std::set<pair<int，int> >-仅使用pair<>::first作为关键字进行查找

#include <iostream>
#include <set>

struct PairCompare {
    bool operator()(const std::pair<int, int>& lhs, const std::pair<int, int>& rhs) const {
        return lhs.first < rhs.first;
    }
};

int main() {
    std::set<std::pair<int, int>, PairCompare> s;
    s.insert(std::make_pair(1, 2));
    s.insert(std::make_pair(3, 4));
    s.insert(std::make_pair(1, 5));

    // 查找
    auto it = s.find(std::make_pair(1, 0));
    if (it != s.end()) {
        std::cout << "Found: (" << it->first << ", " << it->second << ")\n";
    } else {
        std::cout << "Not found\n";
    }

    return 0;
}

#include <iostream>
#include <set>

namespace std {
    template <>
    struct less<pair<int, int>> {
        bool operator()(const pair<int, int>& lhs, const pair<int, int>& rhs) const {
            return lhs.first < rhs.first;
        }
    };
}

int main() {
    std::set<std::pair<int, int>> s;
    s.insert(std::make_pair(1, 2));
    s.insert(std::make_pair(3, 4));
    s.insert(std::make_pair(1, 5));

    // 查找
    auto it = s.find(std::make_pair(1, 0));
    if (it != s.end()) {
        std::cout << "Found: (" << it->first << ", " << it->second << ")\n";
    } else {
        std::cout << "Not found\n";
    }

    return 0;
}