Passing structs by value via ffi in ruby/jruby

I was stumped for a few hours on passing FFI::Structs into external routines, luckily, this post, 'Functions returning structures' (groups/google/ruby-ffi), tipped me off by showing how to return a struct by value:

From the C code:

...
    typedef struct
    { 
        int r, g, b, a; 
    } ALLEGRO_COLOR; 
...

And the correct ruby FFI code:

...
    class ALLEGRO_COLOR < FFI::Struct 
        layout :r, :float, 
               :g, :float, 
               :b, :float, 
               :a, :float 
    end 
    attach_function :al_map_rgb, [:uchar, :uchar, :uchar], ALLEGRO_COLOR.by_value
...

The existing FFI documentation is anemic in this area, and focuses on the more complex, though yet, more often used passing of pointer references to FFI::Structs, making it not obvious what to do with routines to make use of passing in structs 'by value'. Though, had I examined the methods offered by my derived GSL_Complex < FFI::Struct, I might have noticed the 'by_value'. Offering documentation help to Wayne Meissner

Now I have the means to wrapper up the GSL (Gnu-Scientific Library) routines for my gsl4r work. The GSL library routines covering complex numbers pass around structs by value in a big way.

My ruby ffi code:

...
  attach_function :gsl_complex_add, [ GSL_Complex.by_value, GSL_Complex.by_value ], GSL_Complex.by_value
...

Demo:

colby@gks lib $ jirb -r gsl4r
irb(main):001:0> include GSL4r
=> Object
irb(main):002:0> a=GSL_Complex.new
=> (0.0,0.0)
irb(main):003:0> a.set(1,1)
=> (1.0,1.0)
irb(main):004:0> b=GSL_Complex.new
=> (0.0,0.0)
irb(main):005:0> b.set(2,2)
=> (2.0,2.0)
irb(main):006:0> c=GSL4r::gsl_complex_add(a,b)
=> (3.0,3.0)

Now, my next question is, should it be possible to define an initializer for a class that inherits from FFI::Struct?