unorder_set针对int类型的默认哈希函数是什么呢？
我们可以看到，桶的大小是8，我们存进去了0-7这8个不冲突的数字。假如哈希函数是将数字映射到桶中，那么0号桶应保存数字0，其余数字类似。那最后，挨个访问桶时的输出应是0，1，2，3，4，5，6，7。但是从输出结果来看并不是这样，请问为什么呢？
这是我在我的本机functional_hash.h中找到的哈希映射代码。
有关该源码的详细解析，请参见：http://hustsxh.is-programmer.com/posts/82605.html
// functional_hash.h header -*- C++ -*-
// Copyright (C) 2007-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library. This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
// <http://www.gnu.org/licenses/>.
/** @file bits/functional_hash.h
* This is an internal header file, included by other library headers.
* Do not attempt to use it directly. @headername{functional}
*/
#ifndef _FUNCTIONAL_HASH_H
#define _FUNCTIONAL_HASH_H 1
#pragma GCC system_header
#include <bits/hash_bytes.h>
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
/** @defgroup hashes Hashes
* @ingroup functors
*
* Hashing functors taking a variable type and returning a @c std::size_t.
*
* @{
*/
template<typename _Result, typename _Arg>
struct __hash_base
{
typedef _Result result_type _GLIBCXX17_DEPRECATED;
typedef _Arg argument_type _GLIBCXX17_DEPRECATED;
};
/// Primary class template hash.
template<typename _Tp>
struct hash;
template<typename _Tp, typename = void>
struct __poison_hash
{
static constexpr bool __enable_hash_call = false;
private:
// Private rather than deleted to be non-trivially-copyable.
__poison_hash(__poison_hash&&);
~__poison_hash();
};
template<typename _Tp>
struct __poison_hash<_Tp, __void_t<decltype(hash<_Tp>()(declval<_Tp>()))>>
{
static constexpr bool __enable_hash_call = true;
};
// Helper struct for SFINAE-poisoning non-enum types.
template<typename _Tp, bool = is_enum<_Tp>::value>
struct __hash_enum
{
private:
// Private rather than deleted to be non-trivially-copyable.
__hash_enum(__hash_enum&&);
~__hash_enum();
};
// Helper struct for hash with enum types.
template<typename _Tp>
struct __hash_enum<_Tp, true> : public __hash_base<size_t, _Tp>
{
size_t
operator()(_Tp __val) const noexcept
{
using __type = typename underlying_type<_Tp>::type;
return hash<__type>{}(static_cast<__type>(__val));
}
};
/// Primary class template hash, usable for enum types only.
// Use with non-enum types still SFINAES.
template<typename _Tp>
struct hash : __hash_enum<_Tp>
{ };
/// 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)
/// Explicit specialization for signed char.
_Cxx_hashtable_define_trivial_hash(signed char)
/// Explicit specialization for unsigned char.
_Cxx_hashtable_define_trivial_hash(unsigned char)
/// Explicit specialization for wchar_t.
_Cxx_hashtable_define_trivial_hash(wchar_t)
/// Explicit specialization for char16_t.
_Cxx_hashtable_define_trivial_hash(char16_t)
/// Explicit specialization for char32_t.
_Cxx_hashtable_define_trivial_hash(char32_t)
/// Explicit specialization for short.
_Cxx_hashtable_define_trivial_hash(short)
/// Explicit specialization for int.
_Cxx_hashtable_define_trivial_hash(int)
/// Explicit specialization for long.
_Cxx_hashtable_define_trivial_hash(long)
/// Explicit specialization for long long.
_Cxx_hashtable_define_trivial_hash(long long)
/// Explicit specialization for unsigned short.
_Cxx_hashtable_define_trivial_hash(unsigned short)
/// Explicit specialization for unsigned int.
_Cxx_hashtable_define_trivial_hash(unsigned int)
/// Explicit specialization for unsigned long.
_Cxx_hashtable_define_trivial_hash(unsigned long)
/// Explicit specialization for unsigned long long.
_Cxx_hashtable_define_trivial_hash(unsigned long long)
#ifdef __GLIBCXX_TYPE_INT_N_0
_Cxx_hashtable_define_trivial_hash(__GLIBCXX_TYPE_INT_N_0)
_Cxx_hashtable_define_trivial_hash(__GLIBCXX_TYPE_INT_N_0 unsigned)
#endif
#ifdef __GLIBCXX_TYPE_INT_N_1
_Cxx_hashtable_define_trivial_hash(__GLIBCXX_TYPE_INT_N_1)
_Cxx_hashtable_define_trivial_hash(__GLIBCXX_TYPE_INT_N_1 unsigned)
#endif
#ifdef __GLIBCXX_TYPE_INT_N_2
_Cxx_hashtable_define_trivial_hash(__GLIBCXX_TYPE_INT_N_2)
_Cxx_hashtable_define_trivial_hash(__GLIBCXX_TYPE_INT_N_2 unsigned)
#endif
#ifdef __GLIBCXX_TYPE_INT_N_3
_Cxx_hashtable_define_trivial_hash(__GLIBCXX_TYPE_INT_N_3)
_Cxx_hashtable_define_trivial_hash(__GLIBCXX_TYPE_INT_N_3 unsigned)
#endif
#undef _Cxx_hashtable_define_trivial_hash
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); }
template<typename _Tp>
static size_t
hash(const _Tp& __val)
{ return hash(&__val, sizeof(__val)); }
template<typename _Tp>
static size_t
__hash_combine(const _Tp& __val, size_t __hash)
{ return hash(&__val, sizeof(__val), __hash); }
};
// A hash function similar to FNV-1a (see PR59406 for how it differs).
struct _Fnv_hash_impl
{
static size_t
hash(const void* __ptr, size_t __clength,
size_t __seed = static_cast<size_t>(2166136261UL))
{ return _Fnv_hash_bytes(__ptr, __clength, __seed); }
template<typename _Tp>
static size_t
hash(const _Tp& __val)
{ return hash(&__val, sizeof(__val)); }
template<typename _Tp>
static size_t
__hash_combine(const _Tp& __val, size_t __hash)
{ return hash(&__val, sizeof(__val), __hash); }
};
/// Specialization for float.
template<>
struct hash<float> : public __hash_base<size_t, float>
{
size_t
operator()(float __val) const noexcept
{
// 0 and -0 both hash to zero.
return __val != 0.0f ? std::_Hash_impl::hash(__val) : 0;
}
};
/// Specialization for double.
template<>
struct hash<double> : public __hash_base<size_t, double>
{
size_t
operator()(double __val) const noexcept
{
// 0 and -0 both hash to zero.
return __val != 0.0 ? std::_Hash_impl::hash(__val) : 0;
}
};
/// Specialization for long double.
template<>
struct hash<long double>
: public __hash_base<size_t, long double>
{
_GLIBCXX_PURE size_t
operator()(long double __val) const noexcept;
};
// @} group hashes
// Hint about performance of hash functor. If not fast the hash-based
// containers will cache the hash code.
// Default behavior is to consider that hashers are fast unless specified
// otherwise.
template<typename _Hash>
struct __is_fast_hash : public std::true_type
{ };
template<>
struct __is_fast_hash<hash<long double>> : public std::false_type
{ };
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
#endif // _FUNCTIONAL_HASH_H
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