What is the best way to use a HashMap in C++?

Question

I know that STL has a HashMap API, but I cannot find any good and thorough documentation with good examples regarding this.

Any good examples will be appreciated.

Answer 1

Evidence that std::unordered_map uses a hash map in GCC stdlibc++ 6.4

This was mentioned at: https://stackoverflow.com/a/3578247/895245 but in the following answer: What data structure is inside std::map in C++? I have given further evidence of such for the GCC stdlibc++ 6.4 implementation by:

• GDB step debugging into the class
• performance characteristic analysis

Here is a preview of the performance characteristic graph described in that answer:

How to use a custom class and hash function with unordered_map

This answer nails it: C++ unordered_map using a custom class type as the key

Excerpt: equality:

struct Key
{
  std::string first;
  std::string second;
  int         third;

  bool operator==(const Key &other) const
  { return (first == other.first
            && second == other.second
            && third == other.third);
  }
};

Hash function:

namespace std {

  template <>
  struct hash
  {
    std::size_t operator()(const Key& k) const
    {
      using std::size_t;
      using std::hash;
      using std::string;

      // Compute individual hash values for first,
      // second and third and combine them using XOR
      // and bit shifting:

      return ((hash()(k.first)
               ^ (hash()(k.second) << 1)) >> 1)
               ^ (hash()(k.third) << 1);
    }
  };

}