Seg Fault when creating dynamic array of strings - c++

Question

My friend is writing a text-based game and asked me to look at this code that was crashing. I debugged it and it was getting a seg fault when creating a dynamic array. I'm not sure exactly why, I recommended he just avoid pointers altogether and use a vector so hopefully that will solve his problem but I'm curious as to what exactly is going wrong here. Here's his code:
#include <iostream>
#include <fstream>
#include <string>
#include <cstdlib>
#include <ctime>
using namespace std;
class nation
{
public:
void init();
string genName();
string getName();
private:
string myName;
int* myBorderPoints;
};
string nation::getName()
{
return myName;
}
string nation::genName()
{
int listLength = 0, listPos = 0, listRand = 0;
string nameToGen = "";
string* namePartList;
ifstream fileName;
fileName.open("NamePart1.txt");
listLength = fileName.tellg();
namePartList = new string[listLength]; // Seg fault here
while (fileName.good())
{
while (!fileName.eof())
{
getline(fileName,namePartList[listPos]);
listPos += 1;
}
}
listRand = rand() % listLength;
nameToGen += namePartList[listRand];
fileName.close();
listLength = 0;
listPos = 0;
listRand = 0;
nameToGen = "";
fileName.open("NamePart2.txt");
listLength = fileName.tellg();
namePartList = new string[listLength];
while (fileName.good())
{
while (!fileName.eof())
{
getline(fileName,namePartList[listPos]);
listPos += 1;
}
}
listRand = rand() % listLength;
nameToGen += namePartList[listRand];
fileName.close();
return nameToGen;
}
void nation::init()
{
srand(time(NULL));
myName = genName();
}
int main()
{
nation testNation;
testNation.init();
cout << testNation.getName();
return 0;
}

Answer 1

You are calling tellg:
listLength = fileName.tellg();
without having read anything, which depending on whether the file was opening successfully or not will return 0 or -1 and so you will have this called:
namePartList = new string[listLength]
with a probably a undesirable value. I am pretty sure it is returning -1 since allocating a zero sized should be ok.
This also applies later on the code as well, going with std::vector probably makes more sense.