insertion into unordered_map got lost - c++

Question

I'm having the following code, but after run the code, the result is empty, any ideas why the result is empty? the reference of result in function main was passed to myclass, I thought function addToResult will actually add data to result, and I'm expecting a map key = "test", value = "1": "1". I'm kind of new to c++. Thanks!
#include <iostream>
#include <string>
#include <unordered_map>
using LookUpTable = std::unordered_map<std::string, std::string>;
using DLTable = std::unordered_map<std::string, LookUpTable>;
class MyClass
{
public:
MyClass(DLTable& dltable) {
m_dltable = dltable;
};
void addToResult() {
LookUpTable ee;
ee.emplace("1", "1");
m_dltable.emplace("test", ee);
};
private:
DLTable m_dltable;
};
int main ()
{
DLTable result;
MyClass myclass(result);
myclass.addToResult();
std::cout << "myrecipe contains:" << std::endl;
for (auto& x: result) {
std::cout << x.first << ": "<< std::endl;
for (auto& xx : x.second) {
std::cout << xx.first << ": " << xx.second << std::endl;
}
}
std::cout << std::endl;
return 0;
}

Answer 1

Let' look into simplified example:
int a = 0;
int &b = a;
int c = b;
c = 123;
Will last assignment modify a? Of course not. It does not matter how you pass value to c through reference or not c is completely independent variable that just initialized by a reference.
Your case is the same - m_dltable is separate variable and the fact you initialize it using reference does not change anything. (Your case even worse, you did not initialize it by reference, you assigned to it)
In general your approach is wrong. If you want directly access that variable then just make it public, do not try to create convoluted workarounds on how to access it. If you want incapsulation just create members that allow you to iterate over that container. For example return a const reference to it or have begin() and end() methods that return (const) iterators accordingly.