c++ - 关于 C++ Boost 图创建和 vertex_index 属性。

Question

我是 boost 菜鸟。我想知道为什么以下代码编译失败。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。 (我正在关注此页面: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，而且我还在 Stackoverflow ( how provide a vertex_index property for my graph ) 中看到了这个非常相关的问题讨论如何使用 associative_property_map 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，我想了解它失败的原因。任何帮助是极大的赞赏!

编译错误如下:

错误:表达式不可赋值
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，您会收到错误消息，因为当您尝试分配给它时它不是左值。

此外，它实际上返回v。下面是 vertIndx[v] 运行的代码:

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

这里有一个版本，希望能清楚地说明它是如何工作的:

#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;
}