Question

我目前正在查看Boost Dijkstra的文档 - http://www.boost.org/doc/libs/1_52_0/libs/graph/doc/dijkstra_shortest_paths.html;我的目标是在计算距离时修改距离组合以获得“最大”而不是“加”。医生说这个：

IN: distance_combine(CombineFunction cmb)
This function is used to combine distances to compute the distance of a path. The
CombineFunction type must be a model of Binary Function. The first argument typ
of the binary function must match the value type of the DistanceMap property map
and the second argument type must match the value type of the WeightMap property
map. The result type must be the same type as the distance value type.
Default: closed_plus<D> with D=typename property_traits<DistanceMap>::value_type

定义这样的Combine函数的语法是什么？我试过用std :: max摸索，但我的编译器似乎并不满意。

Answer 1

我正在采取懒惰的方式，只是给出一些代码来说明如何做到这一点：）

#include <boost/graph/dijkstra_shortest_paths.hpp>
#include <boost/graph/adjacency_list.hpp>

struct Edge {
        Edge(float weight_) : weight(weight_) {}
        float weight;
};

// simple function
float combine(float a, float b){
        return std::max(a, b);
}

// functor
struct Combine{
        // Some internal state

        float operator()(float a, float b) const {
                return std::max(a, b);
        }
};

int main(int, char**){
        typedef boost::adjacency_list < boost::vecS, boost::vecS, boost::directedS, boost::no_property, Edge > graph_t;
        typedef boost::graph_traits < graph_t >::vertex_descriptor vertex_t;
        graph_t g;
        vertex_t a = boost::add_vertex(g);
        vertex_t b = boost::add_vertex(g);
        vertex_t c = boost::add_vertex(g);
        vertex_t d = boost::add_vertex(g);
        boost::add_edge(a, b, Edge(3), g);
        boost::add_edge(b, c, Edge(3), g);
        boost::add_edge(a, d, Edge(1), g);
        boost::add_edge(d, c, Edge(4), g);

        std::vector<vertex_t> preds(4);

        // Traditional dijsktra (sum)
        boost::dijkstra_shortest_paths(g, a, boost::predecessor_map(&preds[0]).weight_map(boost::get(&Edge::weight,g)));
        assert(preds[c] == d);
        assert(preds[d] == a);

        // Dijkstra with custom combine as a function
        boost::dijkstra_shortest_paths(g, a, boost::predecessor_map(&preds[0]).weight_map(boost::get(&Edge::weight,g)).distance_combine(&combine));
        assert(preds[c] == b);
        assert(preds[b] == a);

        // Dijkstra with custom combine as a functior
        boost::dijkstra_shortest_paths(g, a, boost::predecessor_map(&preds[0]).weight_map(boost::get(&Edge::weight,g)).distance_combine(Combine()));
        // Dijkstra with custom combine as a lambda
        boost::dijkstra_shortest_paths(g, a, boost::predecessor_map(&preds[0]).weight_map(boost::get(&Edge::weight,g)).distance_combine([](float a, float b){return std::max(a,b);}));

        return 0;
}

Answer 2

可能将其参数设为模板可能会让事情变得有点困难......

尝试（其中T是距离的类型）

T comb(T& a, T& b) { return std::max(a, b); }

并通过梳子。

如何在Boost Dijkstra中定义自定义距离？

2 个答案: