什么是在C ++ std :: unordered_map中使用的默认哈希函数？ [英] What is the default hash function used in C++ std::unordered_map?

问题描述

我使用

unordered_map<string, int>

和

unordered_map<int, int>

在每种情况下使用什么哈希函数，在每种情况下什么是碰撞的可能性？
我将在每种情况下分别插入唯一的字符串和唯一的int作为键。

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.

我有兴趣知道hash函数的算法， int键及其冲突状态。

I am interested in knowing the algorithm of hash function in case of string and int keys and their collision stats.

推荐答案

函数对象 std :: hash<> 。

对于所有内置类型和一些其他标准库类型
，如 std :: string 和 std ::线程。请参阅完整列表的链接。

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.

对于在 std :: unordered_map 中使用的其他类型，必须专门化 std :: hash<> 或创建自己的函数对象。

For other types to be used in a std::unordered_map, you will have to specialize std::hash<> or create your own function object.

碰撞的机会是完全依赖于实现的，但是考虑到整数限定在一个定义的范围之间，而字符串在理论上是无限长，我想说有一个更好的机会与字符串碰撞。

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.

对于GCC中的实现，builtin-types的特殊化只返回位模式。以下是 bits / functional_hash.h 中定义的方法：

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)

  /// ...

---

std :: string 的专业化定义为：

#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()); }
    };

进一步的搜索引导我们：

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 是 libstdc ++ 。更多搜索引导我访问此文件，其中说明：

_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/

因此，GCC对字符串使用的默认散列算法是MurmurHashUnaligned2 。

So the default hashing algorithm GCC uses for strings is MurmurHashUnaligned2.

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