【正文】 failbit. The state of the stream is revealed by the bad, fail, eof, and good operations. If any of bad, fail, or eof are true, then testing the stream itself will indicate that the stream is in an error state. Similarly, the good operation returns TRue if none of the other conditions is true. The clear and setstate operations change the state of the condition member. The clear operations put the condition back in its valid state. They are called after we have remedied whatever problem occurred and we want to reset the stream to its valid state. The setstate operation turns on the specified condition to indicate that a problem occurred. setstate leaves the existing state variables unchanged except that it adds the additional indicated state(s). Accessing the Condition State The rdstate member function returns an iostate value that corresponds to the entire current condition state of the stream. Dealing with Multiple States Often we need to set or clear multiple state bits. We could do so by making multiple calls to the setstate or clear functions. Alternatively, we could use the bitwise OR (Section , p. 154) operator to generate a value to pass two or more state bits in a single call. The bitwise OR generates an integral value using the bit patterns of its operands. For each bit in the result, the bit is 1 if the corresponding bit is 1 in either of its operands. 第八章 標準 IO 庫 C++ 的輸入／輸出（ input/output）由標準庫提供。 can be called with an ofstream or ostring stream object. Because the IO types are related by inheritance, we can write one function and apply it to all three kinds of streams: console, disk files, or string streams. International Character Support The stream classes described thus far read and write streams posed of type char. The library defines a corresponding set of types supporting the wchar_t type. Each class is distinguished from its char counterpart by a w prefix. Thus, the types wostream, wistream, and wiostream read and write wchar_t data to or from a console window. The file input and output classes are wifstream, wofstream, and wfstream. The wchar_t versions of string stream input and output are wistring stream, wostring stream, and wstring stream. The library also defines objects to read and write wide characters from the standard input and standard output. These objects are distinguished from the char counterparts by a w prefix: The wchar_t standard input object is named wcin。ll have more to say about inheritance and objectoriented programming in Part IV, but generally speaking, types related by inheritance share a mon interface. When one class inherits from another, we (usually) can use the same operations on both classes. More specifically, when two types are related by inheritance, we say that one class inherits the behavior the interface of its parent. In C++ we speak of the parent as the base class and the inheriting class as a derived class. The IO types are defined in three separate headers: iostream defines the types used to read and write to a console window, fstream defines the types used to read and write named files, and sstream defines the types used to read and write inmemory strings. Each of the types in fstream and sstream is derived from a corresponding type defined in the iostream header. Table lists the IO classes and Figure on the next page illustrates the inheritance relationships among these types. Inheritance is usually illustrated similarly to how a family tree is displayed. The topmost circle represents a base (or parent) class. Lines connect a base class to its derived (or children) class(es). So, for example, this figure indicates that istream is the base c