来

                                                                            ostringstream写到string对象中去；由ostream派生而来

                                                                            stringstream对string对象进行读写；由iostream派生而来