【正文】 寫。 others occur deep within the system and are beyond the scope of a program to correct. The IO library manages a set of condition state members that indicate whether a given IO object is in a usable state or has encountered a particular kind of error. The library also defines a set of functions and flags, listed in Table , that give us access to and let us manipulate the state of each stream. If we enter Borges on the standard input, then cin will be put in an error state following the unsuccessful attempt to read a string of characters as an int. Similarly, cin will be in an error state if we enter an endoffile. Had we entered 1024, then the read would be successful and cin would be in a good, nonerror state. To be used for input or output, a stream must be in a nonerror state. The easiest way to test whether a stream is okay is to test its truth value. The if directly tests the state of the stream. The while does so indirectly by testing the stream returned from the expression in the condition. If that input operation succeeds, then the condition tests true. Condition States Many programs need only know whether a stream is valid. Other programs need more finegrained access to and control of the state of the stream. Rather than knowing that the stream is in an error state, we might want to know what kind of error was encountered. For example, we might want to distinguish between reaching endoffile and encountering an error on the IO device. Each stream object contains a condition state member that is managed through the setstate and clear operations. This state member has type iostate, which is a machinedependent integral type defined by each iostream class. It is used as a collection of bits, much the way we used the int_quiz1 variable to represent test scores in the example in Section (p. 156). Each IO class also defines three const values of type iostate that represent particular bit patterns. These const values are used to indicate particular kinds of IO conditions. They can be used with the bitwise operators (Section , p. 154) to test or set multiple flags in one operation. The badbit indicates a system level failure, such as an unrecoverable read or write error. It is usually not possible to continue using a stream after such an error. The failbit is set after a recoverable error, such as reading a character when numeric data was expected. It is often possible to correct the problem that caused the failbit to be set. The eofbit is set when an endoffile is encountered. Hitting endoffile also sets the 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 bitwi