Implement a MultiKeyMap in C++

This post will discuss how to implement a MultiKeyMap (map with multiple keys) in C++.

A MultiKeyMap is a map that offers support for multiple keys. It is exactly the same as a normal map, except that it needs a container to store multiple keys. A simple solution to implement a MultiKeyMap in C++ is using std::pair for the key. To insert elements into the multimap, use the [] operator.

#include <iostream>

#include <map>

int main()

{

std::map<std::pair<int,int>, int> multimap;

std::pair<int,int> key = std::make_pair(1, 2);

int val = 10;

multimap[key] = val;

std::cout << multimap[key] << std::endl; // 10

return 0;

}

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If a key exists in the map, the [] operator will override the original value with the new value. To retain the original value in such cases, consider using the std::map::insert function.

#include <iostream>

#include <map>

int main()

{

std::map<std::pair<int,int>, int> multimap;

std::pair<int,int> key = std::make_pair(1, 2);

int val = 10;

multimap.insert({key, val});

std::cout << multimap[key] << std::endl; // 10

return 0;

}

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This works fine for a multimap having two keys of the same or different types. To construct a multimap with three keys of the same or different types, we can use std::tuple instead.

#include <iostream>

#include <map>

#include <tuple>

int main()

{

std::map<std::tuple<int,int,int>, int> multimap;

std::tuple<int,int,int> key(1, 2, 3);

int val = 10;

multimap[key] = val;

std::cout << multimap[key] << std::endl; // 10

return 0;

}

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To create a multimap with an arbitrary number of keys of the same type, use the std::vector instead. This can be implemented as follows in C++.

#include <iostream>

#include <map>

#include <vector>

int main()

{

std::map<std::vector<int>, int> multimap;

std::vector<int> key = {1, 2, 3};

int val = 10;

multimap[key] = val;

std::cout << multimap[key] << std::endl; // 10

return 0;

}

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Also See:

Use custom objects as keys to std::map in C++

Use std::pair as key to std::unordered_map in C++

Use struct as key to std::unordered_map in C++

That’s all about implementing a MultiKeyMap in C++.

Implement a MultiKeyMap in C++

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