尝试解决细分错误

Question

I keep on getting a segmentation fault in my C++ code which I have traced with GDB through the thread of execution and do not see where the issue is coming from. My program creates a Graph object from a list of Vertex objects in the primary class AsTheCrowFlies. The actual creation of the Graph object happens in the makeGraph(const std::vector). The line in makeGraph where the problem starts is where I call another function checkEdges(Map map, const Vertex &To, const &From) which looks like this:

  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;

 }

Based on how the output from GDB it looks like when the if statement in the for loop executes and tries to use the overloaded == function in the Vertex class:

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

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

            return true;

         }

    return false; 

   }

it gives the error message: " Cannot access memory at address 0x4a8>" The full list of the error messages when I use the bt command in GDB is:

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

Here is the full version of makeGraph(const Vertex input):

 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;       

}

And the signature of getDestination is

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

I looking at this and not seeing why there would be a segmentation fault here. Please help! I have included the pertinent sections of code where the error seems to be happening at but if you need to some more code to help understand what is going on, please let me know.

Answer 1

The most likely thing is that getDestination() returns a pointer to an invalid/invalidated object or even null .

Anyway, I'd use the debugger. And I would slightly adapt the code such that debugging and inspecting the objects becomes easier. Good luck!

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