BGL的depth_first_search算法有时会对访问者调用back_edge(),即使图中没有循环也是如此。根据后边缘的定义,根据Boost的DFS Visitor Documentation,这不应该发生。请注意,仅当listS用作顶点和边的表示时,这才是可重现的。
下面的代码示例(应该按原样编译)构造一个包含两个节点和一个边的图。它错误地打印“后边缘”。我在这里做错了吗?或者这是一个错误吗?
#include <iostream>
using namespace std;
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/depth_first_search.hpp>
#include <boost/graph/visitors.hpp>
using namespace boost;
typedef boost::property<boost::vertex_index_t,std::size_t> VertexProperties;
typedef boost::adjacency_list<boost::listS,
boost::listS,
boost::bidirectionalS,
VertexProperties> Graph; // Graph object type
typedef boost::graph_traits<Graph>::vertex_descriptor Vertex;
typedef boost::graph_traits<Graph>::edge_descriptor Edge;
class VisitorClass : public dfs_visitor<> {
public:
VisitorClass() {}
template <typename Edge, typename Graph>
void back_edge(Edge, const Graph&) const {
cout << "back edge" << endl;
}
};
int
main() {
Graph g;
Vertex v = add_vertex(g);
Vertex u = add_vertex(g);
bool inserted;
tie(tuples::ignore, inserted) = add_edge(v, u, g);
assert(inserted);
VisitorClass vst;
depth_first_search(g, visitor(vst));
// Should not print "back edge", but does.
return 0;
}
在Mac OS 10.7上使用i686-apple-darwin11-llvm-g ++ - 4.2测试Boost 1.46.1;
使用Debian 2.6.32-34squeeze1上的gcc 4.4.5测试Boost 1.42.0。
答案 0 :(得分:4)
您提交了一个错误,但您没有跟进。
不久之后,你得到了答案:
您没有初始化图表的vertex_index属性。尝试添加一些代码,例如:
graph_traits :: vertices_size_type i = 0;
BGL_FORALL_VERTICES(v,graph,Graph)put(vertex_index,g,v,i ++);
我试过这个(修正错字)并且工作正常:
#include <boost/graph/iteration_macros.hpp>
int main() {
Graph g;
Vertex v = add_vertex(g);
Vertex u = add_vertex(g);
graph_traits<Graph>::vertices_size_type i = 0;
BGL_FORALL_VERTICES(v, g, Graph) put(vertex_index, g, v, i++);
bool inserted;
tie(tuples::ignore, inserted) = add_edge(v, u, g);
assert(inserted);
VisitorClass vst;
depth_first_search(g, visitor(vst));
// Should not print "back edge", but does.
return 0;
}
(至少,它不再打印“后缘”)