c++ Sharing list between main an exterior function - c++

Question

I am making a rabbit population simulator for a challenge I am doing. I have a map of class objects. I also have a list which contains the keys for the map. Each round of the simulator I want to add more objects to the class, and update my map and list. To do this I wrote a separate "creation" function which will randomly generate new class objects and add them to a map and add the keys to a list.
The problem is when I call the creation function, then iterate through the map using the list it shows the map or list is empty (not sure which). If I iterate through the map before leaving the function it only shows the newest objects. If I move the code for the function into the main function, it works correctly (calling it twice gives me the new objects created in the first iteration of the code, and also the objects created by the second iteration).
I am guessing that a new list or map is being created each time the function is called and is overwriting the old list or map. How can I get the list and map to pass between the main function and the creation function?
Here is my code:
#include "stdafx.h"
#include <iostream>
#include "windows.h"
#include <string>
#include <sstream>
#include <array>
#include <time.h>
#include <conio.h>
#include <map>
#include <list>
class Bunny
{
public:
char fname;
int sex, age, color, status;
Bunny(int, int, int, int);
Bunny();
int s() { return (sex); }
int a() { return (age); }
int c() { return(color);}
int st() { return (status);}
int aging(int age) { return (age + 1); }
};
Bunny::Bunny(int s, int a, int c, int st)
{
sex = s;
age = a;
color = c;
status = st;
}
Bunny::Bunny(){}
void creation(std::map<std::string, Bunny> bunnies, std::list<std::string>names, int births);
std::string firstname(int s, int num)
{
std::string name;
if (s == 0)
{
switch (num)
{
case (0) :
name = "Tim";
break;
case (1) :
name = "Tom";
break;
case (2) :
name = "Mark";
break;
case (3) :
name = "Bob";
break;
case (4) :
name = "Rob";
break;
}
}
if (s == 1)
{
switch (num)
{
case (0) :
name = "Suzy";
break;
case (1) :
name = "Linda";
break;
case (2) :
name = "Mary";
break;
case (3) :
name = "Jan";
break;
case (4) :
name = "Julie";
break;
}
}
return (name);
}
void main()
{
int num = rand() % 5;
int n, births = 10;
std::list<std::string>names;
std::map<std::string, Bunny> bunnies;
srand(time(0));
creation(bunnies, names, births);
std::cout << "Number" << "\t" << "Name" << "\t" << "age" << "\t" << "Sex" << "\t" << "Color" << "\t" << "Vampire?" "\n";
n = 0;
for (std::list<std::string>::iterator it = names.begin(); it != names.end(); it++)
{
n++;
std::cout << n << "\t";
std::cout << " " << *it;
std::cout << "\t" << bunnies[*it].a() << "\t" << bunnies[*it].s() << "\t" << bunnies[*it].c() << "\t" << bunnies[*it].st() << "\n";
}
creation(bunnies, names, births);
_getch();
}
/*void year()
{
for (std::list<std::string>::iterator it = names.begin(); it != names.end(); it++)
{
bunnies[*it].aging(bunnies[*it].a())
}
}*/
void creation(std::map<std::string, Bunny> bunnies,std::list<std::string> names,int births)
{
int n;
for (n = 0; n < births; n++)
{
int num = std::rand() % 5;
char id = (std::rand() % 100) + 20;
int s = std::rand() % 2;
std::string f = firstname(s, num) + '_' + id;
int a = 0;
int c = std::rand() % 5;
int st;
if (rand() % 50 == 43) st = 1; else st = 0;
bunnies[f] = Bunny(s, a, c, st);
names.push_front(f);
//std::cout << f << " " << bunnies[f].a() << " " << bunnies[f].c() << "\n";
}
std::cout << "Number" << "\t" << "Name" << "\t" << "age" << "\t" << "Sex" << "\t" << "Color" << "\t" << "Vampire?" "\n";
n = 0;
for (std::list<std::string>::iterator it = names.begin(); it != names.end(); it++)
{
n++;
std::cout << n << "\t";
std::cout << *it;
std::cout << "\t" << bunnies[*it].a() << "\t" << bunnies[*it].s() << "\t" << bunnies[*it].c() << "\t" << bunnies[*it].st() << "\n";
}
}

Answer 1

Your problem is that your main function is passing your map and list by value instead of passing by reference. This means that your creation function is receiving a copy of the existing map/list, rather than a reference to the original one you created. Since it is then only making edits to that copy, any changes it makes will not be reflected in the main function.
Change your creation function from:
void creation(std::map<std::string, Bunny> bunnies, std::list<std::string>names, int births)
to
void creation(std::map<std::string, Bunny>& bunnies, std::list<std::string>& names, int births)