Question

I have an error with an assignment, of which I get a seg fault, that when reading the core dump gives this:

#0  0xb781eb27 in std::basic_string<char, std::char_traits<char>, std::allocator<char> >::size() const () from /usr/lib/libstdc++.so.6

This is the code snippet I believe it is referring to.

bool flightmap::GetNextCity(int& index, int& nextCity){
    bool success = false;
    flightRec tmp, tmp2;
    for (int i = nextCity; (!success) && (i < size); i++)
    {//if Citytmp goes over size, we never found it
        tmp.DestinationCity = GetCityName(i);
        tmp2 = map[index].Retrieve(tmp,success);
    }//end for loop
    if (success)
    {
        nextCity = GetCityNumber(tmp2.DestinationCity);
    }//end if
    return success;
}

Here is the Retrieve function:

flightRec sortedListClass::Retrieve(flightRec& input, bool& success) const{
    nodeptr curr;
    flightRec tmp;
    curr = head;
    if (head == NULL)
    {//If the list is empty
        std::cout << "List empty, operation not preformed" << std::endl;
        success = false;
    }
    /*LINE 167*/ while ((curr!=NULL)&&(!(curr->DestinationCity==input.DestinationCity))) //<- THIS IS LINE 167
    {//Here we first check if curr points to NULL, then we see if DesinationCity and Origin are not yet found.
        curr=curr->ptr;
    }
    if ((curr->DestinationCity==input.DestinationCity))
    {//If we found it, then let's return it...
        tmp.DestinationCity=curr->DestinationCity;
        //tmp.Origin=curr->Origin;
        tmp.flightnumber=curr->flightnumber;
        tmp.cost=curr->cost;
        success = true;
        return tmp;
    }
    else //We didn't and then...damn.
    {
        //std::cout << "Can't find flight to " << input.DestinationCity << std::endl;
        success = false;
    }
}

flightRec:

struct flightRec{
    std::string Origin;
    int flightnumber;
    float cost;
    std::string DestinationCity;
    bool operator <(const flightRec& rhs) const;
    bool operator ==(const flightRec& rhs) const;
    flightRec* ptr;
};
typedef flightRec* nodeptr;

flightMap.h

#ifndef FLIGHTMAP_H
#define FLIGHTMAP_H
#include "sortedListClass.h"
#include "stackClass.h"
#include <fstream>
#include <cstdio>

class flightmap {
public:
    flightmap();
    flightmap(const flightmap& orig);
    virtual ~flightmap();
    void readcities (std::ifstream& in);
    void readflights (std::ifstream& in);
    void display ();
    void isPath (int& in, int& out);
    void MarkVisited (int& index);
    bool IsVisited (int& index);
    void UnvisitAll ();
    bool GetNextCity (int& index, int& nextCity);
    int GetCityNumber(std::string& city);
    std::string GetCityName(int& index);
private:
    int size;
    sortedListClass* map;
    std::string* origin;
    bool* visited;
};

#endif

After typing bt into gdb this is what the output was:

#0  0xb7f4eb27 in std::basic_string<char, std::char_traits<char>, std::allocator<char> >::size() const () from /usr/lib/libstdc++.so.6
#1  0x0804a407 in std::operator==<char> (__lhs=..., __rhs=...) at /usr/lib/gcc/i686-pc-linux-gnu/4.5.2/../../../../include/c++/4.5.2/bits/basic_string.h:2345
#2  0x0804ab32 in sortedListClass::Retrieve (this=0x8053468, input=..., success=@0xbffff0a3) at sortedListClass.cpp:167
#3  0x0804a1ff in flightmap::GetNextCity (this=0xbffff4dc, index=@0xbffff108, nextCity=@0xbffff104) at flightMap.cpp:197
#4  0x08049ba7 in flightmap::isPath (this=0xbffff4dc, in=@0xbffff164, out=@0xbffff160) at flightMap.cpp:110
#5  0x08049314 in FindPath (datafile=..., map=...) at ola6.cpp:51
#6  0x08049190 in main (argc=2, argv=0xbffff5b4) at ola6.cpp:35

Answer 1

bt is correct for a backtrace. You can see there the error starts in sortedListClass::Retrieve line 167. The next entry in the backtrace (working back up to #0) is for operator ==, which means the == on line 167 is the cause of the crash.

There are a few == calls in the code you posted, so if you can edit your post and indicate which line is line 167 we can diagnose further.

Update after line number was added:

This is just a guess, but it’s possible you have forgotten to set the ptr member to NULL at some point earlier in the program, so that in that loop around line 167-168 curr eventually points at a random memory location, causing the crash. It’s also possible that the object at that memory location has since been freed, but ptr hasn’t been updated and is pointing at the now deleted object. Check your code closely where you allocate new flightRec objects and make sure you are always setting ptr correctly.