Which operator should be overloaded in the following code to make the program error free?

#include <iostream>
#include <string>
using namespace std;
class Box{
    int capacity;
public:
    Box(){}
    Box(double capacity){
	this->capacity = capacity;
    }
};
int main(int argc, char const *argv[])
{
    Box b1(10);
    Box b2 = Box(14);
    if(b1 == b2){
	cout<<"Equal";
    }
    else{
 	cout<<"Not Equal";
    }
    return 0;
}