如何获得std :: map的std ::键集

#include<iostream>
#include <map>
#include <set>

std::set<int> getKeys(std::map<int, std::string> m) {
    std::set<int> keys;
    for (auto it = m.begin(); it != m.end(); ++it) {
        keys.insert(it->first);
    }
    return keys;
}

int main() {
    std::map<int, std::string> m = {{1, "one"}, {2, "two"}, {3, "three"}};
    std::set<int> keys = getKeys(m);
    for (auto key : keys) {
        std::cout<< key<< std::endl;
    }
    return 0;
}

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