Trying to print in c++ but receiving "error: invalid operands to binary expression ('std::ostream' (aka 'basic_ostream<char>') and 'Minimum')"

Question

I have to implement this simple Minimum class which keeps track of min and total, however I keep receiving this error: "error: invalid operands to binary expression ('std::ostream' (aka 'basic_ostream') and 'Minimum')". Is there a way to solve this issue? And it looks like it's coming from overloading << operator but I can't see where the problem is. Any help would be appreciated thank you!

#include <iostream>
#include <climits>
using namespace std;


class Minimum {
private : 
    int min;
    int total;
public :
    Minimum(int m = INT_MAX, int t = 0){
        min = m;
        total= t;
    }


    friend ostream& operator<<(ostream& os,  Minimum&  m) {
        os << "Total = " << m.total << ", " << "min = " << m.min;
        return os;
    }


    Minimum& operator+=(int num) {
        total += num;
        if (num < min) {
            num = min;
        }
        return *this;
    }


    Minimum& operator++() {
        total++;
        return *this;
    }


    Minimum operator++(int) {
        Minimum temp = *this;
        total++;
        return *this;
    }


    bool operator==(const Minimum& m) const {
        return ((total == m.total) && (min == m.min));
    }


    bool operator!=(const Minimum& m) const {
        return !(*this == m);
    }

};

int main() {
    Minimum m;
    cout << m << endl;
    m +=8;
    cout << m << endl;
    m +=6;
    cout << m << endl;
    m += 4;
    cout << m << endl;
    m += 5;
    cout << m << endl;
    cout << m++ << endl;
    cout << m << endl;
    cout << ++m << endl;
    cout << m << endl;
    (m += -10) += 3; // 2 calls chained together
    cout << m << endl;
    Minimum copy = m;
    cout << copy << endl;
    if (m != copy)
        cout << "Different" << endl;
    else
        cout << "Equal" << endl;
}

Answer 1

You get this error here:

cout << m++ << endl;

That's because m++ returns a Minimum r-value and the operator you defined expects a non-const l-value to Minimum (ie Minimum& ). Change it to:

friend ostream& operator<<(ostream& os, Minimum const & m)

This works because const l-values bind to r-values.

---

Also, most probably you have a bug in your post-increment operator as you should return the old value:

Minimum operator++(int) {
    Minimum temp = *this;
    total++;
    //return *this;
    return temp;
}