Hash function errors, c++ - c++

Question

Here's the simple example class with hash function I wrote. Function maybe isn't most efficient one, but quality of hash function isn't relevant to me for now.
#include<iostream>
#include<unordered_set>
using namespace std;
class Class{
private:
int num;
public:
Class(int n){num=n;}
Class(){num=0;}
int getNum(){return num;}
friend bool operator==(const Class &k1, const Class &k2);
};
bool operator==(const Class &k1, const Class &k2){
return(k1.num == k2.num);
}
namespace std {
template <>
struct hash<Class>{
size_t operator()(const Class & c) const
{
return(31+c.getNum()*7);
}
};
}
void main(){
unordered_set<Class> set;
set.insert(Class(5));
set.insert(Class(55));
set.insert(Class(4));
set.insert(Class(123));
set.insert(Class(11));
for(unordered_set<Class>::iterator it = set.begin(); it!=set.end(); it++)
cout<<it->getNum()<< endl;
}
This worked fine when all fields of Class were public, but errors appear in hash function at return(31+c.getNum()*7); on c, because I can't call getNum() function. I don't know if there is any problem if in line size_t operator()(const Class & c) const I remove const, so object c is not const?
Also, in the last line, cout<<it->getNum()<< endl, there is error when accessing getNum() function. I don't know how to iterate over unordered_set set and to print num.

Answer 1

Your operator takes a const class but calls a non-const member function. To fix your issue declare getNum as const since it does not modify the class.
class Class{
private:
int num;
public:
Class(int n) : num(n) {}
Class() : num(0) {}
int getNum() const { return num;}
friend bool operator==(const Class &k1, const Class &k2);
};
In the above example also notice that I used member initializer lists to instantiate member variables. Also change the return type from void to int on main and I recommend either returning 0 or EXIT_SUCCESS from <cstdlib> at the end of main.