Understanding Exception Handling

Exceptions are anomalies that occur during the normal flow of a program and prevent it from continuing. These anomalies--user, logic, or system errors--can be detected by a function. If the detecting function cannot deal with the anomaly, it "throws" an exception. A function that "handles" that kind of exception catches it.

In C++, when an exception is thrown, it cannot be ignored--there must be some kind of notification or termination of the program. If no user-provided exception handler is present, the compiler provides a default mechanism to terminate the program.

Exception handling is expensive compared to ordinary program flow controls, such as loops or if-statements. It is therefore better not to use the exception mechanism to deal with ordinary situations, but to reserve it for situations that are truly unusual.

Exceptions are particularly helpful in dealing with situations that cannot be handled locally. Instead of propagating error status throughout the program, you can transfer control directly to the point where the error can be handled.

For example, a function might have the job of opening a file and initializing some associated data. If the file cannot be opened or is corrupted, the function cannot do its job. However, that function might not have enough information to handle the problem. The function can throw an exception object that describes the problem, transferring control to an earlier point in the program. The exception handler might automatically try a backup file, query the user for another file to try, or shut down the program gracefully. Without exception handlers, status and data would have to be passed down and up the function call hierarchy, with status checks after every function call. With exception handlers, the flow of control is not obscured by error checking. If a function returns, the caller can be certain that it succeeded.

Exception handlers have disadvantages. If a function does not return because it, or some other function it called, threw an exception, data might be left in an inconsistent state. You need to know when an exception might be thrown, and whether the exception might have a bad effect on the program state.