我想看看比较器在cpp中是如何工作的。所以当我插入s1时，比较器操作符没有被调用，这是有道理的。但是当s2被插入时，比较器操作符被调用了两次。为什么？当我检查时，我发现两次调用它的第一个参数都是s2，而第二个参数是s1。有人能解释一下吗？

#include <set>
#include <iostream>
#include <string>

using std::string;

// Student Class
class student {
public:
    // To store Name and Roll Number
    string name;
    int rollnum;

    // Overloaded Constructor
    student(string name, int rollnum)
    {
        this->name = name;
        this->rollnum = rollnum;
    }
};

// Comparator Class to compare 2 objects
class studentcompare {
public:
    // Comparator function
    bool operator()(const student& a,
                    const student& b) const
    {
        std::cout << a.name << "::" << b.name << std::endl;
        return a.name < b.name;
    }
};

// Driver Code
int main()
{
    // Object of class student
    student s1("Raj", 23);
    student s2("Prerna", 24);

    std::set<student, studentcompare> s;
    s.insert(s1);
    s.insert(s2);
    return 0;
}

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