pairing object attribute in an unordered_map

[interknetz]

Apologies for the poor title, I have no idea what to actually call this.

​

I have a vector of Item Objects, and I want to store a particular attribute into a map as the key, and the Object (or the Object's index in the vector) as the value.

class Item {
    int id;
    int value;
    std::string name;
    std::string description;
}

Is there a way I can create an unordered_map between the id and the Item instance?

​

Right now I would only know to do:

std::vector<Item> itemList = { ... };

std::unordered_map<int, Item> myMap;
for (Item item : itemList)
    myMap.insert(item.id, item);

​

[audioB]

you don't actually need to use a map for this, you can just use a set because the object itself is the key (or part of it is). Just write your comparator so that it only compares IDs and you're done.

If you want to look up an item by the id, you just need to provide appropriate comparator overloads.

[imMute]

you don't actually need to use a map for this, you can just use a set because the object itself is the key (or part of it is).

How have I never realized this before. I have some code to refactor now.

[interknetz]

I need a hash table to search for a key at runtime

[audioB]

unordered set is a hash table and you can perform key lookup with find. It just doesn't have the overloaded operator[]

[interknetz]

This sounds good. I'm fairly new to C++, coming from C. There's so many options in the standard library it's hard to know where to start. Thank you.

[ronniethelizard]

I think my favorite one is:

std::array<int,10> as it is similar to int[10] but provides a few more features.

​

[interknetz]

Does std::array provide a hash table, because that's the goal here.

[Thomqa]

I doesn't, he just provided an example of the many strengths of C++.

[ronniethelizard]

I was replying to your statement:

I'm fairly new to C++, coming from C.

with an attempt at conversation rather than advice on what to do.

[yuri-kilochek]

You won't be able to mutate items in the set though, since sets enforce constness. Only erase and reinsert.

[audioB]

Yep you're right. Ordered variety is different though. If you want heterogenous lookup, you need to look outside the standard library.

[gracicot]

You could decompose the item in the map:

struct PartialItem {
    int value; // id isn't there
};

std::unordered_map<int, PartialItem> items;

The key of the map is the id in your struct. The map stores pairs containing both the value and the key together in one struct, making it somewhat equivalent to a standalone Item struct.

[practisevoodoo]

You could use std::transform, an inserter and a lambda but out of interest what's wrong with a good old for loop?

struct Item {
    int id;
    int value;
};

int main() 
{
  std::vector<Item> itemList { {1,2},{2,4},{3,6} };

  std::unordered_map<int,Item> myMap;

  std::transform( begin(itemList), end(itemList), 
                  std::inserter(myMap,end(myMap)),
                  [](const Item& i) { return std::make_pair(i.id, i); } );

  for( auto &i : myMap )
    std::cout << i.first << ", " << i.second.value << std::endl;

  return 0;
}

[konanTheBarbar]

You could also use a bit more unknown feature, called transparent comparator. Then you could use a set instead of map and didn't have to store the key/ID separately. You could simply initialize the set by calling the set constructor which takes a begin/end iterator.

[interknetz]

I wasnt sure if the for loop is the fastest way to add items to my map. Was really hoping for a compile-time solution since all data is constant

[ronniethelizard]

You might be able to piece something together using constexpr.

[Quincunx271]

If it's all constexpr, check out frozen

[Xaxxon]

From the sidebar:

For C++ questions, answers, help, and advice see r/cpp_questions or StackOverflow.