Using vector of pointer to objects of another class - oop

Question

I have been trying to do the following to no avail,
In 'used.h',
#ifndef USED_H_
#define USED_H_
#include<iostream>
#include<string>
class used
{
public:
int member=0;
used();
virtual ~used();
};
#endif
In the used.cc,
#include "used.h"
used::used()
{
}
used::~used()
{
}
In 'the_user.h',
#ifndef THE_USER_H_
#define THE_USER_H_
#include<queue>
#include<iostream>
class used; //Class forward declaring
class the_user
{
public:
std::deque<used*> my_queue;
the_user();
~the_user();
};
#endif
Now, I want to access and change 'member' in 'the_user.cc',
#include "used.h"
#include "the_used.h"
#include<iostream>
#include<queue>
using namespace std;
the_user::the_user()
{
deque <used*> my_queue;
my_queue.resize(6);
used* object = new used; <-------marked line
for(unsigned int i=0; i<my_queue.size(); i++)
{
my_queue.push_back(object);
}
cout << my_queue[5] << endl; //Should give 0
my_queue[0]->member=1000;
cout << my_queue[0]->member << endl; //1000
}
in main file(I have only read access),
#include "the_used.h"
#include <iostream>
#include <stdlib.h>
#include <sstream>
using namespace std;
int main()
{
the_used *object = new the_used();
}
Actually, I am getting undefined reference to used::used() at the marked line. What seems to be the problem?
I have tried to use the same for a vector as well but to no avail.
I am not allowed to make changes to the 'int main(){}'.
Any help would be highly appreciated.

Answer 1

Your class declaration doesn't declare any constructor or destructor:
class used
{
public:
int member=0;
};
But in your cpp file you define them. Your compiler should complain already here:
#include "used.h"
used::used()
{
}
used::~used()
{
}
You must declare constructor and destructor in your class:
class used
{
public:
used();
~used();
int member=0;
};
Then here:
my_queue.resize(6);
you will actually create 6 pointers that will be initialized to nullptr. Maybe you're aware of that, since you expect my_queue[5] to return 0.
Then in your loop, everytime you do this:
my_queue.push_back(object);
you will increase the size of my_queue by one, thus make your loop run forever.
Apart from that: Do. Not. Do. using namespace std;. Ever.