C/C++ __gnu_cxx hash_set

In this post, we go about __gnu_cxx::hash_set. As we saw before, we need to define our own HashFcn. We'll also need to define EqualKey if we're using custom data types. Another workaround is to overload the == operator inside the std namespace, then you don't have to provide your own EqualKey. It should look like:

namespace std {
  bool operator==(const dnsRecord& rec1, const dnsRecord& rec2) {
    return rec1.ip == rec2.ip;
  }
}

However, I prefer the first solution, and we'll use it in the code below. We also define our own hasher and pass it to the hash_set declaration instead of specializing the hash template inside the __gnu_cxx namespace as we did in the previous post.

#include <iostream>
#include <ext/hash_set>

using namespace std;
using namespace __gnu_cxx;

class dnsRecord {
public:
  unsigned long ip;
  string domainName;
};

class MyHasher {
public:
  size_t operator()(const dnsRecord &r) const
  {
    return h(r.domainName.c_str());
  };

private:
  __gnu_cxx::hash<char*> h;
};

class MyComparator
{
public:
  bool operator()(const dnsRecord& rec1, const dnsRecord& rec2) const {
    return rec1.ip == rec2.ip;
  }
};

int main(int argc, char *argv[])
{
  hash_set<dnsRecord, MyHasher, MyComparator> hset;
  dnsRecord rec1;
  dnsRecord rec2;
  dnsRecord rec3;
  rec1.ip = 989798l; rec1.domainName = "dname1";
  rec2.ip = 112334l; rec2.domainName = "dname2";
  rec2.ip = 808323l; rec3.domainName = "dname3";
  hset.insert(rec1);
  hset.insert(rec2);
  hset.insert(rec3);

  dnsRecord rec4;
  rec4.ip = 0;
  rec4.domainName = "dname1";

  hash_set<dnsRecord, MyHasher, MyComparator>::iterator it;
  for(it = hset.begin(); it != hset.end(); it++)
    cout << (*it).ip << ", " << (*it).domainName << endl;

  rec4.domainName = "dname3";
  hset.erase(rec4);

  rec4.domainName = "dname3";
  if(hset.find(rec4) == hset.end())
    cout << "Successfully removed dname3" << endl;
  else
    cout << "Error!" << endl;

  return EXIT_SUCCESS;
}