关于 C++ Boost Graph Creation 和 vertex_index 属性。

Question

我是新手。我想知道为什么以下代码中的编译失败。我正在创建一组顶点，并尝试分配我自己的顶点索引和顶点名称。 （我在关注这个页面：http://fireflyblue.blogspot.com/2008/01/boost-graph-library.html。）

我知道Boost 中的vertS 顶点列表不需要显式创建顶点ID，我还在Stackoverflow (how provide a vertex_index property for my graph) 中看到了这个非常相关的问题，该问题讨论了如何使用associative_property_map 进行分配顶点索引。以下虽然 - 获取 vertex_index 映射，并分配键值对 - 似乎是一件相当简单的事情，我想了解它为什么会失败。任何帮助是极大的赞赏！

编译错误如下：

error: expression is not assignable vertIndx[v] = i;

//Define graph
typedef boost::property<boost::vertex_name_t, std::string> sv_namePty;
typedef boost::property<boost::vertex_index_t, int, sv_namePty > sv_indx_n_name_pty;
typedef boost::property<boost::edge_weight_t, int> se_weightPty;
typedef boost::adjacency_list<boost::vecS, boost::vecS, boost::undirectedS, 
        sv_indx_n_name_pty, se_weightPty> ScafGraph;

//descriptors
typedef boost::graph_traits<ScafGraph>::vertex_descriptor SV;
typedef boost::graph_traits<ScafGraph>::edge_descriptor SE;

//Graph Object
ScafGraph SG;

//property accessors
boost::property_map<ScafGraph, 
     boost::vertex_name_t>::type vertName = boost::get(boost::vertex_name, SG);
boost::property_map<ScafGraph, 
     boost::vertex_index_t>::type vertIndx = boost::get(boost::vertex_index, SG);
boost::property_map<ScafGraph, 
     boost::edge_weight_t>::type edgeWeight = boost::get(boost::edge_weight, SG);

//Populate Graph
std::vector<SV> svlist;
for(int i=0; i<4; i++) {
    SV v = boost::add_vertex(SG);
    svlist.push_back(v);
    vertName[v] = std::to_string(i);
    vertIndx[v] = i;
}

Answer 1

表达式vertIndx[v] 按值返回一个顶点。因此，您会收到错误，因为当您尝试分配它时它不是左值。

此外，它实际上返回v。这是vertIndx[v]运行的代码：

inline value_type operator[](key_type v) const { return v; }

这是一个希望清楚其工作原理的版本：

#include <boost\graph\adjacency_list.hpp>

int main()
{
    //Define graph
    typedef boost::adjacency_list
        <
            boost::vecS                                        //! edge list 
          , boost::vecS                                        //! vertex list
          , boost::undirectedS                                 //! undirected graph  
          , boost::property<boost::vertex_name_t, std::string> //! vertex properties : name                
          , boost::property<boost::edge_weight_t, int>         //! edge properties : weight 
        >   ScafGraph;

    //descriptors
    typedef boost::graph_traits<ScafGraph>::vertex_descriptor SV;
    typedef boost::graph_traits<ScafGraph>::edge_descriptor SE;

    //Graph Object
    ScafGraph SG;

    //property accessors
    boost::property_map<ScafGraph,
        boost::vertex_name_t>::type vertName = boost::get(boost::vertex_name, SG);
    boost::property_map<ScafGraph,
        boost::vertex_index_t>::type vertIndx = boost::get(boost::vertex_index, SG);
    boost::property_map<ScafGraph,
        boost::edge_weight_t>::type edgeWeight = boost::get(boost::edge_weight, SG);

    //Populate Graph
    std::vector<SV> svlist;
    for (int i = 0; i < 4; i++) {
        SV v = boost::add_vertex(ScafGraph::vertex_property_type(std::to_string(i)), SG);
        svlist.push_back(v);
        assert(vertName[v] == std::to_string(i));
        assert(vertIndx[v] == i);
    }
    return 0;
}