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
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.
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.