關聯容器的Key

有序容器關鍵字類型要求

有序容器(map,multimap,set,multiset),關鍵字類型必須定義元素比較方法。默認狀況下，標準庫使用關鍵字類型的<運算來比較兩個關鍵字。

關鍵字類型重載了<運算符：

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; }

 

 

定義兩個關鍵字的<的比較運算函數：

#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

爲何有序容器的關鍵字類型有嚴格弱序的要求？得從實現的STL來分析。map的STL實現是基於紅黑樹的數據結構。當使用迭代器遍歷有序容器時，迭代器按照關鍵字的升序遍歷元素。

紅黑樹

 

無序容器的關鍵字類型要求

無序容器不是使用比較運算來組織元素，而是使用一個hash function和關鍵字類型的==運算符。在關鍵字類型的元素沒有明顯的序關係的狀況下，無序容器是很是有用的。

unordered_map的hash函數

#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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