Crystal : class warp or extension methods like CSharp

I exported a struct from a C library which is Void*

lib LibXXX
    alias OldStruct = Pointer(Void)
    fun foo(old : OldStruct)
end

And want to add some methods to LibXXX::Old

I know it's can only be done by a class warp in most language :

struct NewStruct {
    OldStruct old;
    void new_functions();
};

But it's really bothering that I must access field old explicitly. LibXXX.foo(old.old)

I want some transparent way keeping simple representation LibXXX.foo(old), like Inheritance or Extension Methods in CSharp.

I tried :

struct NewStruct < LibXXX::Old
# ...
end

But an compilation error occured:

can't extend non-abstract struct Pointer(Void)

I believe it must could be done using Crystal. But I don't know what's the best practice, so, please.

Solution

The best practice is to have a non-lib wrapper around your object, that wraps all of the existing methods. That's then often a good place to add your new functionality too or any idiomatic abstractions.

lib LibPet
  alias Pet = Void*
  alias Dog = Void*

  fun make_dog : Dog
  fun bark(bar : Dog)
  fun move(pet : Pet)
  fun retreat(pet : Pet)
end

struct Dog
  def initialize
    @dog = LibPet.make_dog
  end

  def bark
    LibPet.bark(@dog)
  end

  def move
    LibPet.move(@dog)
  end

  def retreat
    LibPet.retreat(@dog)
  end

  def at_front
    move
    yield
  ensure
    retreat
  end

  def attack
    at_front { bark }
  end
end