boost :: graph中的DFS,用于更改图形内容

Question

最小的例子:

#include <boost/graph/graph_traits.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/depth_first_search.hpp>

struct vertex
{
    int number;
};
struct edge {};

typedef boost::adjacency_list<boost::listS, boost::vecS, boost::directedS, vertex, edge> graph_t;
typedef boost::graph_traits<graph_t>::vertex_descriptor vertex_t;
typedef boost::graph_traits<graph_t>::edge_descriptor edge_t;

struct vertex_visitor : public boost::default_dfs_visitor
{
    void discover_vertex(vertex_t v, graph_t& g)
    {
        g[v].number = 42;
    }
};

int main()
{
    graph_t g;
    vertex_t v1 = boost::add_vertex(g);
    vertex_t v2 = boost::add_vertex(g);
    boost::add_edge(v1, v2, g);

    vertex_visitor vis;
    boost::depth_first_search(g, boost::visitor(vis));

    return 0;
}

它不起作用,因为图形需要像const在中一样被引用vertex_visitor::discover_vertex().

有没有比编写我自己的DFS算法(或使用const_cast)更好的方法来做我想做的事情？此外,您的解决方案是否允许在发现顶点时添加/删除边和顶点？

Answer 1

看看Boost如何实现connected_components.为了存储组件ID,使用以下访问者:

// This visitor is used both in the connected_components algorithm
// and in the kosaraju strong components algorithm during the
// second DFS traversal.
template <class ComponentsMap>
class components_recorder : public dfs_visitor<>
{
  typedef typename property_traits<ComponentsMap>::value_type comp_type;
public:
  components_recorder(ComponentsMap c, 
                      comp_type& c_count)
    : m_component(c), m_count(c_count) {}

  template <class Vertex, class Graph>
  void start_vertex(Vertex, Graph&) {
    if (m_count == (std::numeric_limits<comp_type>::max)())
      m_count = 0; // start counting components at zero
    else
      ++m_count;
  }
  template <class Vertex, class Graph>
  void discover_vertex(Vertex u, Graph&) {
    put(m_component, u, m_count);
  }
protected:
  ComponentsMap m_component;
  comp_type& m_count;
};

我们的想法是将属性映射传递给访问者的构造函数,稍后用于更新数据.关于您的示例,vertex_visitor可以通过以下方式重写:

template <class PropertyMap>
struct vertex_visitor : public boost::dfs_visitor<>
{
  PropertyMap m_pmap;
  vertex_visitor(PropertyMap pmap) : m_pmap(pmap) {}
  template <class Vertex, class Graph>
  void discover_vertex(Vertex v, const Graph& g)
  {
    boost::put(m_pmap, v, 42);
  }
};

这个访问者的实例化有点令人费解,因为我们需要明确指定属性映射类型:

typedef boost::property_map<graph_t, int vertex::*>::type NumbersProperty;
vertex_visitor<NumbersProperty> vis(boost::get(&vertex::number, g));

根据问题的最后部分,图结构的变异(即顶点和边的添加或删除)使整数器无效,因此这将破坏DFS算法.我认为这正是图表通过const-reference传递的原因.