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ruby-talk:192757 reminded me of SuperStruct. I was going to reply with a pointer to that library, but I realized that the usual

class MyClass < Struct.new(:a,:b)
  def initialize(*a)
    super
    # stuff
  end
end

idiom and SuperStruct still left an important part of the solution space uncovered.

When you inherit from a Struct, the resulting class doesn't use plain instance variables, so you're forced to use the accessors. SuperStruct addresses that while emulating Struct quite closely, and also introduces OpenStruct-like features, so you get hash access with numeric, String and Symbol keys, #members, #to_a, pretty printing... But what if you don't need all that?

I'm guessing the OP would have wanted to do (at least that's what I wanted):

class Foo < InitializedClass.new(:a, :b, :c)
  def initialize(*a)
    super
    @c ||= 10       # this is how we manage default values
  end
  
  def bar
    a + b * c
  end
end

Foo.new(1,2,3).bar                                 # => 7

And while we're at it, why not get named arguments for free too?

Foo.new(:a => 1, :b => 2).bar                      # => 21

This (and a few additional features) takes 56 lines in my first implementation, including #instance_exec, which I already wrote about.

Inheritance

One thing neither Struct nor SuperStruct can do is create a class derived from an existing one. Well, of course module inclusion is but another form of inheritance (and multiple too), but some times you might want the real thing, with matz's blessing.

Say you have

class Baz
  def initialize(foo, bar)
    @foo = a
    @bar = b
  end
end

and want to create a subclass with a number of attributes, but you also need to be able to run Baz#initialize. If we were just subclassing manually,

 super(whatever)

would do, but then there's no automatic accessors nor named arguments. Is it possible at all to get both at once?