c++ - 如何使用boost图形库广度优先搜索创建遍历顶点队列？

Question

我想使用 boost 图形库广度优先搜索来返回从节点 1 开始时访问的顶点队列。我阅读了文档，但仍然无法理解如何实现这一点。

下面的结果将按顺序返回一个队列:1,2,3,4 或 1,3,2,4

typedef boost::property<boost::edge_weight_t, unsigned int> EdgeWeightProperty;
typedef boost::adjacency_list<boost::setS, boost::vecS, boost::undirectedS, 
                              boost::no_property, EdgeWeightProperty> Graph;

//create graph and add edge
Graph g;
boost::add_edge(1,2,6,g);
boost::add_edge(2,3,6,g);
boost::add_edge(3,1,6,g);
boost::add_edge(3,4,6,g);

//Perform breadth first using boost and return result in a queue.

Answer 1

boost 图形库文档定义了一个 BFS Visitor concept哪个国家

Users can define a class with the BFS Visitor interface and pass an object of the class to breadth_first_search(), thereby augmenting the actions taken during the graph search.

基于此和 bfs.cpp example完成您所要求的首选方法是对 boost::visitor 进行子类化或 boost::default_bfs_visitor并覆盖一个或几个

initialize_vertex

discover_vertex

examine_vertex

examine_edge

tree_edge

non_tree_edge

gray_target

black_target

finish_vertex

对于您的用例，我会说最适合子类化 boost::default_bfs_visitor并覆盖 discover_vertex :

discover_vertex(s, g)

Invoked when a vertex is encountered for the first time. s is An object of type boost::graph_traits<Graph>::vertex_descriptor and g is an object of type Graph.

下面是一个具体如何做到这一点的例子，适当处理 std::queue记录顶点的对象。

#include <queue>
#include <iostream>
#include <boost/graph/visitors.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/breadth_first_search.hpp>

typedef boost::property<boost::edge_weight_t, unsigned int> EdgeWeightProperty;
typedef boost::adjacency_list<boost::setS, boost::vecS, boost::undirectedS, 
                              boost::no_property, EdgeWeightProperty> Graph;

class BFSVisitor : public boost::default_bfs_visitor {
public:    
    BFSVisitor(std::queue<Graph::vertex_descriptor>& _visited) : visited(_visited){}
    void discover_vertex(Graph::vertex_descriptor s, const Graph &g) { visited.push(s); }
    std::queue<Graph::vertex_descriptor>& visited;
};

int main() {
    Graph g;
    boost::add_edge(0,1,6,g);
    boost::add_edge(1,2,6,g);
    boost::add_edge(2,3,6,g);
    boost::add_edge(3,1,6,g);
    boost::add_edge(3,4,6,g);

    Graph::vertex_descriptor s = *(boost::vertices(g).first);
    std::queue<Graph::vertex_descriptor> q;
    BFSVisitor vis(q);
    boost::breadth_first_search(g, s, boost::visitor(vis));
    while (!vis.visited.empty()) {
        std::cout << vis.visited.front() << std::endl;
        vis.visited.pop();
    }
}

这将输出

0
1
2
3
4

备注 — Boost 期望图从顶点 0 开始，不是 1 - 因此这条线

boost::add_edge(0,1,6,g);

如果你从顶点开始 1不会有输出。