Question

Given the following code:

struct Item
{
    std::string name;
    int someInt;
    string someString;
    Item(const std::string& aName):name(aName){}
};
std::unordered_map<std::string, Item*> items;
Item* item = new Item("testitem");
items.insert(make_pair(item.name, item);

The item name will be stored in memory two times – once as part of the Item struct and once as the key of the map entry. Is it possible to avoid the duplication? With some 100M records this overhead becomes huge.

Note:
I need to have the name inside the Item structure because I use the hashmap as index to another container of Item-s, and there I don’t have access to the map’s key values.

Answer 1

OK, since you say you are using pointers as values, I hereby bring my answer back to life.

A bit hacky, but should work. Basicly you use pointer and a custom hash function

struct Item
{
    std::string name;
    int someInt;
    string someString;
    Item(const std::string& aName):name(aName){}

    struct name_hash  
    { 
       size_t operator() (std::string* name)
       {
           std::hash<std::string> h;
           return h(*name);
       }
    };
};
std::unordered_map<std::string*, Item*, Item::name_hash> items;
Item* item = new Item ("testitem");
items.insert(make_pair(&(item->name), item);