Converting C to C++ and Dynamic Memory Allocation - c++

Question

So I have recently been tasked with converting a program I wrote in C to C++ for an assignment in an intro to programming course. The program was a basic database program where the user would create a grade book with student IDs, text names, course IDs, text names, and grades. The user would be able to interact with the menu and perform a number of operations. The most important thing was that the program could reallocate more memory when needed and it would do this automatically. So now, I'm trying to convert this program to C++.
We began discussing C++ about two and a half weeks ago in class so I am fairly new to the language. I have given a snippet for the program and what I have done so y'all can better understand my issue. I've included the class for Course and only two functions (the first adds course IDs and text name and the second prints a full list of the courses). I want the bound to start at 25, and if the user wants to put in more courses past this bound, the program will allocate more memory and copy the old entries into the new space. I'm not sure if I don't understand constructors still or what, but whenever the user inputs the 26th course it doesn't get saved. And if I try to print the courses when this happens, I get a segmentation fault.
If anyone can spot what I did wrong I would greatly appreciate it. If there's anything I didn't make clear, let me know and I'll reply as soon as I can. Thank y'all for taking the time to look at my program.
#include <iostream>
#include <fstream>
#include <stdlib.h>
#include <string>
using namespace std;
const int MAX_COURSES=25;
//Class
class Course
{
//Two private member variables
private:
string courseText;
int courseID;
public:
//Constructor
Course(void);
//Member functions
static void addCourse(Course* courses, int *courseCounter, int *timesReallocatedCoursesCounter);
static void printCourses(Course* courses, int *courseCounter, int *timesReallocatedCoursesCounter);
};
//Class implementation
//Constructor
Course::Course(void)
{
//Just providing initial value to the two object variables
courseText;
courseID=-1;
}
//This function isn't a part of any class and will print the grade book menu.
void printMenu()
{
cout << endl;
cout << "GRADE BOOK MENU" << endl;
cout << "1. Add a new course" << endl;
cout << "2. Add a new student" << endl;
cout << "3. Add a student to a course" << endl;
cout << "4. Add grades for a student in a course" << endl;
cout << "5. Print a list of all grades for a student in a course" << endl;
cout << "6. Print a list of all students in a course" << endl;
cout << "7. Compute the average for a student in a course" << endl;
cout << "8. Print a list of all courses" << endl;
cout << "9. Print a list of all students" << endl;
cout << "10. Compute the average for a course" << endl;
cout << "11. Store grade book (to a disk file" << endl;
cout << "12. Load grade book (from a disk file" << endl;
cout << "13. Quit the program" << endl;
cout << "14. Check individual student 2D grade matrix" << endl;
cout << endl;
}
//This method function will add a course to the course list
void Course::addCourse(Course* courses, int *courseCounter, int *timesReallocatedCoursesCounter)
{
int userEnteredCourseID=0;
Course* pointerCourses=NULL;
pointerCourses = courses;
do
{
cout << endl;
cout << "Please enter a course ID number using only integers." << endl;
cout << "There is a limit of only 25 courses allowed in the grade book." << endl;
cout << "To exit to grade book menu at any time, enter a negative integer or 0." << endl;
cout << "A message will be returned to you and no course IDs will be added." << endl;
cin >> userEnteredCourseID;
//Checking for a valid integer input.
while(cin.fail())
{
cout << "Error! User entered something other than an integer." << endl;
cin.clear();
cin.ignore(256,'\n');
cout << "Try entering a valid integer input this time..." << endl;
cin >> userEnteredCourseID;
}
cout << "You have entered the integer " << userEnteredCourseID << endl;
/*Checking if they entered a negative integer value.*/
if(userEnteredCourseID<=0)
{
cout << endl;
cout << "You have entered a negative integer or 0. Nothing has been added" << endl;
cout << "to the course list and you will be returned to the menu." << endl;
}
/*A valid integer was entered.*/
else
{
/*Now need to check if the course has been entered before. Should be noted, only
course ID numbers will be checked, not text names. Two course IDs could exist for
the same course; these ID numbers could represent different times for the same
course.*/
int j;
for(j=0; j<MAX_COURSES+(*timesReallocatedCoursesCounter); j++)
{
/*Checks if user has entered that course ID before.*/
if(courses[j].courseID==userEnteredCourseID)
{
cout << "You've already entered that course ID!" << endl;
userEnteredCourseID=0;
break;
}
/*If the current element doesn't equal the course ID entered, keep traversing the array.*/
else if(courses[j].courseID!=userEnteredCourseID && j<=((MAX_COURSES + (*timesReallocatedCoursesCounter))-2))
{
continue;
}
/*Entire loop was traversed and there weren't any duplicate course IDs, so a new course will be added to array.*/
else
{
//Since the previous bounds for courses used to be 25, we'll leave that part of the code in.
//However, since we now want to allow for infinite inputs, if the user reaches 25
//courses, the program will reallocate memory and allow for more courses to be entered.
//Only memory space for one course will be allocated each time so space isn't wasted.
if(*courseCounter==(MAX_COURSES + (*timesReallocatedCoursesCounter)))
{
courses = new Course[MAX_COURSES + *timesReallocatedCoursesCounter + 1];
for(int m=0; m < MAX_COURSES + *timesReallocatedCoursesCounter; m++)
{
courses[m] = pointerCourses[m];
}
delete[] pointerCourses;
*timesReallocatedCoursesCounter+=1;
courses[*courseCounter].courseID=userEnteredCourseID;
cout << "What would you like the new course to be called?" << endl;
cin.ignore();
getline(cin, courses[*courseCounter].courseText);
cout << "Course " << userEnteredCourseID << " has successfully been registered with the name " << courses[*courseCounter].courseText << endl;
*courseCounter+=1;
cout << "Press enter to continue." << endl;
cin.ignore();
}
else
{
courses[*courseCounter].courseID=userEnteredCourseID;
cout << "What would you like the new course to be called?" << endl;
cin.ignore();
getline(cin, courses[*courseCounter].courseText);
cout << "Course " << userEnteredCourseID << " has successfully been registered with the name " << courses[*courseCounter].courseText << endl;
*courseCounter+=1;
cout << "Press enter to continue." << endl;
cin.ignore();
}
}
}
}
}while(userEnteredCourseID>0);
}
//This method function prints the courses that have been successfully entered by the user.
void Course::printCourses(Course* courses, int *courseCounter, int *timesReallocatedCoursesCounter)
{
if(*courseCounter==0)
{
cout << endl;
cout << "You haven't entered any course IDs successfully yet!" << endl;
}
else
{
int i;
cout << endl;
cout << *courseCounter << " course(s) successfully entered so far." << endl;
cout << "The gradebook started with allowing only 25 courses to be entered." << endl;
cout << "Due to the amount of courses entered by the user, memory has been reallocated" << endl;
cout << *timesReallocatedCoursesCounter << " time(s) (in increments of 1) so entries could be made." << endl;
cout << "This means, a maximum of " << MAX_COURSES+*timesReallocatedCoursesCounter << " courses can be entered before reallocation is needed." << endl;
cout << "The list of the courses entered so far are:" << endl;
for(i=0;i<*courseCounter;i++)
{
cout << endl;
cout << courses[i].courseID << ": " << courses[i].courseText;
}
cout << endl;
}
}
//Main program file.
int main(void)
{
Course *courses;
courses = new Course [MAX_COURSES];
int userInput;
int courseCounter = 0;
int timesReallocatedCoursesCounter = 0;
do
{
/*Function that will re-print grade book menu.*/
printMenu();
cout << "What would you like to do with the menu?" << endl;
cin >> userInput;
while(cin.fail())
{
cout << "Error! User entered something other than an integer." << endl;
cin.clear();
cin.ignore(256,'\n');
cout << "Try entering a valid integer input this time..." << endl;
cin >> userInput;
}
cout << "You have entered the integer " << userInput << endl;
if(userInput==1)
{
cout << "You want to add a new course." << endl;
Course::addCourse(courses, &courseCounter, &timesReallocatedCoursesCounter);
}
else if(userInput==2)
{
cout << "You want to add a new student." << endl;
}
else if(userInput==3)
{
cout << "You want to add a student to a course." << endl;
}
else if(userInput==4)
{
cout << "You want to add a grade for a student in a course." << endl;
}
else if(userInput==5)
{
cout << "You want to print the grades for a student in a course." << endl;
}
else if(userInput==6)
{
cout << "You want to print a list of students in a course." << endl;
}
else if(userInput==7)
{
cout << "You want to compute the average for a student in a course." << endl;
}
else if(userInput==8)
{
cout << "You want to print a list of all courses." << endl;
Course::printCourses(courses, &courseCounter, &timesReallocatedCoursesCounter);
}
else if(userInput==9)
{
cout << "You want to print a list of all students." << endl;
}
else if(userInput==10)
{
cout << "You want to compute the average for a course." << endl;
}
else if(userInput==11)
{
cout << "You want to store the grade book you've been working on to a text file." << endl;
}
else if(userInput==12)
{
cout << "You want to load a grade book you've worked on previously from a text file." << endl;
}
else if(userInput==13)
{
cout << "You want to quit the program." << endl;
cout << "Goodbye, and thank you for using this program!" << endl;
}
else if(userInput==14)
{
cout << "An optional menu function that prints an individual student's 2D array." << endl;
cout << "This array contains all the course IDs for which the student is enrolled on the top row." << endl;
cout << "The rows below the course IDs correspond to grades made by the student in each course." << endl;
}
else
{
cout << "User entered an integer value that doesn't correspond to any menu option." << endl;
cout << "Nothing will happen." << endl;
}
}while(userInput!=13);
delete[] courses;
return 0;
}

Answer 1

but whenever the user inputs the 26th
Maybe because of this:
const int MAX_COURSES=25;
//...
Course *courses;
courses = new Course [MAX_COURSES]
You allocated room for 25 students, not 26. However, this issue would have occurred if you just used arrays:
Course courses[MAX_COURSES];
With an array you may have crashed, but the issue is the same -- a memory overwrite. So either decide -- limit the user to 25 students, or allow any number of students. If it's the latter, then you need to use a dynamic array.
The most important thing was that the program could reallocate more memory when needed and it would do this automatically. So now, I'm trying to convert this program to C++.
If you do with the dynamic array, it would have been advantageous to learn to use std::vector instead of using new[]/delete[]. That is the way dynamic arrays are done using C++. If you instead stick with new[]/delete[] to attempt to resize, all I can tell you is "good luck". I can almost bet you will come back with more issues if you choose the new[]/delete[] path.
The way the vector approach would look is something like this:
#include <vector>
//...
typedef std::vector<Course> CourseVector;
//...
void addCourse(CourseVector& v)
{
Course tempCourse;
// gather all the user information in tempCourse
//...
// now add the course to the array
v.push_back(tempCourse);
}
It took a single line of code to add a course to the vector. A vector is nothing more than a wrapper around a dynamic array. It basically does new[]/delete[] but with one huge advantage - it uses these operations correctly. You never need to get your hands dirty doing this work and making mistakes. The vector knows when to allocate, when to destroy, etc. All you need to do is call the functions to resize(), push_back(), etc.

Answer 2

I think your issue is that you are passing in the courses pointer by value. That is, a new copy of courses is created on the call stack, and you are operating on that copy. The copy is lost after addCourse() is executed. So for the first 25 courses that you add, your program works as normal since the copy of the courses pointer points to the same memory block as the "original". When you try to dynamically increase the size of courses by allocating a new block of larger memory, you are not doing anything to the original pointer in main(). This is why I think you are segfaulting when you print out the list.
A possible solution is to have addCourse() return the larger array, set it to courses in main(), and then deallocate the old array.
The prototype for the new addCourse() function would look like this:
static Course* addCourse(Course* courses, int *courseCounter, int *timesReallocatedCoursesCounter);
Inside the function allocate the larger array using a new Course pointer, and copy the values one by one into the new array. Then, return the new array.
and to call it, use a statement like:
Course* newCourses = addCourse(...);
delete [] courses;
courses = newCourses;