create an instance of your exception with new:

class CustomException < StandardError  def initialize(data)    @data = data  endend# => nil raise CustomException.new(bla: "blupp")# CustomException: CustomException

Solution:

class FooError < StandardError  attr_reader :foo  def initialize(foo)   super   @foo = foo  endend

This is the best way if you follow the Rubocop Style Guide and always pass your message as the second argument to raise:

raise FooError.new('foo'), 'bar'

You can get foo like this:

rescue FooError => error  error.foo     # => 'foo'  error.message # => 'bar'

If you want to customize the error message then write:

class FooError < StandardError  attr_reader :foo  def initialize(foo)   super   @foo = foo  end  def message    "The foo is: #{foo}"  endend

This works well if foo is required. If you want foo to be an optional argument, then keep reading.

Explanation:

Pass your message as the second argument to raise

As the Rubocop Style Guide says, the message and the exception class should be provided as separate arguments because if you write:

raise FooError.new('bar')

And want to pass a backtrace to raise, there is no way to do it without passing the message twice:

raise FooError.new('bar'), 'bar', other_error.backtrace

As this answer says, you will need to pass a backtrace if you want to re-raise an exception as a new instance with the same backtrace and a different message or data.

Implementing FooError

The crux of the problem is that if foo is an optional argument, there are two different ways of raising exceptions:

raise FooError.new('foo'), 'bar', backtrace # case 1

and

raise FooError, 'bar', backtrace # case 2

and we want FooError to work with both.

In case 1, since you've provided an error instance rather than a class, raise sets 'bar' as the message of the error instance.

In case 2, raise instantiates FooError for you and passes 'bar' as the only argument, but it does not set the message after initialization like in case 1. To set the message, you have to call super in FooError#initialize with the message as the only argument.

So in case 1, FooError#initialize receives 'foo', and in case 2, it receives 'bar'. It's overloaded and there is no way in general to differentiate between these cases. This is a design flaw in Ruby. So if foo is an optional argument, you have three choices:

(a) accept that the value passed to FooError#initialize may be either foo or a message.

(b) Use only case 1 or case 2 style with raise but not both.

(c) Make foo a keyword argument.

If you don't want foo to be a keyword argument, I recommend (a) and my implementation of FooError above is designed to work that way.

If you raise a FooError using case 2 style, the value of foo is the message, which gets implicitly passed to super. You will need an explicit super(foo) if you add more arguments to FooError#initialize.

If you use a keyword argument (h/t Lemon Cat's answer) then the code looks like:

class FooError < StandardError  attr_reader :foo  def initialize(message, foo: nil)   super(message)   @foo = foo  endend

And raising looks like:

raise FooError, 'bar', backtraceraise FooError(foo: 'foo'), 'bar', backtrace

Here is a sample code adding a code to an error:

class MyCustomError < StandardError    attr_reader :code    def initialize(code)        @code = code    end    def to_s        "[#{code}] #{super}"    endend

And to raise it:raise MyCustomError.new(code), message