本篇学习unordered_multimap的查找操作和观察器操作，具体函数如下：
count
(C++11)
返回匹配特定键的元素数量(公开成员函数)
find
(C++11)
寻找带有特定键的元素(公开成员函数)
contains
(C++20)
检查容器是否含有带特定键的元素(公开成员函数)
equal_range
(C++11)
返回匹配特定键的元素范围(公开成员函数)
hash_function
(C++11)
返回用于对键散列的函数(公开成员函数)
key_eq
(C++11)
返回用于比较键的相等性的函数(公开成员函数)
代码示例：
#include <iostream>#include <unordered_map>#include <string>using namespace std;void findOpertion(){ //1.count返回匹配特定键的元素数量 std::unordered_multimap<int, std::string> map1; map1.emplace(1, "Hero"); map1.emplace(2, "Archer"); map1.emplace(3, "辛勤的绿茶"); int nkey = 0; for(nkey = 1; nkey < 6; ++nkey) { if(map1.count(nkey) > 0) std::cout << nkey << " is element of map1\n"; else std::cout << nkey << " is not element of map1\n"; } //2.find寻找带有特定键的元素,返回的是迭代器 std::unordered_multimap<int, std::string> map2; map2.emplace(4, "camel"); map2.emplace(5, "iran"); std::unordered_multimap<int, std::string>::iterator it; it = map2.find(4); if(it != map2.end()) std::cout << "find element is " << it->second << std::endl; else std::cout << "not find element\n"; //3.equal_range返回匹配特定键的元素范围,返回容器中所有拥有给定关键的元素范围。范围以二个迭代器定义，一个指向首个不小于 key 的元素， //另一个指向首个大于 key 的元素。首个迭代器可以换用 lower_bound() 获得，而第二迭代器可换用 upper_bound() 获得。 std::unordered_multimap<int, std::string> map3; map3.emplace(1, "scott"); map3.emplace(3, "camel"); map3.emplace(5, "Sky"); map3.emplace(7, "beer"); pair<unordered_multimap<int, string>::iterator, unordered_multimap<int, string>::iterator> it2; pair<unordered_multimap<int, string>::iterator, unordered_multimap<int, string>::iterator> it3; std::unordered_multimap<int,char> map = {{1,'a'},{1,'b'},{3,'d'},{5,'b'}}; auto range = map.equal_range(1); for (auto it = range.first; it != range.second; ++it) { std::cout << it->first << ' ' << it->second << '\n'; } std::cout << range.first->first << " => " << range.first->second << std::endl; std::cout << range.second->first << " => " << range.second->second << std::endl; //print it2 = map3.equal_range(1);//返回的第一个迭代器为传进来的关键字的值， //这个与map里返回的值有差别，如果没有这个关键字，则返回end()迭代器。 //在map里equal_range(2)返回3的迭代器，而在这里返回的是end()迭代器 if(it2.first != map3.end()) { std::cout << "lower bound points to: ";//返回的第一个迭代器为传进来的关键字的值 std::cout << it2.first->first << " => " << it2.first->second << std::endl; } else { std::cout << "not find lower range" << endl; } if(it2.second != map3.end()) { std::cout << "upper bound points to: ";//返回的第二个迭代器首个大于传进来的关键字的值 std::cout << it2.second->first << " => " << it2.second->second << std::endl; } else { std::cout << "not find upper range" << endl; } std::cout << "======================================\n"; it3 = map3.equal_range(3); std::cout << "lower bound points to: "; std::cout << it3.first->first << " => " << it3.first->second << std::endl; std::cout << "upper bound points to: "; std::cout << it3.second->first << " => " << it3.second->second << std::endl; //4.lower_bound返回指向首个等于 key 的元素的迭代器。若找不到这种元素，则返回尾后迭代器 std::unordered_multimap<int, std::string>::iterator lowerIt; //lowerIt = map3.lower_bound(2);//这个key一定要存在unordered_map中，否则返回end()迭代器 //std::cout << "lowerIt->first = " << lowerIt->first << " lowerIt->second = " << lowerIt->second << std::endl; lowerIt = map3.lower_bound(3); std::cout << "lowerIt->first = " << lowerIt->first << " lowerIt->second = " << lowerIt->second << std::endl; //5.upper_bound返回指向首个大于 key 的元素的迭代器。若找不到这种元素，则返回尾后。 std::unordered_multimap<int, std::string>::iterator upperIt; //upperIt = map3.upper_bound(4);//这个key一定要存在unordered_map中，否则返回end()迭代器 //std::cout << "upperIt->first = " << upperIt->first << " upperIt->second = " << upperIt->second << std::endl; upperIt = map3.upper_bound(5); std::cout << "upperIt->first = " << upperIt->first << " upperIt->second = " << upperIt->second << std::endl; //6. key_comp 返回用于比较键的函数 std::unordered_multimap<int, std::string> map4; map4.emplace(1, "scott"); map4.emplace(3, "beer"); map4.emplace(5, "Sky"); map4.emplace(7, "camel"); std::unordered_multimap<int, std::string>::key_compare funCompare = map4.key_comp();//返回一个比较键的函数 bool b1 = funCompare(3, 5); bool b2 = funCompare(5, 3); std::cout << "b1 = " << b1 << " b2 = " << b2 << std::endl; //7. value_comp返回用于在value_type类型的对象中比较键的函数。 std::unordered_multimap<char, int> map5; map5.emplace('1', 12); map5.emplace('3', 8); map5.emplace('5', 54); map5.emplace('7', 6); std::unordered_multimap<char, int>::iterator endIt = map5.end(); auto lastElement = --endIt;//最后一个元素 std::cout << "lastElement.key = " << lastElement->first << " lastElement.value = " << lastElement->second << std::endl; std::unordered_multimap<char, int>::iterator firstIt = map5.begin();//第一个元素迭代器 bool b3 = map5.value_comp()(*firstIt, *lastElement); ++firstIt; bool b4 = map5.value_comp()(*firstIt, *lastElement); std::cout << "b3 = " << b3 << " b4 = " << b4 << std::endl; //8. hash_function 返回对关键哈希的函数 unordered_multimap<int, string> map6; unordered_multimap<int, string>::hasher fn = map6.hash_function(); std::cout << "fn1 = " << 香港vps fn(1) << " fn2 = " << fn(2) << std::endl; //9. key_eq 返回用于比较键的相等性的函数 unordered_multimap<string, string> map7; bool case_insensitive = map7.key_eq()("camle","CAMLE"); std::cout << "map7.key_eq() is "; std::cout << ( case_insensitive ? "case insensitive" : "case sensitive" ); std::cout << std::endl; //case sensitive:区分大小写 //case insensitive不区分大小写}int main(){ findOpertion(); cout << "hello world" << endl; return 0;}运行结果：
?
参考：
https://zh.cppreference.com/w/cpp/container/unordered_multimap
74983074