Vector Iterators Incompatible (Segmentation Fault) - c++

Question

I was trying to implement some Graph Algorithms so continuing by testing I received an error in GNU C++ compiler (Segmentation Fault). In Visual Studio I saw the cause is "vector iterators incompatible". But how this happens? The error is thrown in shortestPathBFS function in the line "getName()" when I try to access a field of visitor object. Visitor is an element of Vertice* queue, so it must not depend on queue iterator in my opinion. If you can explain me why, I will be appreciated.
#include <queue>
#include <stack>
#include <vector>
#include <set>
#include <string>
#include <iostream>
using namespace std;
//#define traverse(container,iterator) \
//for(typeof(container.begin()) iterator = container.begin(); iterator != container.end(); iterator++)
class Edge;
class Vertice
{
private:
string name;
vector<Edge*> incidences;
bool visited;
public:
Vertice() {name = "NULL"; visited = false;}
Vertice(string name) {this->name = name; visited = false;}
void setName(string name) {this->name = name;}
string getName() {return name;}
bool isVisited() {return visited;}
void setVisited() {visited = true;}
void setUnvisited() {visited = false;}
void connectTo(Vertice*);
void connectTo(Vertice*,int);
void printNeighbors();
vector<Vertice*> getNeighbors();
};
class Edge
{
private:
int cost;
Vertice *start,*end;
public:
friend class Vertice;
Edge() {cost = 0;}
Edge(Vertice * start, Vertice * end) {this->start = start; this->end = end;}
Edge(Vertice * start, Vertice * end, int cost)
{this->start = start; this->end = end; this->cost = cost;}
Vertice* getEnd() {return end;}
Vertice* getStart() {return start;}
};
void Vertice::connectTo(Vertice * w)
{
incidences.push_back(new Edge(this,w));
}
void Vertice::connectTo(Vertice* w,int cost)
{
incidences.push_back(new Edge(this,w,cost));
}
vector<Vertice*> Vertice::getNeighbors()
{
vector<Vertice*> temp;
for(vector<Edge*>::iterator it = incidences.begin(); it != incidences.end(); it++)
{
temp.push_back((*it)->getEnd());
}
return temp;
}
void Vertice::printNeighbors()
{
for (vector<Edge*>::iterator i=incidences.begin(); i!= incidences.end(); i++)
{
cout<<(*i)->start->getName()<<"--"<<(*i)->cost<<"--"<<(*i)->end->getName()<<endl;
}
}
class Graph
{
public:
// using set for non-comparable elements are not good
// but this is for exercising
set<Vertice *> vertices;
public:
void initGraph()
{
Vertice *v;
v = new Vertice("IST");vertices.insert(v);
v = new Vertice("ANK");vertices.insert(v);
v = new Vertice("IZM");vertices.insert(v);
v = new Vertice("BER");vertices.insert(v);
v = new Vertice("TOR");vertices.insert(v);
v = new Vertice("BEJ");vertices.insert(v);
v = new Vertice("PER");vertices.insert(v);
(*findByName("IST"))->connectTo(*findByName("ANK"),10);
(*findByName("IST"))->connectTo(*findByName("IZM"),5);
(*findByName("IST"))->connectTo(*findByName("BER"),61);
(*findByName("IZM"))->connectTo(*findByName("ANK"),3);
(*findByName("IZM"))->connectTo(*findByName("TOR"),98);
(*findByName("IZM"))->connectTo(*findByName("BER"),70);
(*findByName("BER"))->connectTo(*findByName("ANK"),59);
(*findByName("BER"))->connectTo(*findByName("TOR"),91);
(*findByName("ANK"))->connectTo(*findByName("PER"),77);
(*findByName("ANK"))->connectTo(*findByName("BEJ"),151);
(*findByName("BEJ"))->connectTo(*findByName("TOR"),48);
(*findByName("TOR"))->connectTo(*findByName("ANK"),100);
(*findByName("PER"))->connectTo(*findByName("BEJ"),162);
(*findByName("TOR"))->connectTo(*findByName("PER"),190);
(*findByName("BEJ"))->connectTo(*findByName("PER"),163);
}
set<Vertice*>::iterator findByName(string name)
{
for(set<Vertice*>::iterator it = vertices.begin(); it != vertices.end(); it++)
{
if ((*it)->getName() == name)
{
return it;
}
}
return vertices.end();
}
int shortestPathBFS(Vertice * start, Vertice * finish)
{
queue<Vertice *> q;
q.push(start);
Vertice *visitor;
while(!q.empty())
{
visitor = q.front();q.pop();
visitor->setVisited();
cout<<"BFS : "<<visitor->getName()<<endl;
if (visitor->getName() == finish->getName())
{
break;
}
for(vector<Vertice*>::iterator it = (visitor->getNeighbors()).begin(); it != (visitor->getNeighbors()).end(); it++ )
{
if (!(*it)->isVisited())
{
q.push((*it));
}
}
}
return 0;
}
void printAll()
{
for(set<Vertice*>::iterator it = vertices.begin(); it != vertices.end(); it++)
{
(*it)->printNeighbors();
}
}
};
int main(int argc, char **argv)
{
Graph g;
g.initGraph();
g.printAll();
g.shortestPathBFS(*(g.findByName("IST")),*(g.findByName("PER")));
return 0;
}

Answer 1

The culprit is this line in Graph::shortestPathBFS:
for(vector<Vertice*>::iterator it = (visitor->getNeighbors()).begin(); it != (visitor->getNeighbors()).end(); it++ )
The problem is that you cannot compare iterators from two different containers (even if the containers are the same type), but visitor->getNeighbors() returns a new object each time it is invoked. Consequently, it is initialized from one object then compared to an iterator from a different object.
Rewrite the loop as:
vector<Vertice*> neighbors = visitor->getNeighbors();
for(vector<Vertice*>::iterator it = neighbors.begin(); it != neighbors.end(); ++it)
{
if (!(*it)->isVisited())
{
q.push((*it));
}
}