The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017
#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 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;
//...
}