8.2 Checking the Stream State

Let's first examine how you can check for errors using the stream state. A stream has several member functions for this purpose, which are summarized with their effects in Table 9:

Table 9 -- Stream member functions for error checking

*ios_base* member function	Effect
`bool good()`	True if no error flag is set
`bool eof()`	True if `eofbit` is set
`bool fail()`	True if `failbit` or `badbit` is set
`bool bad()`	True if `badbit` is set
`bool operator!()`	As `fail()`
`operator void*()`	Null pointer if `fail()` and non-null value otherwise
`iostate rdstate()`	Value of stream state

It is a good idea to check the stream state in some central place, for example:

if (!cout) error();

The state of cout is examined with operator!(), which will return true if the stream state indicates an error has occurred.

An ostream can also appear in a boolean position to be tested as follows:

if (cout << x) // okay!

The magic here is the operator void*() that returns a non-zero value when the stream state is non-zero.

Finally, the explicit member functions can also be used:

if (cout << x, cout.good()) // okay!;

Note that there is a difference between good() and operator!(). The function good() takes all flags into account; operator!() and fail() ignore eofbit.

OEM Release, June 1998