1.8.5.1 An example of multiple inheritance

The inheritance rules may lead to unexpected results in the case of multiple inheritance. For example, consider the following IDL:

// IDL definition of IFoo

import "unknwn.idl";

[ uuid(7D9EB760-E4E5-11D5-BF02-000347024BE1) ]

interface IFoo : IUnknown

{

  HRESULT meth1();

  HRESULT meth2();

  HRESULT meth3();

}

and these three (partial) implementations of the interface i-foo .

1. An implementation with no definition of meth2:

(define-com-implementation foo-impl-1 ()

  ()

  (:interfaces i-foo))

(define-com-method meth1 ((this foo-impl-1))

  s_ok)

(define-com-method meth3 ((this foo-impl-1))

  s_ok)

2. An implementation with no definition except meth2 :

(define-com-implementation foo-impl-2 ()

  ()

  (:interfaces i-foo))

(define-com-method meth2 ((this foo-impl-2))

  s_ok)

3. A combined implementation, inheriting from steps 1 and 2 .

(define-com-implementation foo-impl-12 (foo-impl-1

                                        foo-impl-2)

  ()

  (:interfaces i-foo))

In step 3 , the class foo-impl-12 implements the interface i-foo , but inherits all the i-foo method definitions from foo-impl-1 , which is the first class in the class precedence list that implements that interface. These method definitions include the "unimplemented" definition of meth2 in foo-impl-1 , which hides the definition in the other superclass foo-impl-2 . As a result, when the following form is evaluated with p-foo created from an instance of foo-impl-12 :

(let ((object (make-instance 'foo-impl-12)))

  (with-temp-interface (p-foo)

      (nth-value 1 (query-object-interface

                    foo-impl-12

                    object

                    'i-foo))

    (with-com-interface (call-p-foo i-foo) p-foo

      (values (call-p-foo meth1)

              (call-p-foo meth2)

              (call-p-foo meth3)))))

the three values are S_OK , E_NOTIMPL and S_OK .