13.4. Stream Input

Now let’s consider stream input. Formatted and unformatted input capabilities are provided by istream. The stream extraction operator (>>) normally skips white-space characters (such as blanks, tabs and newlines) in the input stream; later we’ll see how to change this behavior. After each input, the stream extraction operator returns a reference to the stream object that received the extraction message (e.g., cin in the expression cin >> grade). If that reference is used as a condition (e.g., in a while statement’s loop-continuation condition), the stream’s overloaded void * cast operator function is implicitly invoked to convert the reference into a non-null pointer value or the null pointer based on the success or failure, respectively, of the last input operation. A non-null pointer converts to the bool value true to indicate success and the null pointer converts to the bool value false to indicate failure. When an attempt is made to read past the end of a stream, the stream’s overloaded void * cast operator returns the null pointer to indicate end-of-file.

Each stream object contains a set of state bits used to control the stream’s state (i.e., formatting, setting error states, etc.). These bits are used by the stream’s overloaded void * cast operator to determine whether to return a non-null pointer or the null pointer. Stream extraction causes the stream’s failbit to be set if data of the wrong type is input and causes the stream’s badbit to be set if the operation fails. Sections 13.7–13.8 discuss stream state bits in detail, then show how to test these bits after an I/O operation.