[hawick] Support passing std::reference_wrapper to boost::hawick_circuits

Reported as ldionne/hawick_circuits#1.

doc/hawick_circuits.html

@@ -49,6 +49,10 @@ edges are not desired.</p>
 
   <p>For example, if a circuit <code>u -&gt; v -&gt; w -&gt; u</code> exists in the graph, the
   visitor will be called with a sequence consisting of <code>(u, v, w)</code>.</p>
+
+  Note that if <code>visitor</code> is a <code>std::reference_wrapper</code>,
+  it will be unwrapped before the <code>.cycle</code> method being called on it.
+  This allows passing a visitor that shouldn't be copied.
 </blockquote>
 
 <p><strong>IN:</strong> <code>VertexIndexMap const&amp; vim = get(vertex_index, graph)</code></p>

include/boost/graph/hawick_circuits.hpp

@@ -21,6 +21,7 @@
 #include <boost/tuple/tuple.hpp> // for boost::tie
 #include <boost/type_traits/remove_reference.hpp>
 #include <boost/utility/result_of.hpp>
+#include <functional> // for std::reference_wrapper
 #include <set>
 #include <utility> // for std::pair
 #include <vector>
@@ -89,6 +90,18 @@ namespace hawick_circuits_detail
         return std::find(boost::begin(c), boost::end(c), v) != boost::end(c);
     }
 
+    template < typename T >
+    struct unwrap_reference_wrapper {
+        typedef T type;
+    };
+
+#if __cplusplus >= 201103L
+    template < typename T >
+    struct unwrap_reference_wrapper<std::reference_wrapper<T> > {
+        typedef T& type;
+    };
+#endif
+
     /*!
      * @internal
      * Algorithm finding all the cycles starting from a given vertex.
@@ -301,8 +314,9 @@ namespace hawick_circuits_detail
 
         typedef std::vector< Vertex > Stack;
         typedef std::vector< std::vector< Vertex > > ClosedMatrix;
+        typedef typename unwrap_reference_wrapper<Visitor>::type VisitorNoRef;
 
-        typedef hawick_circuits_from< Graph, Visitor, VertexIndexMap, Stack,
+        typedef hawick_circuits_from< Graph, VisitorNoRef, VertexIndexMap, Stack,
             ClosedMatrix, GetAdjacentVertices >
             SubAlgorithm;
 

test/hawick_circuits.cpp

@@ -6,6 +6,8 @@
 
 #include "cycle_test.hpp"
 #include <boost/graph/hawick_circuits.hpp>
+#include <cstddef>
+#include <functional>
 #include <iostream>
 
 struct call_hawick_circuits
@@ -26,6 +28,20 @@ struct call_hawick_unique_circuits
     }
 };
 
+struct not_copyable
+{
+    not_copyable() { }
+
+    template < typename Path, typename Graph >
+    void cycle(Path const&, Graph const&)
+    {
+
+    }
+
+private:
+    not_copyable(not_copyable const&);
+};
+
 int main()
 {
     std::cout << "---------hawick_circuits---------\n";
@@ -33,4 +49,24 @@ int main()
 
     std::cout << "\n\n---------hawick_unique_circuits---------\n";
     cycle_test(call_hawick_unique_circuits());
+
+    // Make sure we can pass a reference_wrapper to the algorithm.
+#if __cplusplus >= 201103L
+    {
+        typedef boost::adjacency_list<boost::vecS, boost::vecS, boost::directedS> Graph;
+        typedef std::pair<std::size_t, std::size_t> Pair;
+        Pair edges[3] = {
+            Pair(0, 1), // a->b
+            Pair(1, 2), // b->c
+            Pair(2, 0), // c->a
+        };
+
+        Graph G(3);
+        for (int i = 0; i < 3; ++i)
+            add_edge(edges[i].first, edges[i].second, G);
+
+        not_copyable visitor;
+        boost::hawick_circuits(G, std::ref(visitor));
+    }
+#endif // >= C++11
 }