17.6. Constructors, Destructors and Exception Handling

First, let’s discuss an issue that we’ve mentioned but not yet resolved satisfactorily: What happens when an error is detected in a constructor? For example, how should an object’s constructor respond when it receives invalid data? Because the constructor cannot return a value to indicate an error, we must choose an alternative means of indicating that the object has not been constructed properly. One scheme is to return the improperly constructed object and hope that anyone using it would make appropriate tests to determine that it’s in an inconsistent state. Another scheme is to set some variable outside the constructor. The preferred alternative is to require the constructor to throw an exception that contains the error information, thus offering an opportunity for the program to handle the failure.

Before an exception is thrown by a constructor, destructors are called for any member objects whose constructors have run to completion as part of the object being constructed. Destructors are called for every automatic object constructed in a try block before the exception is caught. Stack unwinding is guaranteed to have been completed at the point that an exception handler begins executing. If a destructor invoked as a result of stack unwinding throws an exception, the program terminates. This has been linked to various security attacks.

If an object has member objects, and if an exception is thrown before the outer object is fully constructed, then destructors will be executed for the member objects that have been constructed prior to the occurrence of the exception. If an array of objects has been partially constructed when an exception occurs, only the destructors for the constructed objects in the array will be called.