2.ios类 stream fstreambase strstreambase+ ostream ifstream istrstream ofstream bstrstreamconstream istream withassign fstreamstrstream bstream withassign iostream iostream withassign2. ios 类 ios istream fstreambase strstreambase ostream ifstream istrstream ofstream ostrstream constream istream_withassign fstream ostream_withassign iostream_withassign strstream iostream 4