1、iterator(迭代器）

#include <map>
#include <iostream>
//...
std::map<int, string> m;
//...
for (auto it = m.begin(); it != m.end(); ++it) {
	//key
	std::cout << it->first << std::endl;
	//value 
	std::cout << it->second << std::endl;
}

2、range for(范围for语句）,c++ ver >= 11

#include <map>
#include <iostream>
//...
std::map<int, string> m;
//...
for (const auto &value : m) {
	//value 
	std::cout << value << std::endl;
}

3、structured binding declaration && range for(结构化绑定声明 && 范围for语句) ,c++ver >= 17

#include <map>
#include <iostream>
//...
std::map<int, string> m;
//...
for (const auto &[key, value] : m) {
	//key
	std::cout << key << std::endl;
	//value 
	std::cout << value << std::endl;
}

1、只取key

#include <map>
#include <iostream>
#include <functional>
//...
std::map<int, string> m;
//...
using namespace std::placeholder;
for (const auto &[key, _1] : m) {
	//key
	std::cout << key << std::endl;
}

2、只取value

#include <map>
#include <iostream>
#include <functional>
//...
std::map<int, string> m;
//...
using namespace std::placeholder;
for (const auto &[_1, value] : m) {
	//value 
	std::cout << value << std::endl;
}