std_unordered_multimap.md

Characteristics

• Unordered Container: std::unordered_multimap does not maintain any ordering of its key-value pairs. They are organized into buckets based on their hash values.

• Allows Duplicate Keys: Unlike std::unordered_map, this container allows for multiple key-value pairs with identical keys.

• Dynamic Size: The container can grow or shrink dynamically.

• Hash-Based: It uses a hash table for its underlying implementation. This typically provides average constant-time complexity for search, insertion, and deletion.

Basic usage

#include <iostream>
#include <unordered_map>

int main() {
    std::unordered_multimap<int, std::string> mmap;

    // Insert elements
    mmap.insert({1, "apple"});
    mmap.insert({1, "orange"});  // Duplicate keys allowed
    mmap.insert({2, "banana"});

    // Access and print elements
    auto range = mmap.equal_range(1);
    for (auto it = range.first; it != range.second; ++it) {
        std::cout << it->first << ": " << it->second << std::endl;
    }
    // Output could be:
    // 1: apple
    // 1: orange

    // Remove an element
    mmap.erase(1);  // Removes all key-value pairs with key 1

    return 0;
}

When to Use std::unordered_multimap

• When you need a hash table-based key-value store with fast look-ups.
• When you need to allow duplicate keys.
• When you don't need the key-value pairs to be sorted.

Limitations

• No direct access by index.
• Keys must be hashable, meaning you have to provide a hash function for custom types.

Common Functions

Element Management

• insert({key, value}): Inserts a key-value pair.
• emplace(args): Constructs and inserts a key-value pair in-place.
• erase(key): Removes all key-value pairs with a specific key.
• clear(): Clears all key-value pairs.

Element Lookup

• find(key): Returns an iterator to the first occurrence of the key, or end() if not found.
• count(key): Returns the number of key-value pairs with a specific key.

Map Properties

• size(): Returns the number of key-value pairs.
• empty(): Checks if the map is empty.

Iteration

• begin(), end(): Provide iterators to traverse through the elements.