Library/IterSource: allow for mix-in extension of the IterSource interface

...at least when using a wrapped Lumiera Iterator as source.
Generally speaking, this is a tricky problem, since real mix-in interfaces
would require the base interface (IterSource) to be declared virtual.

Which incurres a performance penalty on each and every user of IterSource,
even without any mix-in additions. The tricky part with this is to quantify
the relevance of such a performance penalty, since IterSource is meant
to be a generic library facility and is a fundamental building block
on several component interfaces within the architecture.

src/lib/iter-source.hpp

@@ -229,13 +229,12 @@ namespace lib {
    * is passed to one of the IterSource's builder functions, thereby
    * erasing the specific type information of the template parameter IT
    */
-  template<class IT>
+  template<class IT,  class ISO = IterSource<typename IT::value_type>>
   class WrappedLumieraIter
-    : public IterSource<typename IT::value_type>
+    : public ISO
     , boost::noncopyable
     {
-      using _Base = IterSource<typename IT::value_type>;
-      using   Pos = typename _Base::Pos;
+      using Pos = typename ISO::Pos;
       
       IT src_;
       

tests/library/iter-tree-explorer-test.cpp

@@ -856,15 +856,15 @@ namespace test{
       verify_IterSource()
         {
           class PrivateSource
-//          : public IterSource<uint>       /////////////////////////////////////TODO linearised Mix-in problem
+            : public IterSource<uint>
             {
             public:
               virtual PrivateSource* expandChildren()  const  =0;
             };
           
           class VerySpecivicIter
-            : public WrappedLumieraIter<NumberSequence>
-            , public PrivateSource
+            : public WrappedLumieraIter<NumberSequence
+            ,        PrivateSource     >
             {
             public:
               VerySpecivicIter(uint start)