varusing.cpp

#include <string>
#include <unordered_set>

class Customer
{
private:
  std::string name;
public:
  Customer(std::string const& n) : name(n){}
  std::string getName() const {return name;}
};

struct CustomerEq{
  bool operator()(Customer const& c1, Customer const& c2) const{
    return c1.getName() == c2.getName();
  }
};

struct CustomerHash{
  std::size_t operator()(Customer const& c) const {
    return std::hash<std::string>()(c.getName());
  }
};

// Define a class that combines operator() for variadic base classes
template<typename... Bases>
struct Overloader : Bases...
{
  using Bases::operator()...;//OK since C++17
};

int main()
{
  //Combine hasher and equality for customers in one type
  using CustomerOP = Overloader<CustomerHash, CustomerEq>;

  std::unordered_set<Customer, CustomerHash, CustomerEq> coll1;
  std::unordered_set<Customer, CustomerOP, CustomerOP> coll2;
}