`[]`运算符导致地图上的编译错误

Question

I am trying to get an element from a map in a for loop. Following the example on cppreference I try this:

#include <iostream>
#include <map>

using namespace std;

int main()
{
    map<int, int> mapping;

    mapping.insert(pair<int, int>(11,1));
    mapping.insert(pair<int, int>(12,2));
    mapping.insert(pair<int, int>(13,3));

    for (const auto &it : mapping)
        mapping[it]++;


    cout << "array: ";
    for (const auto &it : mapping)
        cout << it.second << " ";

    return 0;
}

Which gives the following compilation error with gcc:

main.cpp: In function 'int main()':
main.cpp:15:16: error: no match for 'operator[]' (operand types are 'std::map<int, int>' and 'const std::pair<const int, int>')
         mapping[it]++;

If I understand it correctly the problem is that the auto is resolved to a std::pair<const int, int> for which no [] operator is defined. I was wondering whether there is a way to get this to work.

See for the full compilation error here

Answer 1

How about just

for (auto &it : mapping)
    ++it.second;

for your first loop?