What is the purpose of unordered_map in C++?

The unordered_map is a container in the C++ standard library used to store key-value pairs. It functions similarly to a dictionary or hash table, allowing for quick retrieval of values based on keys.

unordered_map uses a hash table to implement it, which allows for fast lookup operations. Unlike the map container, unordered_map does not sort the keys in order, making it more efficient for lookup operations. It can be used in scenarios that require fast lookup of key-value pairs, such as counters and caches.

When using an unordered_map, both the key and value can be of any type, as long as they meet specific requirements. This map provides a range of member functions and operators that make it easy to insert, remove, access, and modify key-value pairs.

For example, the following code demonstrates the basic usage of unordered_map:

#include <iostream>
#include <unordered_map>

int main() {
    std::unordered_map<int, std::string> map;

    // 插入键值对
    map.insert({1, "apple"});
    map.insert({2, "banana"});
    map.insert({3, "orange"});

    // 访问和修改值
    std::cout << map[1] << std::endl; // 输出: apple
    map[1] = "kiwi";
    std::cout << map[1] << std::endl; // 输出: kiwi

    // 查找键值对
    if (map.find(2) != map.end()) {
        std::cout << "Found key 2: " << map[2] << std::endl; // 输出: Found key 2: banana
    }

    // 删除键值对
    map.erase(3);

    // 遍历所有键值对
    for (const auto& pair : map) {
        std::cout << "Key: " << pair.first << ", Value: " << pair.second << std::endl;
    }

    return 0;
}

Output result:

apple
kiwi
Found key 2: banana
Key: 1, Value: kiwi
Key: 2, Value: banana

In conclusion, unordered_map offers an efficient way to store and look up key-value pairs, making it suitable for scenarios that require quick access and modification of data.