The Application and Performance Testing of unordered_map in Real-World Projects

# 1. Introduction to unordered_map

The `unordered_map` is an associative container in the C++ Standard Template Library (STL) that offers quick lookup, insertion, and deletion operations. Unlike the `map`, the `unordered_map` uses a hash table internally, which means that the elements are stored in an unordered fashion. This results in better performance for lookup operations. Additionally, `unordered_map` allows for the storage of key-value pairs where the key is unique and the value can be duplicated.

Utilizing `unordered_map` can enhance program efficiency, especially in scenarios that require frequent lookup operations. The time complexity for insertion and deletion operations is O(1), and the average time complexity for lookups is also O(1). This makes `unordered_map` an important choice for projects that handle vast amounts of data and require fast lookup capabilities.

# ***paring unordered_map with Other Containers

The `unordered_map` is an efficient hash table container capable of quickly locating and inserting elements. When selecting a container, it's crucial to consider the characteristics of each based on specific requirements. Next, we will compare the pros and cons of `unordered_map` with other common containers.

### Comparing unordered_map with map

#### Internal Implementation Principle

The `unordered_map` employs a hash table, achieving constant time complexity O(1) for lookup and insertion operations, whereas the `map` uses a red-black tree with a time complexity of O(log n). Hence, `unordered_map` outperforms in terms of locating elements or inserting a large amount of data.

#### Performance Comparison for Lookup and Insertion

The `unordered_map` excels in efficiency for lookup and insertion operations, especially as the dataset grows. In contrast, the `map` is better at maintaining the order of elements, making it suitable for scenarios that require sorting by key.

### Comparing unordered_map with unordered_set

#### Scenario Comparison

The `unordered_map` is ideal for storing key-value pairs, enabling fast retrieval of values through keys; the `unordered_set`, on the other hand, is suitable for storing individual values primarily used for checking the existence of a value.

#### Memory Usage Comparison

When storing the same amount of data, `unordered_map` typically occupies more memory because each element must store a key-value pair, whereas `unordered_set` only needs to store a single value, thus saving on memory usage.

In summary, selecting the appropriate container based on different scenarios and requirements can better leverage its advantages.

# 3. Applications of unordered_map in Large Projects

### Problems Solved by Using unordered_map

#### Example One

##### Problem Description
In a large social platform, there is a need to implement quick lookups of user-friend lists to support the management of social relationships. Traditional data structures fail to meet the requirements for real-tim