c++ Posterior Decrement Operator Overloading Value Passing Problems

Question

I'm using vs2022 and I'm confused about the c++ post-decrement operator overloading value passing, why does overloading a function that returns a value instead of a reference also affect the object itself?

I tried to return a reference, aka MyInteger& operator--(int), and the output is consistent with a value return, both changing the value of the object itself. Why does the post-decrement operator have to return a value instead of a reference, and why does returning a value also change the value of the object itself?

#include <iostream>
using namespace std;

class MyInteger {
    friend ostream& operator<<(ostream& cout, MyInteger myint);
public:
    MyInteger() {
        m_int = 0;
    }
    MyInteger& operator--() {
       m_int--;
        return *this;
    }
    MyInteger operator--(int) {
        MyInteger myint  = *this;
        //myint = *this;
        m_int--;
        return myint;
    }
private:
    int m_int;
};

ostream& operator<<(ostream& cout, MyInteger myint) {
    cout << myint.m_int;
    return cout;
}

void test() {
    MyInteger myint;
    cout << myint << endl;
    cout << --myint << endl;
    cout << myint << endl;
    cout << myint-- << endl;
    cout << (myint--)-- << endl;
    cout << myint << endl;
}

int main()  
{
    test();
    system("pause");
    system("cls");
    return 0;
}