Exception Handling

Exception handling in Ruby is very similar to other languages.

Raising an Exception

Raising an exception in Ruby is trivially easy. We use raise.

raise "A Error Occurred"

This will raise the default RuntimeException.

Raising a Specific Exception

We can also raise a specific type of exception:

value = "Hi there"
raise TypeError, 'Expected a Fixnum' if value.class != Fixnum

Rescuing Exceptions

We can rescue exceptions easily. Put the code that might raise an exception in a begin, rescue end block. If an exception occurs, control will be passed to the rescue section.

begin
  raise "A problem occurred"
rescue => e
  puts "Something bad happened"
  puts e.message
end

Rescuing Specific Exceptions

We can rescue different types of exceptions

value = "Hi there"

begin
  raise TypeError, 'Expected a Fixnum' if value.class != Fixnum
  raise "A problem occurred"
rescue TypeError => e
  puts "A Type Error Occurred"
  puts e.message
rescue => e
  puts "an unspecified error occurred"
end

The Ruby Exception Hierarchy

Here are the built in exceptions available in Ruby:

Exception
 NoMemoryError
 ScriptError
   LoadError
   NotImplementedError
   SyntaxError
 SignalException
   Interrupt
 StandardError
   ArgumentError
   IOError
     EOFError
   IndexError
   LocalJumpError
   NameError
     NoMethodError
   RangeError
     FloatDomainError
   RegexpError
   RuntimeError
   SecurityError
   SystemCallError
   SystemStackError
   ThreadError
   TypeError
   ZeroDivisionError
 SystemExit
 fatal

Defining Your Own Exception

You can define your own exceptions like so:

class MyNewError < StandardError
end

You can then raise your new exception as you see fit.

Exception Exercises

Try these exercises to get a feel for exception handling in Ruby.

Raising an Argument Error

Extend your kitten class from yesterday. Lets assume your kitten needs an age (0 will not do) Raise an argument error if age is not set in the initialiser

Raise a Type Error

Your kittens age must be a Fixnum. Check for this, if it is not, throw a Type Error

Catching a Division By Zero Error

Can your kitten do maths? If not, write a divide function now that accepts two values and divides them. Catch the division by zero error, and if it occurs, return nil.