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

EN

#include 
#include 
#include 
#include 

int main()
{
    using key_type = std::pair;
    std::set s { {1, -1}, {1, 3}, {2, 10}, {3, 42} };
    auto it = s.upper_bound (key_type (2, std::numeric_limits::min ()));
    std::cout << "(" << it->first << "; " << it->second << ")\n";
}

auto fn = [](const pair&a, const pair &b) {
  return a.first < b.first;
};
set, decltype(fn)> my_set(fn);

#include 
#include 
#include 

using namespace std;

int main()
{
    auto fn = [](const pair&a, const pair &b) {
      return a.first < b.first;
    };
    set, decltype(fn)> my_set(fn);
    
    my_set.insert({1, 123});
    my_set.insert({4, 456});
    my_set.insert({7, 789});
    
    auto iter = my_set.find({4, 0});
    if (iter != my_set.end()) {
      cout << "first: " << iter->first << ", second: " << iter->second << "\n";
    } else {
      cout << "not found\n";
    }

    return 0;
}

first: 4, second: 456