ruby Exception and StandardError

It is important to understand the place of StandardError in the hierarchy of ruby exceptions:

class TestException1 < StandardError
end

class TestException2 < Exception
end

begin
begin
raise TestException1
rescue
puts $!.class.name
end

begin
raise TestException2
rescue
puts $!.class.name
end
rescue Exception
puts "Unhandled Exception #{$!.class.name}"
end

 

The result of this code is:

TestException1
Unhandled Exception TestException2

This behavior is consistent with the way ruby exceptions are designed; Errors which you can generally deal with are subclassed from StandardError, while errors which indicate a failure in the semantic meaning of the code or its execution inherit directly from Exception

 

class Object
def self.subclasses(direct = false)
classes = []

ObjectSpace.each_object(Class) do |c|
next unless c.superclass == self
classes << c
end

return classes
end
end

puts "Direct descendants of Exception: \n\t" + Exception.subclasses.join("\n\t")
puts "Direct descendants of StandardError: \n\t" + StandardError.subclasses.join("\n\t")

 

This gives us a list of exception classes:

# Direct descendants of Exception:
NoMemoryError
ScriptError
StandardError
SignalException
fatal
SystemExit
# Direct descendants of StandardError:
SystemStackError
LocalJumpError
IOError
RegexpError
ZeroDivisionError
ThreadError
SystemCallError
SecurityError
RuntimeError
NameError
RangeError
IndexError
ArgumentError
TypeError

 

Generally, if you are making your own exception class for non-fatal execution errors, you should inherit from StandardError or one of its descendants.