depth_first_search: Return visitor by value

test/Jamfile.v2

@@ -44,6 +44,7 @@ alias graph_test_regular :
     [ run csr_graph_test.cpp : : : : : <variant>release ]
     [ run dag_longest_paths.cpp ]
     [ run dfs.cpp ]
+    [ run dfv_test.cpp ]
     [ run undirected_dfs.cpp ]
     [ compile dfs_cc.cpp ]
     [ compile dijkstra_cc.cpp ]

test/dfv_test.cpp

@@ -0,0 +1,96 @@
+//=======================================================================
+// Copyright 2022 Ralf Kohrt
+
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//=======================================================================
+#include <boost/graph/adjacency_list.hpp>
+#include <boost/graph/depth_first_search.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+using namespace boost;
+
+// Set up the vertex names
+enum vertex_id_t { u, v, w, x, y, z, N };
+
+
+struct counting_dfs_visitor
+{
+  template<typename Vertex, typename Graph>
+  void initialize_vertex(Vertex v, const Graph& g)
+  {
+    ++vertex_events;
+  }
+
+  template<typename Vertex, typename Graph>
+  void start_vertex(Vertex v, const Graph& g)
+  {
+    ++vertex_events;
+  }
+
+  template<typename Vertex, typename Graph>
+  void discover_vertex(Vertex v, const Graph& g)
+  {
+    ++vertex_events;
+  }
+
+  template<typename Edge, typename Graph>
+  void examine_edge(Edge e, const Graph& g)
+  {
+    ++seen_edges;
+  }
+
+  template<typename Edge, typename Graph>
+  void tree_edge(Edge e, const Graph& g)
+  {}
+
+  template<typename Edge, typename Graph>
+  void back_edge(Edge e, const Graph& g)
+  {}
+
+  template<typename Edge, typename Graph>
+  void forward_or_cross_edge(Edge e, const Graph& g)
+  {}
+
+  template<typename Vertex, typename Graph>
+  void finish_vertex(Vertex v, const Graph& g)
+  {
+    ++vertex_events;
+  }
+
+  size_t vertex_events = 0;
+  size_t seen_edges = 0;
+
+};
+
+void
+test_dfv_returns_copied_visitor()
+{
+  typedef adjacency_list<listS,
+                         vecS,
+                         undirectedS,
+                         // Vertex properties
+                         property<vertex_color_t, default_color_type> >
+    Graph;
+  typedef typename boost::property_map< Graph, boost::vertex_color_t >::type
+    ColorMap;
+
+  // Specify the edges in the graph
+  typedef std::pair<int, int> E;
+  E edge_array[] = { E(u, v), E(u, w), E(u, x), E(x, v), E(y, x),
+                     E(v, y), E(w, y), E(w, z), E(z, z) };
+  Graph g(edge_array, edge_array + sizeof(edge_array) / sizeof(E), N);
+
+  ColorMap color = get(boost::vertex_color, g);
+  counting_dfs_visitor visitor_copy = depth_first_visit(g, vertex(u, g), counting_dfs_visitor(), color);
+  BOOST_TEST(visitor_copy.vertex_events == 6u*2u + 1u); // discover_vertex + finish_vertex for each vertex and once start_vertex
+  BOOST_TEST(visitor_copy.seen_edges == 2u*9u);
+}
+
+int
+main(int argc, char* argv[])
+{
+  test_dfv_returns_copied_visitor();
+  return boost::report_errors();
+}