What is the default hash function used in C++ std::unordered_map?

Question

I am using

unordered_map

and

unordered_map

What hash function is use

Answer 1

Though the hashing algorithms are compiler-dependent, I'll present it for GCC C++11. @Avidan Borisov astutely discovered that the GCC hashing algorithm used for strings is "MurmurHashUnaligned2," by Austin Appleby. I did some searching and found a mirrored copy of GCC on Github. Therefore:

The GCC C++11 hashing functions used for unordered_map (a hash table template) and unordered_set (a hash set template) appear to be as follows.

• Thanks to Avidan Borisov for his background research which on the question of what are the GCC C++11 hash functions used, stating that GCC uses an implementation of "MurmurHashUnaligned2", by Austin Appleby (http://murmurhash.googlepages.com/).
• In the file "gcc/libstdc++-v3/libsupc++/hash_bytes.cc", here (https://github.com/gcc-mirror/gcc/blob/master/libstdc++-v3/libsupc++/hash_bytes.cc), I found the implementations. Here's the one for the "32-bit size_t" return value, for example (pulled 11 Aug 2017)

Code:

// Implementation of Murmur hash for 32-bit size_t.
size_t _Hash_bytes(const void* ptr, size_t len, size_t seed)
{
  const size_t m = 0x5bd1e995;
  size_t hash = seed ^ len;
  const char* buf = static_cast(ptr);

  // Mix 4 bytes at a time into the hash.
  while (len >= 4)
  {
    size_t k = unaligned_load(buf);
    k *= m;
    k ^= k >> 24;
    k *= m;
    hash *= m;
    hash ^= k;
    buf += 4;
    len -= 4;
  }

  // Handle the last few bytes of the input array.
  switch (len)
  {
    case 3:
      hash ^= static_cast(buf[2]) << 16;
      [[gnu::fallthrough]];
    case 2:
      hash ^= static_cast(buf[1]) << 8;
      [[gnu::fallthrough]];
    case 1:
      hash ^= static_cast(buf[0]);
      hash *= m;
  };

  // Do a few final mixes of the hash.
  hash ^= hash >> 13;
  hash *= m;
  hash ^= hash >> 15;
  return hash;
}

For additional hashing functions, including djb2, and the 2 versions of the K&R hashing functions (one apparently terrible, one pretty good), see my other answer here: https://stackoverflow.com/a/45641002/4561887.