lang/lambdahash.cpp

The following code example is taken from the book
C++20 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2021
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <iostream>
#include <string>
#include <unordered_set>

class Customer
{
  std::string name;
public:
  Customer(const std::string& n)
   : name{n} {
  }
  Customer(const char* n)
   : name{n} {
  }
  //...
  std::string getName() const {
    return name;
  }
  bool operator== (const Customer& c) const {
    return name == c.name;
  }
  //...
};
  
int main()
{
  // create hash table with user-defined hash function:
  std::unordered_set<Customer,
                     decltype([] (const Customer& c) {
                                return std::hash<std::string>{}(c.getName());
                              })> coll;  // OK since C++20

  coll.insert({"Tom", "Andrea", "Sam"});
  coll.insert({"Kim", "Hans", "Sarah", "Bill"});
  
  for (const auto& c : coll) {
    std::cout << c.getName() << '\n';
  }
}