title: How to map passing objects by value in the IDL file for Djinni
body: <p>I'm trying to generate the wrapping classes for an existing c++ Library (with source code) to be used in ObjectiveC, I need to know how to write the idl interface for the methods that takes the same class as an argument 
  for example:</p>
  
  <p>In C++ header file for BigNumber class :</p>
  
  <pre><code>BigNumber add(BigNumber other);
  </code></pre>
  
  <p>I tried to write the same interface as an input to method:
  bignumber.djinni </p>
  
  <pre><code>bignumberinterface =
      interface +c {
              static create(number: string): bignumberinterface;
          add(other: bignumberinterface): bignumberinterface;
      }:
  </code></pre>
  
  <p>But this generates the headers with the BigNumber as an input pointer.
  So I got the error that :
  Parameter type 'BigNumber' is an abstract class</p>