I am using
unordered_map<string, int>
and
unordered_map<int, int>
What hash function is used in each case and what is chance of collision in each case? I will be inserting unique string and unique int as keys in each case respectively.
I am interested in knowing the algorithm of hash function in case of string and int keys and their collision stats.
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.