1. map

1. map<key, value>， key即first， value即second。 
map::iterator it = map.find(key);
if(it != map.end())
{
  it->first;  // key
  it->second; // value
}
2. map[key] = value 解析： 不存在key，添加元素<key, value>；存在key，则赋值value给该元素的second

so, add new item, we can do it like this:

if(map.end() == map.find(key))
{
  map[key] = value;
}

demo:

std::map<int, std::string> map001;
map001[1] = "001";
map001[2] = "002";
map001[3] = "003";
map001[4] = "004";
map001[5] = "005";

int map_key = 1;
if (map001.end() != map001.find(map_key)) {
	printf("find pair, key: %d, value: %s\n", map_key, map001[map_key].c_str());
	map001.erase(map_key);
}

std::map<int, std::string>::const_iterator map_it = map001.find(2);
if (map_it != map001.end()) {
	printf("find pair, key: %d, value: %s\n", map_it->first, map_it->second.c_str());
	map001.erase(map_it);
}

std::for_each(map001.begin(), map001.end(), [&](const std::pair<int, std::string>& item) {
	printf("key: %d, value: %s\n", item.first, item.second.c_str());
});

代码库：map用例(Server类)

2. hash_map

2.1 hash相关概念

hash table

参考文档：

2.2 std::hash_map

参考文档： C++中使用STL的hashmap

2.2.1 std::hash_map<std::string, ..>

#include <string>
#include <hash_map>
#include <iostream>

using namespace std;

class T_ {
public:
	T_(int a = 0) : m_nNum(a) {}
private:
	int m_nNum;
};

struct string_less : public binary_function<std::string, std::string, bool>  {
public:
    result_type operator()(const first_argument_type& _Left, const second_argument_type& _Right) const {
        return(_Left.compare(_Right) < 0 ? true : false);
    }
};

int main() {
	std::hash_map<std::string, T_*, hash_compare<std::string, string_less>> hmp2;
	hmp2["123"] = new T_(12);
	hmp2["456"] = new T_(15);
	if(hmp2.end() == hmp2.find("123")) {
		hmp2["123"] = new T_(12);
	}
	return 0;
}

2.2.2 std::hash_map<自定义类, ..>

#include <string>
#include <hash_map>
#include <iostream>

using namespace std;

/* interface */
class IHashable {
public:
	virtual ~IHashable() {} // virtual destructor
	virtual size_t hash_value() const = 0; // pure virtual funciton
	virtual bool operator<(const IHashable& val) const = 0; // pure virtual funciton
};

class CTest : public IHashable   
{    
public:   
    CTest(int value = 0, const std::string& message = "") 
		: m_value(value), m_message(message) {}

	~CTest() {}

    // 哈希函数,可以采用其他哈希算法
    virtual size_t hash_value() const {  
        return ((size_t)m_value ^ _HASH_SEED);
    }  
              
    virtual bool operator<(const IHashable& val) const {   
        return (m_value < ((CTest&)val).m_value);   
    }   

private:   
    int m_value;   
    std::string m_message;  
}; 

template<class _Tkey>   
class HashCompare : public hash_compare<_Tkey>   
{   
public:   
    size_t operator()(const _Tkey& _Key) const {   
        return(_Key.hash_value());   
    }  
  
    bool operator()(const _Tkey& _Keyval1, const _Tkey& _Keyval2) const {   
        return (this->_Getcomp()(_Keyval1, _Keyval2));
    }   
};  

int main() {
	std::hash_map<CTest, std::string, HashCompare<IHashable>> hmp;
	hmp[CTest(10, "helloworld")] = "12306";
	if(hmp.end() == hmp.find(CTest(8, "wonderful"))) {
		hmp[CTest(8, "wonderful")] = "12308";
	}
	return 0;
}

备注： 上述例子如需深入研究，需要具体分析STL源码，此处篇幅有限，不详述。

2.3 哈希函数

参考文档： 常见的hash函数

3. hash_map用例

参考文档：