From 4c59e16f06bece310f7d628b9dab6ef530eab3ac Mon Sep 17 00:00:00 2001
From: Ichthyostega <prg@ichthyostega.de>
Date: Sat, 29 Aug 2015 18:56:19 +0200
Subject: [PATCH] add the visitor-predicate as a second option

contrary to the Visitor, accepting a Predicate is const,
and -- of course -- the Predicate invocation returns bool.

This can be used to implement comparison operators or
search functions on Variant based data structures.
---
 src/lib/variant.hpp | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/src/lib/variant.hpp b/src/lib/variant.hpp
index 5884134fd..d675e233f 100644
--- a/src/lib/variant.hpp
+++ b/src/lib/variant.hpp
@@ -224,8 +224,9 @@ namespace lib {
           
           virtual ~Buffer() {}           ///< this is an ABC with VTable
           
-          virtual void dispatch (Visitor&)  =0;
-          virtual operator string()  const  =0;
+          virtual void dispatch (Visitor&)         =0;
+          virtual bool dispatch (Predicate&) const =0;
+          virtual operator string()          const =0;
         };
       
       
@@ -317,6 +318,15 @@ namespace lib {
               typeDispatcher.handle (this->access());
             }
           
+          bool
+          dispatch (Predicate& visitor)  const
+            {
+              using Dispatcher = VFunc<bool>::template ValueAcceptInterface<const TY>;
+              
+              Dispatcher& typeDispatcher = visitor;
+              return typeDispatcher.handle (this->access());
+            }
+          
           /** diagnostic helper */
           operator string()  const;
         };
@@ -452,6 +462,12 @@ namespace lib {
         {
           buffer().dispatch (visitor);
         }
+      
+      bool
+      accept (Predicate& visitor)  const
+        {
+          return buffer().dispatch (visitor);
+        }
     };