c++ - 尝试解决段错误

Question

我一直在我的 C++ 代码中遇到段错误，我通过执行线程使用 GDB 对其进行了跟踪，但看不到问题出在哪里。我的程序根据主类 AsTheCrowFlies 中的 Vertex 对象列表创建一个 Graph 对象。 Graph 对象的实际创建发生在 makeGraph(const std::vector) 中。 makeGraph 中问题开始的那一行是我调用另一个函数 checkEdges(Map map, const Vertex &To, const &From) 的地方，它看起来像这样:

  bool Graph::checkEdges(Map &map, const Vertex &To, const Vertex &From) {

      if(map.find(To)!=map.end()) {

        std::vector<Edge> edges = map[To];

        for (auto itr = edges.begin(); itr!=edges.end(); ++itr) {

               if(itr->getDestination()==From)
                  return true;

          }

     }

   return false;

 }

根据 GDB 的输出，当 for 循环中的 if 语句执行并尝试使用 Vertex 类中重载的 == 函数时:

     bool Vertex::operator == (const Vertex &other) const {

        if (name==other.name && latitude==other.latitude && 
         longitude == other.longitude) {

            return true;

         }

    return false; 

   }

它给出了错误消息:“无法访问地址 0x4a8 处的内存>” 当我在 GDB 中使用 bt 命令时，错误消息的完整列表是:

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7b8f6f3 in std::string::size() const ()
from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
(gdb) bt
#0  0x00007ffff7b8f6f3 in std::string::size() const ()
from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#1  0x000000000040a729 in std::operator==<char> (__lhs=" ",
__rhs=<error reading variable: Cannot access memory at address 0x4a8>)
at /usr/include/c++/4.8/bits/basic_string.h:2495

#2  0x000000000041a30d in Vertex::operator== (this=0x671b58, other=...)
at Vertex.cpp:50

#3  0x0000000000415fb7 in Graph::checkEdges (this=0x7fffffffe880,
map=std::unordered_map with 50 elements = {...}, To=..., From=...)
at Graph.cpp:364

#4  0x00000000004157cf in Graph::makeGraph (this=0x7fffffffe880,
input=std::vector of length 50, capacity 50 = {...}) at Graph.cpp:161

#5  0x0000000000414bea in Graph::setVertices (this=0x7fffffffe880,
v=std::vector of length 50, capacity 64 = {...}) at Graph.cpp:38

#6  0x0000000000402b62 in AsTheCrowFlies::menu (this=0x7fffffffe800,
filename=0x7fffffffec48 "cap_cities.txt") at AsTheCrowFlies.cpp:17

#7  0x000000000041a05c in main (argc=2, argv=0x7fffffffe9c8) at main.cpp:24

这里是 makeGraph(const Vertex input) 的完整版本:

 Map Graph::makeGraph(std::vector<Vertex> input) {

   Map new_graph;

   size_t size, n, mid,index;

   size  = input.size();

   n = size - 1;

   mid = ((size + size%2)/2)-1;

   if (size<1)
return new_graph;


double mean,max,min,median; //double variables for mean, median, maximum, minimum, sum of squares and standard deviation created

    //vector used to hold the distances between adjacent vertices
std::vector<double> dist(size); 


    for (int i = 0; i < size; i++) {

        mean=0.0; //set average/mean to "0"


        if(new_graph.find(input[i])==new_graph.end()) { //if the current Vertex has no map entry, add it here

      std::vector<Edge> edges(size);            
      new_graph[input[i]] = edges;         

        }

       //Create and sort duplicate list with each Vertex moved to 
         the front in turn         

       std:vector<Vertex> list(size);

       list = input;

       sort(list.begin(),list.end(),[&](const Vertex &v1, const Vertex &v2){return findMinDist(input[i].getLatitude(),input[i].getLongitude(),v1.getLatitude(),v1.getLongitude()) < findMinDist(input[i].getLatitude(),input[i].getLongitude(),v2.getLatitude(),v2.getLongitude()) ;}); 


       //Gets mean and fills distance tracking vector


       for (int j = 0; j < size; j++) {

        dist[j]=findMinDist(input[i].getLatitude(),input[i].getLongitude(),input[j].getLatitude(),input[j].getLongitude());     
        mean+=dist[j]/size;//accumulates mean value

            }        


      min = dist[1];



      max = dist[n];


  if(size%2==0) { //calculates median distance value for...

    median = (dist[mid]+dist[mid+1])/2; //...even sized vectors and...

      } else {median = dist[mid];} //...odd sized vectors


     if(mean >= median) { //index value is calucated for...

    index = (mid+1)*(mean-min)/(max-min); //...case where mean is greater than or equal to median and...


      } else {index = (mid+1)*(median-min)/(max-min);}  //...case where mean is less than median and...


  for (int k = 0; k < index; k++) { //

         if(!checkEdges(new_graph,list[0],list[k])){

    new_graph[list[0]].push_back(Edge(list[0],list[k],dist[k]));

         }


         if(k!=0) {

             if(new_graph.find(list[k])==new_graph.end()) {//SEGMENTATION
                                                            //FAULT STARTS 
                                                            //HERE

        std::vector<Edge> edges(size);

                    new_graph[list[k]]=edges;

                    new_graph[list[k]].push_back(Edge(list[k],list[0],dist[k]));

            } else {

        if(!checkEdges(new_graph,list[k],list[0])) {

        new_graph[list[k]].push_back(Edge(list[k],list[0],dist[k]));    

                   }

               }

        }

       }


     dist.clear(); 

   }

return new_graph;       

getDestination的签名是

  Vertex& Edge::getDestination() {return destination;}

我看着这个，但不明白为什么这里会出现段错误。请帮忙!我已经包含了似乎发生错误的代码的相关部分，但如果您需要更多代码来帮助理解发生了什么，请告诉我。

Answer 1

最有可能的是 getDestination() 返回一个指向无效/无效对象甚至 null 的指针。

无论如何，我会使用调试器。我会稍微调整代码，以便调试和检查对象变得更容易。祝你好运!

       Vertex &edgeFrom = itr->getDestination();
       if(edgeFrom==From)
          return true;