Making a class to provide Lists interface - c++

Question

Im trying to make a class to use in a Library, and im not sure if its even possible to do. The idea of this class, is to provide methods to insert, search and delete items on a list, but my main problem is that, as i dont know the type of object that i want to place in a list, I dont know how to work with it. I thought I could place a void pointer, and then make it point to a structure, but I havent been able to make it work.
Lets suppose my class is something like this:
class ListManager
{
private:
void* FirstItem;//This would point to the first item of the list
void* LastItem;//This would point to the last item of the list
public:
void AddItemToList(void* Item);
void RemoveItemFromList(void* Item);
}
So, the idea would be that, from my program, I can define a structure like
struct Employee
{
*char Name;
int Id;
int PhoneNumber;
}
And then, use this class, to be able to add/delete Employees. So in this case, the void* pointers, should be pointing to a struct of the type Employee. Nevertheless, i want to make my class work for any type of struct.
I dont know if i explained exactly what I want to do, I tried several ways of doing this, and failed on all of them.
Im going to post a code of how I would like the class to work, if I havent explained myself correctly
ListManager *Worker;
Worker=new(ListManager);
Employee *Item;
Item=new (Employee);
Item->Id=126;
Item->PhoneNumber=42154872;
Worker->AddItemToList(Item);
/*At this point, FirstItem and LastItem should point to the Item i just created*/
Could someone point me in the right direction, as how to make a class work with a structure, without knowing the type of structure?
Thanks in advance

Answer 1

You need templates!
here's a simple interface that you can start working with.
template <typename T>
class ListManager
{
public:
void addItemToList(const T& item);
void removeItemFromList(const T& item);
}
Now T is your type, and you'd declare a ListManager like this:
ListManager<Employee> manager;
I would suggest you also to look at the stl documentation/implementation of a list at: http://www.cplusplus.com/reference/list/list/
You have also the concept of iterators to dig into.
Also, try to use values instead of pointers. With the interface I gave you, you would store the actual value in the list and not a pointer, so the list owns the object and you won't need to manage your memory manually.

Answer 2

See this Tutorial:
http://www.tutorialspoint.com/cplusplus/cpp_templates.htm
Your Example could be look like:
#include <iostream>
using namespace std;
template <class T>
class ListManager
{
private:
void* FirstItem;//This would point to the first item of the list
void* LastItem;//This would point to the last item of the list
public:
void AddItemToList(const T& Item){
std::cout << Item << std::endl;
};
void RemoveItemFromList(const T& Item){};
};
int main() {
ListManager<std::string> mgr;
mgr.AddItemToList("Test");
return 0;
}
Output:
Test
Working Example:
http://ideone.com/FCAtcJ

Answer 3

There really are no interfaces in C++, but you could use a class template just the same way:
template <typename T>
class ListInterface {
public:
void add(T * item) { list.push_back( item); }
void remove(T * item) { list.erase(std::find(list.begin(), list.end(), item)); }
T * get(int index) { return list[index]; }
int size() const { return list.size(); }
private:
std::vector<T *> list;
};
class Manager : public ListInterface<Employee> {
// ...
};
Naturally, this is a barebone example, you really outta do some checking in the remove() and get() methods.

Answer 4

There are multiple solutions to this problem.
Simply use std::list
std::list<Employee> manager;
If 1. is not possible you can define your own list interface using template
template <class T>
class ListManager
{
private:
class Iterator {
public:
T& item;
Iterator* next;
Iterator* prev;
Iterator(T& i, Iterator* n, Iterator* p) : item(i), next(n), prev(p) {}
}
Iterator* FirstItem = NULL;//This would point to the first item of the list
Iterator* LastItem = NULL;//This would point to the last item of the list
public:
void AddItemToList(T& Item) {
if(LastItem) {
Iterator* it = new Iterator{Item, NULL, LastItem};
LastItem->next = it;
LastItem = it;
} else {
Iterator* it = new Iterator{Item, NULL, NULL};
FirstItem = it;
LastItem = it;
}
}
void RemoveItemFromList(const T& Item) {
for(Iterator* it = FirstItem; it != NULL; it = it->next) {
if(it->item == Item) {
if(it->prev)
it->prev->next = it->next;
else
FirstItem = it->next;
if(it->next)
it->next->pref = it->pref;
else
LastItem = it->pref;
delete it;
break;
}
}
}
};
If 1. and 2. is not possible because you need need multiple types inside your list. You can either use a list of Anys (like boost::Any) or a unsafe version with void*:
class ListManager
{
private:
class Iterator {
public:
void* item;
Iterator* next;
Iterator* prev;
Iterator(void* i, Iterator* n, Iterator* p) : item(i), next(n), prev(p) {}
}
Iterator* FirstItem = NULL;//This would point to the first item of the list
Iterator* LastItem = NULL;//This would point to the last item of the list
public:
template<class T>
void AddItemToList(T& Item) {
if(LastItem) {
Iterator* it = new Iterator{&Item, NULL, LastItem};
LastItem->next = it;
LastItem = it;
} else {
Iterator* it = new Iterator{&Item, NULL, NULL};
FirstItem = it;
LastItem = it;
}
}
template<class T>
void RemoveItemFromList(const T& Item) {
for(Iterator* it = FirstItem; it != NULL; it = it->next) {
if(it->item == &Item) {
if(it->prev)
it->prev->next = it->next;
else
FirstItem = it->next;
if(it->next)
it->next->pref = it->pref;
else
LastItem = it->pref;
delete it;
break;
}
}
}
};
Code is not tested