ability to pick up the attribute type from the closure/functor

The actual trick to make it work is to use decltype on the function operator
http://stackoverflow.com/questions/7943525/is-it-possible-to-figure-out-the-parameter-type-and-return-type-of-a-lambda/7943765#7943765

In addition, we now pick up the functor by template type and
store it under that very type. For one, this cuts the size
of the generated class by a factor of two. And it gives the
compiler the ability to inline a closure as much as is possible,
especially when the created Binder / Mutator lives in the same
reference frame the closure taps into.

LICENSE

@@ -15,6 +15,13 @@ For a copy of the GPL Version 2 see the file COPYING
   in addition they are also dual-licensed under *CC-By-SA 3*
 
 
+- at various places, the implementation draws on solutions
+  and comments published at http://Stackoverflow.com[Stackoverflow] --
+  according to the http://stackexchange.com/legal[License Agreement] page,
+  any ``subscriber content'' on that site is provided under *CC-By-SA 3*..
+  We attribute significant usages of solutions published this way
+  in the doxygen comments of the relevant source code entities.
+
 
 - Parts of the Implementation are heavily inspired by +
   *The Loki Library*  (C) 2001 by *Andrei Alexandrescu*

src/lib/diff/tree-mutator.hpp

@@ -135,26 +135,46 @@ namespace diff{
   
   namespace {
     
-    template<class PAR, typename VAL>
+    /**
+     * Type rebinding helper to pick up the actual argument type.
+     * Works both for functors and for lambda expressions
+     * @remarks Solution proposed 10/2011 by \link http://stackoverflow.com/users/224671/kennytm user "kennytm" \endlink
+     *          in this \link http://stackoverflow.com/questions/7943525/is-it-possible-to-figure-out-the-parameter-type-and-return-type-of-a-lambda/7943765#7943765
+     *          answer on stackoverflow \endlink
+     */
+    template<typename FUN>
+    struct _ClosureType
+      : _ClosureType<decltype(&FUN::operator())>
+      { };
+
+    template<class C, class RET, class ARG>
+    struct _ClosureType<RET (C::*)(ARG)  const>
+      {
+        typedef ARG ArgType;
+        typedef RET ReturnType;
+      };
+    
+    
+    template<class PAR, class CLO>
     struct ChangeOperation
       : PAR
       {
-        using Closure = function<void(VAL)>;
-        
         ID attribID_;
-        Closure change_;
+        CLO change_;
         
         virtual void
         setAttribute (ID id, Attribute& newValue)
           {
+            using ValueType = typename _ClosureType<CLO>::ArgType;
+            
             if (id == attribID_)
-              change_(newValue.get<VAL>());
+              change_(newValue.get<ValueType>());
             
             else // delegate to other closures (Decorator-style)
               PAR::setAttribute(id, newValue);
           }
         
-        ChangeOperation(ID id, Closure clo, PAR const& chain)
+        ChangeOperation(ID id, CLO clo, PAR const& chain)
           : PAR(chain)
           , attribID_(id)
           , change_(clo)
@@ -169,19 +189,17 @@ namespace diff{
           : PAR(par)
           { }
         
-        template<typename VAL>
+        template<class CLO>
-        using Change = ChangeOperation<PAR,VAL>;
+        using Change = ChangeOperation<PAR,CLO>;
-        template<typename VAL>
-        using Closure = typename Change<VAL>::Closure;
         
         
         /* ==== binding API ==== */
         
-        template<typename VAL>
+        template<typename CLO>
-        Builder<Change<VAL>>
+        Builder<Change<CLO>>
-        change (Literal attributeID, Closure<VAL> closure)
+        change (Literal attributeID, CLO closure)
           {
-            return Change<VAL> (attributeID, closure, *this);
+            return Change<CLO> (attributeID, closure, *this);
           }
       };
 

tests/library/diff/generic-tree-mutator-test.cpp

@@ -92,7 +92,7 @@ namespace test{
           string localData;
           auto mutator =
           TreeMutator::build()
-            .change<string>("data", [&](string val)
+            .change("data", [&](string val)
               {
                 cout << "\"data\" closure received something "<<val<<endl;
                 localData = val;