C ++无法引用地图键

Question

#include <algorithm>
#include <iostream>
#include <vector>
#include <map>
#include <string>

using namespace std;

int main() {
    int steps;
    map<string, string> countries;
    cin >> steps;
    for (int i = 0; i < steps; ++i) {
        string command;
        cin >> command;
        if(command == "CHANGE_CAPITAL") {
            for(auto& s : countries) {
                string& old_country = s.first;
                string& old_capital = s.second;
            }
        } 
    }
}

Hello! When I try to compile this code it gives me error:

binding value of type 'basic_string<...>' to reference to type 'basic_string<...>' drops 'const' qualifier

for the string

string& old_country = s.first;

Why does this happen? (it doesn't give this error for the next string - where I assing "s.second" by reference).

Compiler is ISO C++ 1y (-std=c++1y).

Thank you.

Answer 1

const string& old_country = s.first;

Or even better:

const auto& old_country = s.first;

Side note: for readability, add the const even for auto.

The pair for your map is:

std::pair<const std::string, string>

because the key cannot be modified due to the tree constraint.