14.3.1.2 Handling Output Errors

Generally, you don’t have to check for errors when you overload operator<< because the iostream library is arranged to propagate errors.

When an error occurs, the iostream where it occurred enters an error state. Bits in the iostream’s state are set according to the general category of the error. The inserters defined in iostream ignore attempts to insert data into any stream that is in an error state, so such attempts do not change the iostream’s state.

In general, the recommended way to handle errors is to periodically check the state of the output stream in some central place. If there is an error, you should handle it in some way. This chapter assumes that you define a function error, which takes a string and aborts the program. error is not a predefined function. See 14.3.9 Handling Input Errors for an example of an error function. You can examine the state of an iostream with the operator !,which returns a nonzero value if the iostream is in an error state. For example:

if (!cout) error("output error");

There is another way to test for errors. The ios class defines operator void *(), so it returns a NULL pointer when there is an error. You can use a statement like:

if (cout << x) return; // return if successful

You can also use the function good, a member of ios:

if (cout.good()) return; // return if successful

The error bits are declared in the enum:

enum io_state {goodbit=0, eofbit=1, failbit=2,
badbit=4, hardfail=0x80};

For details on the error functions, see the iostream man pages.