C++ persistence of inserting local variable into a std::map by reference

Question

I'm new to C++ and trying to understand a simple example of inserting a list of integers into a map.

#include <iostream>
#include <map>
#include <list>
using namespace std;

map<string, list<int>> m;

void insert(list<int>& list_to_insert)
{
    m.insert({"ABC", list_to_insert});
}

void setup()
{
    std::list<int> local_list = { 7, 5, 16, 8 };
    insert(local_list);
}

int main()
{
    setup();
    cout << m["ABC"].size(); // PRINTS 4
}

As far as my understanding, local_list is a variable only known to the setup function. When I pass in a reference to the insert function, I expect it to work. However, at the end of setup, I expect local_list to be removed from the stack frame. However, in my main function, when I call size() , I see that it has in fact persisted throughout. I am not sure I understand how local_list gets persisted at the end of setup() . What is exactly happening here?

Answer 1

map<string, list<int>> m;

contains lists, not references to lists. So your

m.insert({"ABC", list_to_insert});

will create a copy of the passed list.

PS: why-is-using-namespace-std-considered-bad-practice