C++ Multiple Maps Inside Map

Question

In python I can have a dictionary inside a dictionary like so:

dict = {  'a': { 1:1, 2:2, 3:3 }, 
          'b': { 1:1, 2:2, 3:3 }, 
          'c': { 1:1, 2:2, 3:3 }   }

In C++ I have to use a map inside another map:

std::map< std::string, std::map<int, int> > map1

But how do I achieve the same structure as the python example? Haven't been able to find any examples of this.

std::map< std::string, std::map<int, int, int> > ??

std::map<std::string, std::map<int, int>, std::map<int, int>, std::map<int, int> > ??

Answer 1

If you mean an initialization of the map object then it could look like

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

int main() 
{
    std::map< std::string, std::map<int, int> > m =
    {
        { "a", { { 1, 1 }, { 2, 2 }, { 3, 3 } } },
        { "b", { { 1, 1 }, { 2, 2 }, { 3, 3 } } },
        { "c", { { 1, 1 }, { 2, 2 }, { 3, 3 } } },
    };

    for ( const auto &p1 : m )
    {
        std::cout << p1.first << ": ";
        for ( const auto &p2 : p1.second )
        {
            std::cout << "{ " << p2.first << ", " << p2.second << " } ";
        }
        std::cout << '\n';
    }

    return 0;
}

The program output is

a: { 1, 1 } { 2, 2 } { 3, 3 } 
b: { 1, 1 } { 2, 2 } { 3, 3 } 
c: { 1, 1 } { 2, 2 } { 3, 3 } 

Answer 2

The syntax for placing keypairs into maps is different in C++ than it is in Python. In C++ the std::map container exposes the emplace(key, value) method in order to do something like dict[key] = value. It is also slightly different because C++ is strongly typed to you must provide the right type for the key and the value (as defined in the template)

To do the equivalent of your python code with std::map you would do this:

std::map<std::string, std::map<int, int> dict
std::map<int, int> my_map;
my_map.emplace(1, 0); // Add a key of 1 with a value of 0 to the map
dict.emplace("My key", my_map); // Add the map to the other map with a key of "My map"