无法解决错误：会员功能＆＃39; getDV＆＃39;不可行：＆＃39;这个＆＃39;参数的类型为＆＃39; const Vertex＆＃39;，但函数未标记为const

Question

我正在尝试写一个重载的＆lt;运营商，我不断收到此错误

1. 错误：成员函数'getDV'不可行：'this'参数有   输入'const Vertex'，但函数未标记为const

我已经回去并尝试将我的变量dv和函数getDV（）更改为const 但这也没有帮助。我觉得我只是在两个不同的错误之间骑自行车。任何帮助都会很棒，你们呀！谢谢！

friend bool operator < (const Vertex & v1, const Vertex & v2);

bool operator < (const Vertex & v1, const Vertex & v2)
{
  return v1.getDV() > v2.getDV();
}


///HERE'S MY VERTEX CLASS//////
#include "Vertex.h"
#include <iostream>
#include <cstdlib>
#include <algorithm>
#include <vector>
#include <fstream>
using namespace std;
const bool myFunction(Vertex & x, Vertex & y);
void shortestPath(vector<Vertex> & v);


///overload the less than operator in order to use the stl sort for vector
///print out the path for each vertex

int main()
{

    /////READ ALL THE STUFF INTO THE GRAPH////
    ifstream file;
    file.open("graph.txt");
    cout << "Opening file..." << endl;
    if(!file)
    {
        cout << "System failed to open file.";
    }
    else
    {
        cout << "File successfully opened" << endl;
    }

    int numVertices;
    int numEdges;
    int num;
    int adjacentVertex;
    int weight;

    file >> numVertices;
    vector<Vertex> vertices(numVertices + 1);

    for(int i=1;i<=numVertices;i++)
    {   
        file >> numEdges;
        Vertex newVertex;
        //Using the i counter variable in the outer for loop to identify
        //the what vertex what are currently looking at in order to read in the correct adjacent vertex and weight
        newVertex.setVertexNum(i);

        for(int j=1;j<numEdges;j++)
        {
            file >> adjacentVertex;
            file >> weight;
            newVertex.setAdjacentVertex(adjacentVertex, weight);
        }
        vertices.push_back(newVertex);
    }

    for(int i=0;i<numVertices+1;i++)
    {
        cout<< "V" << i <<":  ";
        for(int j=0;j<vertices[i].getAdjacentVertices().size();j++)
        {
            cout << "V" << vertices[i].getAdjacentVertices()[j].getAdjVertex() << " " << vertices[i].getAdjacentVertices()[j].getWeight() << endl;
        }
    }





    //CALL THE SHORTEST PATH FUNCTION ON THE GRAPH/////



}

/*
const bool myFunction(const Vertex & x, const Vertex & y)
{
    return (x.getDV() >= y.getDV());
}
*/

bool operator < (const Vertex & v1, const Vertex & v2)
{
    return v1.getDV() > v2.getDV();
}

void shortestPath(vector<Vertex> & v)
{
    sort(v.begin(), v.end());
    for(int i=0;i<v.size();i++)
    {
        cout << v[i].getDV()<< endl;
    }
}