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Ruby Programming
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Raising Exceptions

So far we've been on the defensive, handling exceptions raised by others. It's time to turn the tables and go on the offensive. (There are those that say your gentle authors are always offensive, but that's a different book.)

You can raise exceptions in your code with the Kernel::raise method.

raise
raise "bad mp3 encoding"
raise InterfaceException, "Keyboard failure", caller

The first form simply reraises the current exception (or a RuntimeError if there is no current exception). This is used in exception handlers that need to intercept an exception before passing it on.

The second form creates a new RuntimeError exception, setting its message to the given string. This exception is then raised up the call stack.

The third form uses the first argument to create an exception and then sets the associated message to the second argument and the stack trace to the third argument. Typically the first argument will be either the name of a class in the Exception hierarchy or a reference to an object instance of one of these classes.[Technically, this argument can be any object that responds to the message exception by returning an object such that object.kind_of?(Exception) is true.] The stack trace is normally produced using the Kernel::caller method.

Here are some typical examples of raise in action.

raise

raise "Missing name" if name.nil?

if i >= myNames.size   raise IndexError, "#{i} >= size (#{myNames.size})" end

raise ArgumentError, "Name too big", caller

In the last example, we remove the current routine from the stack backtrace, which is often useful in library modules. We can take this further: the following code removes two routines from the backtrace.

raise ArgumentError, "Name too big", caller[1..-1]
Ruby Programming
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