15.9. Tying an Output Stream to an Input Stream

Interactive applications generally involve an istream for input and an ostream for output. When a prompting message appears on the screen, the user responds by entering the appropriate data. Obviously, the prompt needs to appear before the input operation proceeds. With output buffering, outputs appear only when the buffer fills, when outputs are flushed explicitly by the program or automatically at the end of the program. C++ provides member function tie to synchronize (i.e., "tie together") the operation of an istream and an ostream to ensure that outputs appear before their subsequent inputs. The call

cin.tie( &cout );

ties cout (an ostream) to cin (an istream). Actually, this particular call is redundant, because C++ performs this operation automatically to create a user's standard input/output environment. However, the user would tie other istream/ostream pairs explicitly. To untie an input stream, inputStream, from an output stream, use the call

inputStream.tie( 0 );