I have a map that is part of an object that I want to make sure that I never accidentally mess up; however, when compiling, I get the following error:

./test.cpp: In member function ‘std::map<int, mapValue>& myClass::getMap() const’:
./test.cpp:22:49: error: binding reference of type ‘std::map<int, mapValue>&’ to ‘const std::map<int, mapValue>’ discards qualifiers
   22 |     map<key, mapValue> &getMap() const { return m; }
      |  

Here is my code:

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

struct mapValue
{
  int num;
  string data;
};

class myClass
{
  public:
    typedef int key; //hopefully this helps future people searching for a similar error

    void adjustMap(int index, mapValue mv)
    {
      m[index] = mv;
    }

    map<key, mapValue> &getMap() const { return m; }

  private:
    map<key, mapValue> m = {
      {0, {0, "Hi!"}},
      {1, {0, "Hello"}},
      {2, {1, "Goodbye"}}
    };
};

void operateWithMapData(const myClass &c) //I don't want to accidentally mess up
                                          //the map or anything in c
{
  const auto &m = c.getMap(); //Seriously, I do NOT want to edit this map.

  for(auto &kv : m)
  {
    cout << kv.first << ":" << kv.second.data << ":" << kv.second.num << endl;
  }
}

int main()
{
  myClass c;
  c.adjustMap(5, {8, "Bye"});

  operateWithMapData(c);

  return EXIT_SUCCESS;
}

I want to make sure that when I’m working on operateWithMapData that I don’t accidentally insert, change, or delete any existing values.

Normally, I know that when I make c a const reference, then I’m not:

1. pointlessly coping over large amounts of data (pass by reference part)
2. making it immutable (the const part)

If I were to just do that and remove const from myClass::getMap() and leave it in operateWithMapData then:

1. It still doesn’t compile
2. That could mean I have access to a mutable version of the raw map that I could screw up somewhere else.

I guess my question is two parts:

1. How do I correctly accomplish what I want to?
2. Can I get a good detailed explanation on what this error means so if I see something like it in the future, then I can fix it on my own.

Here are some related questions, but I don’t think they solve my problem..

Binding to reference of type discards qualifiers

How to return a class object by reference in C++?

Error: Binding reference of type ‘&’ to ‘const discards qualifiers’