I am using
unordered_map
and
unordered_map
What hash function is use
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.
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<const char*>(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<unsigned char>(buf[2]) << 16;
[[gnu::fallthrough]];
case 2:
hash ^= static_cast<unsigned char>(buf[1]) << 8;
[[gnu::fallthrough]];
case 1:
hash ^= static_cast<unsigned char>(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.
The function object std::hash<> is used.
Standard specializations exist for all built-in types, and some other standard library types
such as std::string
and std::thread
. See the link for the full list.
For other types to be used in a std::unordered_map
, you will have to specialize std::hash<>
or create your own function object.
The chance of collision is completely implementation-dependent, but considering the fact that integers are limited between a defined range, while strings are theoretically infinitely long, I'd say there is a much better chance for collision with strings.
As for the implementation in GCC, the specialization for builtin-types just returns the bit pattern. Here's how they are defined in bits/functional_hash.h
:
/// Partial specializations for pointer types.
template<typename _Tp>
struct hash<_Tp*> : public __hash_base<size_t, _Tp*>
{
size_t
operator()(_Tp* __p) const noexcept
{ return reinterpret_cast<size_t>(__p); }
};
// Explicit specializations for integer types.
#define _Cxx_hashtable_define_trivial_hash(_Tp) \
template<> \
struct hash<_Tp> : public __hash_base<size_t, _Tp> \
{ \
size_t \
operator()(_Tp __val) const noexcept \
{ return static_cast<size_t>(__val); } \
};
/// Explicit specialization for bool.
_Cxx_hashtable_define_trivial_hash(bool)
/// Explicit specialization for char.
_Cxx_hashtable_define_trivial_hash(char)
/// ...
The specialization for std::string
is defined as:
#ifndef _GLIBCXX_COMPATIBILITY_CXX0X
/// std::hash specialization for string.
template<>
struct hash<string>
: public __hash_base<size_t, string>
{
size_t
operator()(const string& __s) const noexcept
{ return std::_Hash_impl::hash(__s.data(), __s.length()); }
};
Some further search leads us to:
struct _Hash_impl
{
static size_t
hash(const void* __ptr, size_t __clength,
size_t __seed = static_cast<size_t>(0xc70f6907UL))
{ return _Hash_bytes(__ptr, __clength, __seed); }
...
};
...
// Hash function implementation for the nontrivial specialization.
// All of them are based on a primitive that hashes a pointer to a
// byte array. The actual hash algorithm is not guaranteed to stay
// the same from release to release -- it may be updated or tuned to
// improve hash quality or speed.
size_t
_Hash_bytes(const void* __ptr, size_t __len, size_t __seed);
_Hash_bytes
is an external function from libstdc++
. A bit more searching led me to this file, which states:
// This file defines Hash_bytes, a primitive used for defining hash
// functions. Based on public domain MurmurHashUnaligned2, by Austin
// Appleby. http://murmurhash.googlepages.com/
So the default hashing algorithm GCC uses for strings is MurmurHashUnaligned2.