Using map for inventory system

Question

Is there any practical way to get objects to work with maps? (I don't really know much about maps so sorry if this is a bad question). I'm interested in using a map for an inventory system that will contain item objects. An item object has a name, description, and money value. The key would be the item's name, and the linking variable would be the the quantity of items I have for that particular item.

And if a map doesn't work, does anyone have a good alternative to this type of system? I need something keeping track of the quantity of each type of item I have.

You might want to learn C++ through tutorial. We can hardly teach you C++ at a Q&A site. See [this list of great C++ books](http://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list). — sbi, Sep 28 '10 at 18:22
If I knew everything about C++, I wouldn't need to go to a Q&A site. been going through tutorials, reading books, and trying to get help from fellow programmers :) — graydragon10, Sep 28 '10 at 19:22
perhaps some grounding in abstract data types and algorithms might help here as well. Your describing an uncertain problem with a final technical option you have chosen, when you don't seem to have background knowledge in how to pick the right technical solution. 'Inventory system' for a car yard, online book store, computer game, home dvd catalogue? what are the inputs/outputs, how many synchronus changes and how often? — Greg Domjan, Sep 28 '10 at 19:59
I figure the "game development" tag would make that clear :-/ I described it as an inventory system and that I wanted to link the name of the item with its quantity via a map. Knowing these two pieces of the puzzle, it isn't so much what the items do or how they would be shown to the user that matters but getting the logic and syntax sorted out. — graydragon10, Sep 28 '10 at 20:09

score 4 · Accepted Answer · edited Jul 14 '15 at 12:34

The C++ standard library template map is just a storage container so it can definitely be used with objects. The map will take your object as its templated argument parameter.

A map would work well for your inventory system. Use something like:

#include <pair>
#include <map>
#include <string>
#include <iostream>

class Item {
  public:
   Item(void) {}
   ~Item(void) {}
   Item(std::string new_name) {
      my_name=new_name;
   }
   void setName(std::string new_name) {
      my_name= new_name;
   }
   std::string getName(void) {
      return my_name;
   }
  private:
   std::string my_name;
};  

class Item_Manager {
  public:
   Item_Manager(void) {}
   ~Item_Manager(void) {}  
   void addItem(Item * my_item, int num_items) {
      my_item_counts.insert( std::pair<std::string,int>(Item.getName(),num_items) );
   }
   int getNumItems(std::string my_item_name) {
      return my_item_counters[my_item_name];
   }
  private: 
   std::map<std::string, int> my_item_counts;
};

main () {
   Item * test_item = new Item("chips");
   Item * test_item2 = new Item("gum");
   Item_Manager * my_manager = new Item_Manager();

   my_manager->addItem(test_item, 5);
   my_manager->addItem(test_item2,10);
   std::cout << "I have " << my_manager->getNumItems(test_item->getName())
             << " " << test_item->getName() << " and " 
             << my_manager->getNumItems(test_item2->getName())
             << " " << test_item2->getName() << std::endl;

   delete test_item;
   delete test_item2;
   delete my_manager;
}

Here's a reference on the stdlib map and its functions: http://www.cplusplus.com/reference/stl/map/

Look at the function pages for examples of how to iterate through/index a map, etc.

I wanted and it complained to me. is there a good workaround? — graydragon10, Sep 28 '10 at 18:10
Your Item class should not be in the namespace standard. You're confusing assignment and declaration. See my above edits in a sec. — Jason R. Mick, Sep 28 '10 at 18:14
graydragon10: the type parameters of `std::map` are decided compile time. I suspect `getName` returns a `std::string`? If so you should declare it `std::map`. — Skurmedel, Sep 28 '10 at 18:15
Alright I made a full class driven example that I believe satisfies your above listed use request! — Jason R. Mick, Sep 28 '10 at 18:33
this looks great. I'm just wondering where my_item_counters is coming from? I don't see where it was declared. — graydragon10, Sep 28 '10 at 19:45

score 3 · Answer 2 · answered Sep 28 '10 at 18:00

3

If you're talking about std::map, it's a template which can work with any type of object, as long as a way to compare objects for ordering is provided. Since your key (the name) is a string, it will work right out of the box with std::map

struct Item
{
    std::string description;
    int value;
};


int main()
{
    // associate item names with an item/quantity pair
    std::map<std::string, std::pair<Item, int> > itemmap;
}

answered Sep 28 '10 at 18:00

Charles Salvia

52,325
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the problem is that my Item is a class (trying to make it reusable for future games, and I plan on subclassing from it) – graydragon10 Sep 28 '10 at 18:04
1

@graydragon - you can create a class that has a reference to an item and an int for the quantity. the basic structure will still be the same - namely a map with key as item name pointing to one of these classes. – NG. Sep 28 '10 at 18:06
@graydragon. Seem my example above for a map that takes a class as its templated parameter. – Jason R. Mick Sep 28 '10 at 18:10
I think I understand what he might mean. As you can't put objects of a derived type in a map lest they be truncated (A derives from B, putting a B in `map` will give surprising results to the uninitiated.) You have to use pointers, or some kind of other reference if you want to mix instances with a common base. Maybe this is what graydragon wants to avoid? Or perhaps he has misunderstood the workings of the type system. Just grasping straws here :) – Skurmedel Sep 28 '10 at 18:10

score 1 · Answer 3 · answered Sep 28 '10 at 18:01

1

I need something keeping track of the quantity of each type of item I have.

How about std::vector<std::pair<Item, int> >?

answered Sep 28 '10 at 18:01

fredoverflow

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Would this work if I only want each item type to appear in the list once? like apples - 5, oranges - 2, etc. – graydragon10 Sep 28 '10 at 18:08

Using map for inventory system

3 Answers3