Key associative containers

Ordered keyword type container requirements

Ordered containers (map, multimap, set, multiset ), keyword type must define the elements of the comparative method. By default, the standard key type of library < operations to compare two keywords.

Keyword Type overloaded < operator:

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;

}