1. 简介
#include <unordered_map>template < class Key, // unordered_map::key_type class T, // unordered_map::mapped_type class Hash = std::hash<Key>, // unordered_map::hasher class Pred = std::equal_to<Key>, // unordered_map::key_equal class Alloc = std::allocator<std::pair<const Key, T>> // unordered_map::allocator_type> class unordered_map;unordered_map 具备如下性质：
- 唯一性：键是惟一的；
- 无序性：键值对是无序存储的，元素被存储在桶中，如果两个元素领有雷同哈希值的键，则它们会被存储于同一桶中；
- 具备常数工夫复杂度（均匀来说）的搜寻、插入、删除操作。
2. 自定义键类型
如果键是自定义类型，则须要提供相应的哈希函数，来计算键的哈希值。
办法1：显式提供相应的模板参数
#include <iostream>#include <unordered_map>#include <string>struct Key{ std::string first; std::string second;};struct KeyHash{ std::size_t operator()(const Key& k) const { return std::hash<std::string>()(k.first) ^ (std::hash<std::string>()(k.second) << 1); }};struct KeyEqual{ bool operator()(const Key& lhs, const Key& rhs) const { return lhs.first == rhs.first && lhs.second == rhs.second; }};int main(){ Key k1{ "John", "Doe" }, k2{ "Mary", "Sue" }; std::unordered_map<Key, std::string, KeyHash, KeyEqual> m = { { k1, "example"}, { k2, "another"} }; std::cout << m[k1] << '\n'; // example}办法2：显式特化 std::hash<> 模板
template<class Key>struct hash;#include <iostream>#include <unordered_map>#include <string>struct Foo{ Foo(int val_) : val(val_) {} int val; // unordered_map::key_equal bool operator==(const Foo& rhs) const { return val == rhs.val; }};namespace std{ template<> struct hash<Foo> { std::size_t operator()(const Foo& f) const { return std::hash<int>{}(f.val); } };}int main(){ std::unordered_map<Foo, std::string> m = { { Foo(1), "One"}, { 2, "Two"}, { 3, "Three"} }; std::cout << m[Foo(1)] << '\n'; // One}3. 容器大小
std::unordered_map<int, char> m;bool isEmpty = m.empty(); // Test whether container is emptysize_t n = m.size(); // Return container sizesize_t limit = m.max_size(); // Return maximum size/*Sets the number of buckets in the container (bucket_count) to the most appropriate to contain at least 30 elements.*/m.reserve(30);4. 拜访元素
std::unordered_map<std::string, std::string> m;m["Bakery"] = "Barbara"; // 存在时更新值；不存在时插入键值对std::string v = m["Bakery"]; // 存在时返回对应的值；不存在时插入键值对（应用值类型的默认构造函数来创立值）m.at("Hello") = "World"; // 存在时更新值；不存在时抛出 out_of_range 异样std::string v = m.at("Hello"); // 存在时返回对应的值；不存在时抛出 out_of_range 异样5. 查找元素
std::unordered_map<std::string, double> m = { {"mom", 5.4}, {"dad", 6.1}, {"bro", 5.9}};auto it = m.find("mom");if (it == m.end()){ std::cout << "not found";}else{ std::cout << "key=" << it->first << ", value=" << it->second;}bool exists = m.contains("mom"); // 是否存在（C++20）6. 插入元素
emplace：应用给定的实参原地结构元素，能够防止不必要的拷贝。只有不存在相应的键才会插入。
struct Person{ std::string m_name; double m_salary; Person() = default; Person(const std::string& name, double salary) : m_name(name), m_salary(salary) {}};int main(){ std::unordered_map<int, Person> m; /* 返回值类型为 pair<iterator, bool> 其中，迭代器执行新插入的元素，或已存在的元素 bool 示意是否插入胜利 */ auto p = m.emplace(1, Person(std::string("Mike"), 12345.6)); if (p.second) { std::cout << "插入胜利\n"; } else { std::cout << "已存在，原值为：" << p.first->second.m_salary; }}insert：只有不存在相应的键才会插入。
std::unordered_map<std::string, double> m;/*返回值类型为 pair<iterator, bool>其中，迭代器执行新插入的元素，或已存在的元素bool 示意是否插入胜利*/auto p = m.insert({ "sugar", 0.8 });if (p.second){ std::cout << "插入胜利\n";}else{ std::cout << "已存在，原值为：" << p.first->second;}7. 删除元素
std::unordered_map<std::string, std::string> m;m.erase("France"); // 删除指定键的元素，不存在时也没事auto it = m.find("France");if (it != m.end()){ m.erase(it); // 删除指定地位的元素}m.clear(); // 清空容器8. 遍历容器
遍历元素
std::unordered_map<std::string, std::string> m{ { "Australia", "Canberra" }, { "U.S.", "Washington" }, { "France", "Paris" } };for (auto it = m.begin(); it != m.end(); it++){ if (it->first[0] == 'U') { std::cout << it->first << "->" << it->second << '\n'; }}遍历桶
std::unordered_map<std::string, std::string> m{ {"house","maison"}, {"apple","pomme"}, {"tree","arbre"}, {"book","livre"}, {"door","porte"}, {"grapefruit","pamplemousse"}};size_t bucketCount = m.bucket_count();std::cout << "bucket count: " << bucketCount << '\n';for (size_t i = 0; i < bucketCount; i++){ std::cout << "bucket #" << i << " has " << m.bucket_size(i) << " elements.\n";}std::cout << '\n';for (size_t i = 0; i < bucketCount; i++){ std::cout << "bucket #" << i << " contains: "; for (auto it = m.begin(i); it != m.end(i); it++) { std::cout << "[" << it->first << ":" << it->second << "] "; } std::cout << '\n';}bucket count: 8bucket #0 has 1 elements.bucket #1 has 1 elements.bucket #2 has 1 elements.bucket #3 has 1 elements.bucket #4 has 0 elements.bucket #5 has 1 elements.bucket #6 has 0 elements.bucket #7 has 1 elements.bucket #0 contains: [book:livre]bucket #1 contains: [door:porte]bucket #2 contains: [grapefruit:pamplemousse]bucket #3 contains: [house:maison]bucket #4 contains:bucket #5 contains: [tree:arbre]bucket #6 contains:bucket #7 contains: [apple:pomme]