Change a map by reference, not by value

map.cpp

#include <iostream>
#include <map>

int main() {
  std::map<int, int> m;
  auto display_m = [&m](const char *msg) {
    std::cout << msg << " ";
    for (auto [k, v] : m) {
      std::cout << "m[" << k << "] = " << v << std::endl;
    }
  };
  {
    m[4] = 2;
    display_m("After init");
  }

  {
    for (auto [k, v] : m) {
      v = 4;
    }
    display_m("key-value, by value");
  }

  {
    for (auto p : m) {
      p.second = 5;
    }
    display_m("pair, by value");
  }

  {
    for (auto &p : m) {
      p.second = 5;
    }
    display_m("pair, by reference");
  }
}
// After init m[4] = 2
// key-value, by value m[4] = 2
// pair, by value m[4] = 2
// pair, by reference m[4] = 5