C++ map容器插入具有相同键的键值对的覆盖问题

map容器插入键值对的方法一般有两种

1. map["key"] = value;
2. map.insert(make_pair<>("", ""));

 

#include <map>
#include <iostream>

using namespace std;

int main()
{
  map<double, double> mp1;
  mp1[1.0] = 1.1;
  mp1[1.0] = 1.2;
  cout << mp1.size() << endl;
  cout << mp1.begin()->second << endl;
  return 0;
}
 

输出结果为

1
1.2

 

使用方法二插入相同键的键值对时，后一组的键值对不会插入map容器，即不会覆盖前一组键值对。代码如下：

 

 

#include <map>
#include <iostream>

using namespace std;

int main()
{
  map<double, double> mp1;
  mp1.insert(make_pair<double, double>(2.0, 2.1));
  mp1.insert(make_pair<double, double>(2.0, 2.2));
  cout << mp1.size() << endl;
  cout << mp1.begin()->second << endl;
  return 0;
}
 

 

输出结果为

 

1
2.1

 

 

 

 

 

 

 

#include <string>
#include <iostream>
//查询性能最高
//允许重复的,hash_map
#include <unordered_map>
#include <algorithm>
using namespace std;


int main()
{
    //允许重复的映射
    unordered_multimap<string, double>mymap{ {"a1",113},{ "a2",143 },{ "a3",1123 } };

    mymap.insert(pair<string, double>("a4", 345));
    mymap.insert(pair<string, double>("a4", 315));
    mymap.insert(pair<string, double>("a4", 325));
    mymap.insert(pair<string, double>("a4", 335));

    /*mymap.insert(unordered_multimap<string, double>::value_type("a5", 3425));*/

    /*for (auto i : mymap)
    {
        cout << i.first << "  " << i.second << endl;
    }*/

    /*auto it = mymap.find("a1");
    if (it != mymap.end())
    {
        cout << it->second << endl;
    }*/

    //查找所有
    auto it = mymap.equal_range("a4");

    for_each(it.first, it.second, [](unordered_multimap<string, double>::value_type &x) {cout << x.first << "  " << x.second << endl; });
    cin.get();
    return 0;
}

 

 

root@ubuntu:~/c++# g++ -std=c++11  multimap.cpp -o  multimap
root@ubuntu:~/c++# ./multimap 
a4  335
a4  325
a4  315
a4  345

 

#include <string>
#include <iostream>
//查询性能最高
//允许重复的,hash_map
#include <unordered_map>
#include <algorithm>
using namespace std;


int main()
{
    //允许重复的映射
    unordered_map<string, double>mymap{ {"a1",113},{ "a2",143 },{ "a3",1123 } };

    mymap.insert(pair<string, double>("a4", 345));
    mymap.insert(pair<string, double>("a4", 315));
    mymap.insert(pair<string, double>("a4", 325));
    mymap.insert(pair<string, double>("a4", 335));

    /*mymap.insert(unordered_multimap<string, double>::value_type("a5", 3425));*/

    /*for (auto i : mymap)
    {
        cout << i.first << "  " << i.second << endl;
    }*/

    /*auto it = mymap.find("a1");
    if (it != mymap.end())
    {
        cout << it->second << endl;
    }*/

    //查找所有
    auto it = mymap.equal_range("a4");

    for_each(it.first, it.second, [](unordered_multimap<string, double>::value_type &x) {cout << x.first << "  " << x.second << endl; });
    cin.get();
    return 0;
}

 

root@ubuntu:~/c++# g++ -std=c++11  multimap.cpp -o  multimap
root@ubuntu:~/c++# ./multimap 
a4  345

root@ubuntu:~/c++# cat multimap.cpp

 

 

#include <unordered_map>
#include <string>
#include<iostream>
using namespace std;
 
 
class Node
{
public:
        Node(int age, string name);
        ~Node();
 
        bool operator==(const Node &n) const;
public:
        std::string m_strName;
        int m_iAge;
};
 
Node::Node(int age, string name) :m_strName(name),m_iAge(age)
{
}
 
Node::~Node()
{
}
 
bool Node::operator==(const Node & n) const 
{
        if (n.m_iAge==m_iAge && m_strName==n.m_strName)
        {
                return true;
        }
        return false;
}
 
struct KeyHasher
{
        std::size_t operator()(const Node& k) const
        {
                using std::size_t;
                using std::hash;
                using std::string;
                return ((hash<string>()(k.m_strName)) ^ (hash<int>()(k.m_iAge) << 1));
        }
};
 
int  main( )
{
        std::unordered_map<Node, int, KeyHasher> myMap;
        myMap.insert(pair<Node, int>(Node(24, "kobe"), 24));
        //遍历输出+迭代器的使用
        auto iter = myMap.begin();//auto自动识别为迭代器类型unordered_map<int,string>::iterator
        while (iter!= myMap.end())
         {  
                cout << iter->second << endl;  
                ++iter;  
        }  
        auto iterator = myMap.find(Node(24, "kobe"));//find()返回一个指向2的迭代器
        if (iterator != myMap.end())
            cout << iterator->first.m_strName << "  "<< iterator->second << endl; 
        myMap.insert(pair<Node, int>(Node(24, "kobe"), 40));
        iterator = myMap.find(Node(24, "kobe"));//find()返回一个指向2的迭代器
        if (iterator != myMap.end())
            cout << iterator->first.m_strName << "  "<< iterator->second << endl; 
} 
 

 

root@ubuntu:~/c++# ./unorder2 
24
kobe  24
kobe  24