Memory & arrays - Unhandled exception at 0x77b715de (ntdll.dll)

#include "datahandler.h"
#include "gradebook.h"
#include<iostream>
#include<fstream>
#include<iomanip>
#include<cstdlib>
#include<cctype>
#include<string>
#include<vector>
#include<sstream>

using namespace std;

//FUNCTION PROTOTYPES
void displayMenu();


int main(){
	//create a local data-handler object to get a few preliminary
	//necessaries out of the way
	DataHandler dh;
	std::string classID;
	//create a 2D vector to hold all the class names so we can
	//deal with them individually:
	std::vector<std::vector<std::string>> vecClassNames;

	//fill the vector with all the information from Classes.txt
	dh.fillVector("Classes.txt", vecClassNames);

	//I've made this a const int since we won't be providing the user
	//the option to add any more classes than those that already exist in Classes.txt
	const int classCount = vecClassNames.size();

	//now we can create a vector of GradeBooks for each class:
	std::vector<GradeBook> gradeBooks (classCount);
	for(unsigned int i=0; i<gradeBooks.size(); i++){
		//since we KNOW that [i][0] holds the class ID, and [i][1] holds the Class Name
		//we can pass those valuse in order to the GradeBook constructor
		gradeBooks[i] = GradeBook(vecClassNames[i][0], vecClassNames[i][1]);
	}

GradeBook::GradeBook(std::string ID, std::string ClassName){
	//fill a vector with Student Informaiton for this particular gradebook
	std::string filename = "StudentInfo.txt";
	className = ClassName;
	classID = ID;

	//fill the vecStudents vector
	dh.fillVector(ID, filename, vecStudents);

	//fill a vector with assignment information for this particular gradebook
	filename = "Assignments.txt";
	dh.fillVector(ID, filename, vecClassAssignments);
	
	//fill a vector with student grade information for this particular gradebook
	filename = "StudentGrades.txt";
	dh.fillVector(filename, vecStudents, vecStudentGrades);
}

//move back to beginning of file to read
input.seekg(0, std::ios::beg);

//read one line into string row
for(std::string row; getline(input, row, row_delim); ){
     //add an item to the first dimension of vector rows
     rows.push_back(Rows::value_type());

     //feed the row into an input stringstream
     std::istringstream ss(row);

     //loop through the string and get each field
     for(std::string field; getline(ss, field, field_delim);){
          //add the field to the second dimension of the vector
          rows.back().push_back(field);
     }
     //assign the ID we need from that row to a test string
     std::string id = rows.back().at(IDindex);
     
     //if it isn't not the ID we're looking for, pop the entire
     //row off of the vector
     if (id != ID)
          rows.pop_back();
}

std::string id = rows.back().at(IDindex)