C++11 新特性： unordered_map 与 map 的对比

unordered_map和map类似，都是存储的key-value的值，可以通过key快速索引到value。不同的是unordered_map不会根据key的大小进行排序，

存储时是根据key的hash值判断元素是否相同，即unordered_map内部元素是无序的，而map中的元素是按照二叉搜索树存储，进行中序遍历会得到有序遍历。

所以使用时map的key需要定义operator<。而unordered_map需要定义hash_value函数并且重载operator==。但是很多系统内置的数据类型都自带这些，

那么如果是自定义类型，那么就需要自己重载operator<或者hash_value()了。

结论：如果需要内部元素自动排序，使用map，不需要排序使用unordered_map

map使用案例：

#include<string>  
#include<iostream>  
#include<map>  
  
using namespace std;  
  
struct person  
{  
    string name;  
    int age;  
  
    person(string name, int age)  
    {  
        this->name =  name;  
        this->age = age;  
    }  
  
    bool operator < (const person& p) const  
    {  
        return this->age < p.age;   
    }  
};  
  
map<person,int> m;  
int main()  
{  
    person p1("Tom1",20);  
    person p2("Tom2",22);  
    person p3("Tom3",22);  
    person p4("Tom4",23);  
    person p5("Tom5",24);  
    m.insert(make_pair(p3, 100));  
    m.insert(make_pair(p4, 100));  
    m.insert(make_pair(p5, 100));  
    m.insert(make_pair(p1, 100));  
    m.insert(make_pair(p2, 100));  
      
    for(map<person, int>::iterator iter = m.begin(); iter != m.end(); iter++)  
    {  
        cout<<iter->first.name<<"	"<<iter->first.age<<endl;  
    }  
      
    return 0;  
} 

输出为：(根据age进行了排序的结果)

Tom1    20
Tom3    22
Tom4    23
Tom5    24
因为Tom2和Tom3的age相同，由我们定义的operator<只是比较的age，所以Tom3覆盖了Tom2，结果中没有Tom2。

如果运算符<的重载是如下

bool operator < (const person &p)const{
    return this->name < p.name;  
}

输出结果： 按照 那么进行的排序，如果有那么相同则原来的那么会被覆盖

Tom1    20

Tom2    22

Tom3    22

Tom4    23

Tom5    24

#include<string>  
#include<iostream>  
#include<map>  
  
using namespace std;  
  
struct person  
{  
    string name;  
    int age;  
  
    person(string name, int age)  
    {  
        this->name =  name;  
        this->age = age;  
    }  
  
   bool operator < (const person &p)const
   {
    return this->name < p.name;  
   } 
};  
  
map<person,int> m;  
int main()  
{  
    person p1("Tom1",20);  
    person p2("Tom2",22);  
    person p3("Tom3",22);  
    person p4("Tom4",23);  
    person p5("Tom5",24);  
    person p5_2("Tom5",25); 
    m.insert(make_pair(p3, 100));  
    m.insert(make_pair(p4, 100));  
    m.insert(make_pair(p5, 100));  
    m.insert(make_pair(p1, 100));  
    m.insert(make_pair(p2, 100));  
    m.insert(make_pair(p5_2, 100));  
    for(map<person, int>::iterator iter = m.begin(); iter != m.end(); iter++)  
    {  
        cout<<iter->first.name<<"	"<<iter->first.age<<endl;  
    }  
      
    return 0;  
} 

覆盖

root@ubuntu:~/c++# ./test
Tom1    20
Tom2    22
Tom3    22
Tom4    23
Tom5    24

unordered_map使用案例：

#include <iostream>
#include <unordered_map>

#include <boost/functional/hash.hpp>

struct KeyData
{
  int id;
  int age;

  // equality
  bool operator == (const KeyData &other) const {
    return (id == other.id) && (age == other.age);
  }
};

struct KeyDataHasher
{
  std::size_t operator () (const KeyData &key) const 
  {
    // The following line is a stright forward implementation. But it can be
    // hard to design a good hash function if KeyData is complex.

    //return (key.id << 32 | key.age); // suppose size_t is 64-bit and int is 32-bit

    // A commonly used way is to use boost
    std::size_t seed = 0;
    boost::hash_combine(seed, boost::hash_value(key.id));
    boost::hash_combine(seed, boost::hash_value(key.age));
    return seed;
  }
};

using namespace std;

int main(int argc, const char *argv[])
{
  KeyData k1{0, 30}, k2{1, 1}, k3{2, 0};

  // Print the hash results
  KeyDataHasher hasher;
  cout << hasher(k1) << endl 
    << hasher(k2) << endl
    << hasher(k3) << endl;

  // Construct an unordered_map using KeyData, which maps
  // KeyData -> int, using KeyDataHasher as hash function
  typedef std::unordered_map<KeyData, int, KeyDataHasher> KeyDataHashMap;
  KeyDataHashMap mapping{ {k1, 1}, {k2, 2}, {k2, 3} };

  for (auto &kv : mapping) {
    cout << "id:" << kv.first.id 
      << " age:" << kv.first.age 
      << " value:" << kv.second << endl;
  }

  return 0;
}

重复的元素覆盖了

root@ubuntu:~/c++# g++ -std=c++11 custom_hash.cpp -o custom_hash
root@ubuntu:~/c++# ./custom_hash 
12332917005744336841
18297303017781972032
1128965059474579952
id:1 age:1 value:2
id:0 age:30 value:1
root@ubuntu:~/c++# 

#include<string>  
#include<iostream>  
#include<unordered_map>  
#include <boost/functional/hash.hpp>
using namespace std;  
  
struct person  
{  
    string name;  
    int age;  
  
    person(string name, int age)  
    {  
        this->name =  name;  
        this->age = age;  
    }  
  
    bool operator== (const person& p) const  
    {  
        return name==p.name && age==p.age;  
    }  
};  
struct KeyDataHasher
{
size_t operator()(const person& p) const 
{  
    size_t seed = 0;  
    boost::hash_combine(seed, boost::hash_value(p.name));  
    boost::hash_combine(seed, boost::hash_value(p.age));  
    return seed;  
}  
};
  
int main()  
{  
    typedef std::unordered_map<person,int,KeyDataHasher> umap;  
    umap m;  
    person p1("Tom1",20);  
    person p2("Tom2",22);  
    person p3("Tom3",22);  
    person p4("Tom4",23);  
    person p5("Tom5",24);  
    m.insert(umap::value_type(p3, 100));  
    m.insert(umap::value_type(p4, 100));  
    m.insert(umap::value_type(p5, 100));  
    m.insert(umap::value_type(p1, 100));  
    m.insert(umap::value_type(p2, 100));  
      
    for(umap::iterator iter = m.begin(); iter != m.end(); iter++)  
    {  
        cout<<iter->first.name<<"	"<<iter->first.age<<endl;  
    }  
      
    return 0;  
}  

root@ubuntu:~/c++# g++ -std=c++11 map_test.cpp  -o test
root@ubuntu:~/c++# ./test
Tom2    22
Tom1    20
Tom5    24
Tom3    22
Tom4    23

#include <iostream>
#include <unordered_map>
#include <utility>
 
typedef std::pair<std::string, std::string> pair;
 
struct pair_hash
{
    template <class T1, class T2>
    std::size_t operator() (const std::pair<T1, T2> &pair) const {
        return std::hash<T1>()(pair.first) ^ std::hash<T2>()(pair.second);
    }
};
 
int main()
{
    std::unordered_map<pair, int, pair_hash> unordered_map =
    {
        {{"C++", "C++11"}, 2011},
        {{"C++", "C++14"}, 2014},
        {{"C++", "C++17"}, 2017},
        {{"Java", "Java 7"}, 2011},
        {{"Java", "Java 8"}, 2014},
        {{"Java", "Java 9"}, 2017}
    };
 
    for (auto const &entry: unordered_map)
    {
        auto key_pair = entry.first;
        std::cout << "{" << key_pair.first << "," << key_pair.second << "}, "
                  << entry.second << std::endl;
    }
 
    return 0;
}

  
int main()  
{  
    typedef std::unordered_map<person,int,KeyDataHasher> umap;  
    umap m;  
    person p1("Tom1",20);  
    person p2("Tom2",22);  
    person p3("Tom3",22);  
    person p4("Tom4",23);  
    person p5("Tom5",24);  
    m.insert(umap::value_type(p3, 100));  
    m.insert(umap::value_type(p4, 100));  
    m.insert(umap::value_type(p5, 100));  
    m.insert(umap::value_type(p1, 100));  
    m.insert(umap::value_type(p2, 100));  
    m.insert(umap::value_type(p2, 100));  
      
    for(umap::iterator iter = m.begin(); iter != m.end(); iter++)  
    {  
        cout<<iter->first.name<<"	"<<iter->first.age<<endl;  
    }  
      
    return 0;  
}  

覆盖

root@ubuntu:~/c++# ./test
Tom2    22
Tom1    20
Tom5    24
Tom3    22
Tom4    23

#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
#include <utility>
 
typedef std::pair<std::string, std::string> pair;
 
int main()
{
    std::unordered_map<pair, int, boost::hash<pair>> unordered_map =
    {
        {{"C++", "C++11"}, 2011},
        {{"C++", "C++14"}, 2014},
        {{"C++", "C++17"}, 2017},
        {{"Java", "Java 7"}, 2011},
        {{"Java", "Java 8"}, 2014},
        {{"Java", "Java 9"}, 2017}
    };
 
    for (auto const &entry: unordered_map)
    {
        auto key_pair = entry.first;
        std::cout << "{" << key_pair.first << "," << key_pair.second << "}, "
                  << entry.second << std::endl;
    }
 
    return 0;
}

#include <iostream>
#include <boost/functional/hash.hpp>
#include <unordered_map>
#include <utility>
 
typedef std::pair<std::string, std::string> pair;
 
int main()
{
    std::unordered_map<pair, int, boost::hash<pair>> unordered_map =
    {
        {{"C++", "C++11"}, 2011},
        {{"C++", "C++14"}, 2014},
        {{"C++", "C++17"}, 2017},
        {{"Java", "Java 7"}, 2011},
        {{"Java", "Java 8"}, 2014},
        {{"Java", "Java 9"}, 2017}
    };
 
    for (auto const &entry: unordered_map)
    {
        auto key_pair = entry.first;
        std::cout << "{" << key_pair.first << "," << key_pair.second << "}, "
                  << entry.second << std::endl;
    }
 
    return 0;
}

root@ubuntu:~/c++# g++ -std=c++11 hash.cpp -o test
root@ubuntu:~/c++# ./test
{Java,Java 8}, 2014
{Java,Java 9}, 2017
{Java,Java 7}, 2011
{C++,C++17}, 2017
{C++,C++14}, 2014
{C++,C++11}, 2011
root@ubuntu:~/c++#

#include <iostream>
#include <unordered_map>
#include <utility>
 
typedef std::pair<std::string, std::string> pair;
 
struct pair_hash
{
    template <class T1, class T2>
    std::size_t operator() (const std::pair<T1, T2> &pair) const {
        return std::hash<T1>()(pair.first) ^ std::hash<T2>()(pair.second);
    }
};
 
int main()
{
    std::unordered_map<pair, int, pair_hash> unordered_map =
    {
        {{"C++", "C++11"}, 2011},
        {{"C++", "C++14"}, 2014},
        {{"C++", "C++17"}, 2017},
        {{"Java", "Java 7"}, 2011},
        {{"Java", "Java 8"}, 2014},
        {{"Java", "Java 9"}, 2017}
    };
 
    for (auto const &entry: unordered_map)
    {
        auto key_pair = entry.first;
        std::cout << "{" << key_pair.first << "," << key_pair.second << "}, "
                  << entry.second << std::endl;
    }
 
    return 0;
}

Use std::pair as key to std::unordered_map in C++