An exception is an object that represents some kind of exceptional condition; it indicates that something has gone wrong. This could be a programming error—attempting to divide by zero, attempting to invoke a method on an object that does not define the method, or passing an invalid argument to a method. Or it could be the result from some kind of external condition—making a network request when the network is down, or trying to create an object when the system is out of memory.
When one of these errors or conditions occurs, an exception is
raised (or thrown). By
default, Ruby programs terminate when an exception occurs. But it is
possible to declare exception handlers. An exception handler is a block
of code that is executed if an exception occurs during the execution of
some other block of code. In this sense, exceptions are a kind of control
statement. Raising an exception transfers the flow-of-control to
exception handling code. This is like using the
break statement to exit from a loop. As we’ll
see, though, exceptions are quite different from the
break statement; they may transfer control out
of many enclosing blocks and even up the call stack in order to reach
the exception handler.
Ruby uses the
raise to raise exceptions, and uses a
rescue clause to handle exceptions.
Exceptions raised by
instances of the
or one of its many subclasses. The
catch methods described earlier in this chapter are ...