关联容器的Key

有序容器关键字类型要求

有序容器(map,multimap,set,multiset),关键字类型必须定义元素比较方法。默认情况下，标准库使用关键字类型的<运算来比较两个关键字。

关键字类型重载了<运算符：

using std::endl;

using std::map;

using std::string;

struct Student{

   unsigned int age;

   string name;

   Student(unsigned int _age, string _name){

      this->age = _age;

      this->name = _name;

   }

   bool operator<(const Student &B)const;

};

bool Student::operator<(const Student &B)const{

      return (this->age < B.age) || (this->age == B.age && this->name < B.name);

}

map<Student,string> mapStudents;

int main()

{

   mapStudents[Student(4, "Alpha")] = string("Alpha");

   mapStudents[Student(3, "Bob")] = string("Bob");

   mapStudents[Student(3, "Alex")] = string("Alex");

   for (auto it = mapStudents.begin(); it != mapStudents.end();++it ){

      cout<<it->second.c_str()<<endl;

   }

    return 0;

}

定义两个关键字的<的比较运算函数：

#include <iostream>

#include <map>

#include <string>

using namespace std;

using std::cout;

using std::endl;

using std::map;

using std::string;

struct Student{

   unsigned int age;

   string name;

   Student(unsigned int _age, string _name){

      this->age = _age;

      this->name = _name;

   }

};

struct studentOrder{

   bool operator()(const Student &A, const Student &B){

      return (A.age < B.age) || (A.age == B.age && A.name < B.name);

   }

};

map<Student,string, studentOrder> mapStudents;

int main()

{

   mapStudents[Student(4, "Alpha")] = string("Alpha");

   mapStudents[Student(3, "Bob")] = string("Bob");

   mapStudents[Student(3, "Alex")] = string("Alex");

   for (auto it = mapStudents.begin(); it != mapStudents.end();++it ){

      cout<<it->second.c_str()<<endl;

   }

   return 0;

}

map的STL

为什么有序容器的关键字类型有严格弱序的要求？得从实现的STL来分析。map的STL实现是基于红黑树的数据结构。当使用迭代器遍历有序容器时，迭代器按照关键字的升序遍历元素。

红黑树

无序容器的关键字类型要求

无序容器不是使用比较运算来组织元素，而是使用一个hash function和关键字类型的==运算符。在关键字类型的元素没有明显的序关系的情况下，无序容器是非常有用的。

unordered_map的hash函数

#include <iostream>

#include <unordered_map>

#include <string>

using namespace std;

using std::cout;

using std::endl;

using std::unordered_map;

using std::string;

struct AppServer {

    int id;

    string svrName;

    AppServer(unsigned int _id, string name)

    {

        id = _id;

        svrName = name;

    }

    bool operator==(const AppServer &other) const

    {

        return ((id == other.id) && (svrName == other.svrName));

    }

};

namespace std {

    template <>

    struct hash<AppServer>

    {

        size_t operator()(const AppServer& app) const

        {

            return hash<int>()(app.id);

        }

    };

}

unordered_map<AppServer,string> svrOwner;

int main()

{

   svrOwner.insert({AppServer(4, string("LBS")), string("Ali")});

   svrOwner.insert({AppServer(2, string("MT")), string("Baidu")});

   svrOwner.insert({AppServer(1, string("MAP")), string("Google")});

   for (auto it = svrOwner.begin(); it != svrOwner.end();++it ){

      cout<<it->second.c_str()<<endl;

   }

    return 0;

}

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