[c++11]hash<T>哈希结构模板

一、哈希结构模板hash简介

<functional>

template<class _Kty>
struct _Bitwise_hash : public unary_function<_Kty, size_t>
{...};

hash<Key>()(k1) == hash<Key>()(k2)

hash<Key>()(k1) == hash<Key>()(k2)

1.0/std::numeric_limits<size_t>::max()

unordered_set，unordered_multiset，unordered_map

unordered_multimap

二、成员

成员类型

argument_type

result_type

size_t

公有成员方法

size_t operator()(T &t)

三、扩展和应用

template<> struct hash<bool>;
template<> struct hash<char>;
template<> struct hash<signed char>;
template<> struct hash<unsigned char>;
template<> struct hash<char16_t>;
template<> struct hash<char32_t>;
template<> struct hash<wchar_t>;
template<> struct hash<short>;
template<> struct hash<unsigned short>;
template<> struct hash<int>;
template<> struct hash<unsigned int>;
template<> struct hash<long>;
template<> struct hash<long long>;
template<> struct hash<unsigned long>;
template<> struct hash<unsigned long long>;
template<> struct hash<float>;
template<> struct hash<double>;
template<> struct hash<long double>;
template< class T > struct hash<T*>;

std::hash<std::string>
std::hash<std::u16string>
std::hash<std::u32string>
std::hash<std::wstring>

std::hash<std::error_code>

std::hash<std::bitset>
std::hash<std::unique_ptr>
std::hash<std::shared_ptr>
std::hash<std::type_index>
std::hash<std::vector<bool>>
std::hash<std::thread::id>

四、代码示例

#include <iostream>
#include <functional>//
#include <string>
#include <stdlib.h>

using std::hash;//
using std::string;
using std::cout;

//自定义类型
class S
{
public:
string first_name;
string last_name;
};
//自己封装一个哈希函数对象的类型，内部使用了hash结构模板
class MyHash
{
public:
size_t operator()(const S &s) const
{
size_t h1 = hash<string>()(s.first_name);
size_t h2 = hash<string>()(s.last_name);
return h1 ^ (h2 << 1);
}
};
//也可以用自定义类实例化一个hash结构模板
template<>
class hash < S >
{
public:
size_t operator()(const S &s) const
{
size_t h1 = hash<string>()(s.first_name);
size_t h2 = hash<string>()(s.last_name);
return h1 ^ (h2 << 1);
}
};

int main()
{
cout << "计算string的散列值的示例：\n";
string str = "Meet the new boss...";
hash<string> hash_fn;
size_t str_hash = hash_fn(str);

cout << str_hash << '\n';

cout << "\n计算自定义类型S的散列值的示例：\n";
string s1 = "Hubert";
string s2 = "Farnsworth";
hash<string> h1;
S obj_S;
obj_S.first_name = s1;
obj_S.last_name = s2;

cout << "hash(s1) = \t" << h1(s1) << "\n"
<< "hash(s2) = \t" << hash<string>()(s2) << "\n"
<< "MyHash(obj_S) = " << MyHash()(obj_S) << "\n"
<< "hash(obj_S) = \t" << hash<S>()(obj_S) << "\n";

::system("pause");
return 0;
}