为什么这种运算符重载方法在这里不起作用？

Question

// program in c++ to use priority_queue with class
#include <iostream>
#include <queue>
using namespace std;

#define ROW 5
#define COL 2

class Person {

public:
    int age;

    float height;

    // this is used to initialize the variables of the class
    Person(int age, float height)
        : age(age), height(height)
    {
    }

    bool operator < (const Person& p1) {
        return (this->height > p1.height);
    }
};


// bool operator<(const Person& p1, const Person& p2)
// {
//  return p1.height > p2.height;
// }

// struct myCmp
// {
//     bool operator() (const Person& p1, const Person& p2) {
//         return p1.height > p2.height;
//     }
// };


int main()
{

    priority_queue<Person> Q;

    float arr[ROW][COL] = { { 30, 5.5 }, { 25, 5 },
            { 20, 6 }, { 33, 6.1 }, { 23, 5.6 } };

    for (int i = 0; i < ROW; ++i) {

        Q.push(Person(arr[i][0], arr[i][1]));

        // insert an object in priority_queue by using
        // the Person class constructor
    }

    while (!Q.empty()) {

        Person p = Q.top();

        Q.pop();

        cout << p.age << " " << p.height << "\n";
    }
    return 0;
}

Note! The other commented methods in the code seem to work except this gives me an error.

The above code is to make a priority queue with the intention that the persons with lower height are kept at the top. I tried using struct method for defining the custom compare function which seems to work fine. Explicit operator overloading like in the above comments also works.

Expected output:

25 5

30 5.5

23 5.6

20 6

33 6.1

ERROR:

no match for 'operator<' (operand types are 'const Person' and 'const Person') { return __x < __y; }

Answer 1

The error message:

no match for 'operator<' (operand types are 'const Person' and 'const Person')

This tells you that the lhs and rhs object of operator are const objects. and the compiler can not find an operator that will work two const Person objects.

If I look at your implementation:

bool operator < (const Person& p1) {
    return (this->height > p1.height);
}

I see that the right-hand value p1 can be a const reference. But the left-hand value (the owner of the method) is being treated as non cost. So this implementation does not match the requirements needed.

But we know this operator is not changing the state of the object so we can simply mark this as a const member function.

bool operator < (const Person& p1) const {
                             //    ^^^^^    Add the const here.
    return (this->height > p1.height);
}