C++ map empty after inserting data. - c++

Question

Firstly, Happy new year to those who come across my question.
I'm currently learning C++ and I have a class project to complete.
In a nutshell, my code (so far) is supposed to instantiate student objects with details read from a txt file (name, reg-number, and a map of ) and add the student to a list.
I then read a second txt file (consisting of reg-number, course code and mark) and check if there is a match of student reg numbers between whats read and the list.
if there is a match, I should insert the marks read from the txt file into the map (part of the student object), such that afterwards, each student has a map containing the courses taken and marks achieved
I seem to be creating the list fine, then I use a stringstream to read the second file and loop through the list to compare reg-numbers.
if there is a match I then call the add mark method to add marks to the map.
Here's the thing.. If after I completed the map inserting, I loop and print a map of a student, the map is empty. for all students. To confirm this I used map.size().
I have tried many ways to understand and rectify the issue but it seems i'm missing the point of something. Instinct tells me that the add mark method is copying a reference to the variable passed, which is then destroyed by the stringstream in the main method, thus showing no data in the map. unfortunately, I can't change any code within the header files, only implement what's declared.
after reading the std library docs for strings, maps etc, and attempting numerous ways of correcting the behaviour, I'm at a loss.
Any advice would be greatly appreciated so I can continue with the project and better understand what is happening. I have added the files below. there is also a Person base class but I havnt changed this as it consists of only a setter and getter.
Many thanks in advance.
student Header:
#ifndef _STUDENT_H_
#define _STUDENT_H_
#include <string>
#include <map>
#include <stdexcept>
#include "Person.h"
using namespace std;
class NoMarkException: public exception
{
};
class Student: public Person
{ public:
// constructor should initialise name and registration number using arguments
// and initialise marks map to be empty
Student(const string &name, int regNo);
// method to return registration number
int getRegNo() const;
// method to add the mark to the map
// if a mark for the module is already present it should be overwritten
void addMark(const string& module, float mark);
// method to retrieve the mark for a module
// should throw NoMarkException if student has no mark for that module
float getMark(const string &module) const throw (NoMarkException);
private:
int regNo;
map<string, float> marks; // keys are modules, values are marks in range 0.0 to 100.0
// friend function to output details of student to stream
// should output name, regno, and minimum, maximum and average marks on a single line
// if the student has no marks "has no marks" should be output instead of the marks
friend ostream& operator<<(ostream &str, const Student &s);
};
#endif
Student.cpp file:
#include <iostream>
#include "Student.h"
#include "Person.h"
using namespace std;
//constructor makes a student object and initialises the map; marks.
Student::Student(const string &name, int regNo) : Person(name) {
this->name = name;
this->regNo = regNo;
map<string, float> marks;
}
//implemented from the header file. Returns the registration number.
int Student::getRegNo() const {
return regNo;
}
// implemented as per header file request. adds marks to the map. if a mark exists, then it is overwritten.
void Student::addMark(const string &module, float mark) {
marks[module] = mark;
cout << "added: " << marks[module]<< endl;
}
//used to find marks in a map.
float Student::getMark(const string &module) const throw (NoMarkException) {
auto search = marks.find(module);
//line to test the map size after using addMark.
cout << "size after inputted values: "<< marks.size();
return marks[module];
}
main.cpp file
#include <iostream>
#include <string>
#include <fstream>
#include <sstream>
#include <list>
#include "Student.h"
using namespace std;
//main method to obtain a file name from a user and read the file.
//method passes each .. to student.cpp
int main() {
//setting up variables required
string fileName;
const string fileEnd = ".txt";
string line;
string line2;
ifstream file;
int reg;
int reg2;
string studName;
string lastName;
float mark;
string module;
list<Student> listOfStudents;
cout << "Please enter a file name to access..." << std::endl;
cin >> fileName;
fileName += fileEnd;
// opening file an reading its contents. values are prepared and sent to the Student constructor. Fills a list
// with student objects created with variables read from the studs.txt file.
//checks file is found and exits the program if not
file.open(fileName);
if (!file) {
cerr << "Unable to open file " << fileName << endl;
exit(1);
}
while (getline (file, line)) {
stringstream stream (line);
stream >> reg >> studName >> lastName;
studName += (' ' + lastName);
cout << "Student: " << studName << " has been created." << endl;
listOfStudents.push_front(Student(studName, reg));
}
file.close();
cout << "The list of students has been created :)" << endl << endl;
cout << "Please enter the name of the next file to open"<< endl;
cout << listOfStudents.size()<<endl;
// opening second file. If file not found, exit with an error code.
// otherwise read each line, separate the three words into variables, then loop through the previously created list
//and look for a match of regNo. if true then call the addMark method to add the module and mark to the student's map.
cin >> fileName;
fileName += fileEnd;
file.open(fileName);
if (!file) {
cerr << "Unable to open file " << fileName << endl;
exit(1);
}
while(getline(file, line))
{
istringstream line_stream(line);
line_stream >> reg2 >> module >> mark;
for(Student stud : listOfStudents){
if(stud.getRegNo() == reg2){//out << "Match reg2: " << reg2 << " with stud: " << stud.getName() <<stud.getRegNo()<< endl;
stud.addMark(module,mark); }
}
}
//testing the get mark method of student class. with a module that is in the map. expecting a result
for(Student s :listOfStudents){
cout << s.getMark("CE151") << endl;
}
return 0;
}

Answer 1

You create a temporary copy of Student each time you're going to addMark, and discard it afterwards. You need not to copy it, but use reference, like this:
for (auto& stud: listOfStudents) /* stud.addMark */
It would also be a good idea to avoid copying Student when you print the results:
for (const auto& stud: listOfStudents) /* stud.getMark */
Aaaand one more thing:
float getMark(const string &module) const throw (NoMarkException);
Dynamic exception specification is deprecated in C++11 and removed in later standards. It is considered to be a bad practice and should be avoided.

Answer 2

The answer by user: grungegurunge is pretty much the one you are looking for. However, after looking at your class. I noticed that in the private section for the member variables you have declared map<string, float> marks which is okay. Yet when I look at your class's constructor after you set the name and regNo it appears that you are declaring another map<string, float> named marks where this one is local to the constructor only and does nothing. You create local memory on the stack with automatic storage, never use it, then it gets destroyed after the object goes out of scope; it is unnecessary to have it declared here.
//constructor makes a student object and initialises the map; marks.
Student::Student(const string &name, int regNo) : Person(name) {
this->name = name;
this->regNo = regNo;
// map<string, float> marks; // this line is not needed.
}