How to Create a Map of Maps in C++?

In C++, maps are associative containers provided by the STL library of C++, that allow the users to store data in key-value pairs where the keys must be unique. In this article, we will learn how to create a map of maps in C++.

For Example,

Input:
myMap1={1, "C++"};
myMap2={1, "Java"};
myMap3={2, "Python"};

Output:
Map Elements: { {1: {1, "C++"} },
                               {2: {1, "Java"} },
                               {3: {2, "Python"} }

Map of Maps in C++

To create a std::map of maps first, we need to declare a map where the key is of the desired type and the value is of the type map. We can do that by passing the value template parameter as a map.

Syntax to Declare Map of Maps

map < keytype, map <innerKeyType, valueType> > myMap

C++ Program to Create a Map of Maps

The below program demonstrates how we can create a map of maps in C++ STL.

C++

// C++ program to illustrate how to create a map of maps
#include <iostream>
#include <map>
using namespace std;

int main()
{
    // Creating a map of maps
    map<int, map<int, string> > myMap;

    // Adding key-value pairs to the map of maps
    myMap[1][1] = "one";
    myMap[1][2] = "two";
    myMap[2][1] = "two";
    myMap[2][2] = "four";

    // Printing the map of maps
    cout << "Map of maps:" << endl;
    for (const auto& pair1 : myMap) {
        for (const auto& pair2 : pair1.second) {
            cout << pair1.first << ", " << pair2.first
                 << " => " << pair2.second << endl;
        }
    }

    return 0;
}

Output

Map of maps:
1, 1 => one
1, 2 => two
2, 1 => two
2, 2 => four

Time Complexity: O(N * M log(N*M) ), here N is the number of maps in the map and M is the average size of the inner maps
Auxiliary Space: O(N * M)