Interface allowing a class to have optional exceptions.
A class derives from this one to acquire a standard interface for disabling exceptions, possibly only temporarily. By default, exceptions are enabled.
Note that all methods are const even though some of them change our internal flag indicating whether exceptions should be thrown. This is justifiable because this is just an interface class, and it changes the behavior of our subclass literally only in exceptional conditions. This Jesuitical interpretation of "const" is required because you may want to disable exceptions on const subclass instances.
If it makes you feel better about this, consider that the real change isn't within the const OptionalExceptions subclass instance. What changes is the code wrapping the method call on that instance that can optionally throw an exception. This outside code is in a better position to say what "const" means than the subclass instance.