Application Examples of Combining unordered_map with Custom Objects

# 1. Introduction to unordered_map

#### 1.1 What is an unordered_map
An unordered_map is an associative container in the C++ Standard Template Library (STL) that offers fast lookup, insertion, and deletion operations. Unlike the map, which is sorted, the unordered_map is implemented using a hash table and does not maintain any order.

#### 1.2 Characteristics of unordered_map
- Stores data in key-value pairs
- Based on a hash table, with average time complexity of O(1) for lookup, insertion, and deletion
- Does not sort keys in any particular order
- Supports efficient element lookup operations
- Keys must be unique, values can be duplicates

The unordered_map excels at handling large amounts of data and is particularly well-suited for scenarios with frequent querying and insertion. It provides us with an efficient data structure option that can play a significant role in many applications.

# 2. Designing Custom Objects

- #### 2.1 Analyzing the need for custom object design
In real-world development, there are times when we need to store a specific data type that the standard library's data structures cannot fully satisfy. This is when we require custom objects. Custom objects can flexibly define data structures according to our needs, allowing for better organization and manipulation of data.

- #### 2.2 Defining the data structure of custom objects
Firstly, we need to determine the data members of the custom object. Taking a student object as an example, we can define data members such as name, student ID, age, and grades. We then need to select appropriate data types for these data members and design the internal structure of the object, using classes or structures to encapsulate these data members.

- #### 2.3 Implementing constructors for custom objects
Constructors are special member functions that are automatically called when an object is created, used for initializing the object's data members. In custom objects, we need to implement constructors to initialize each data member. For instance, in a student object, we can write a constructor that accepts information such as name, student ID, age, and grades, and assigns them to the corresponding data members of the object.

class Student:
     def __init__(self, name, student_id, age, score):
         self.name = name
         self.student_id = student_id
         self.age = age
         self.score = score

graph LR
     A[Define Custom Object Data Members] --> B[Choose Appropriate Data Types]
     B --> C[Design Object Internal Structure]
     C --> D[Encapsulate Data Members with Classes or Structures]

When defining custom objects, it's essential to clearly understand the data requirements, select appropriate data types for encapsulation, and design constructors to initialize the object's data members. This approach better meets the data handling needs for specific scenarios.

# ***bining unordered_map with Custom Objects

- #### 3.1 How to use custom objects as the value of an unordered_map
- ##### 3.1.1 Advantages of using custom objects as the value of an unordered_map
Using custom objects as the value of an unordered_map can make the code more modular, enhancing readability and maintainability. Moreover, custom objects can encapsulate complex data structures and operations, making the values of the unordered_map more flexible and extensible.
- ##### 3.1.2 Creating an unordered_map with custom objects as values
First, define a custom object, such as a class representing student information. Then, use an unordered_map container to store these student information objects, with keys as student IDs and values as corresponding student information objects.

- #### 3.2 How to use custom objects as the key of an unordered_map
- ##### 3.2.1 Precautions when using custom objects as the key of an unordered_map
When custom objects are used as the key of an unordered_map, it is crucial to ensure that they can correctly perform hash calculations and size comparisons. This requires custom objects to implement hash functions and equality comparison functions to ensure that the unordered_map can perform lookups and insertions correctly.
- ##### 3.2.2 Creating an unordered_map with custom objects as keys
First, overload the hash function and equality comparison function within the custom object's class. Then, define an unordered_map object that uses this custom object as a key, and perform corresponding insertion, lookup, and other operations.

- #### 3.3 Tips for using custom objects in unordered_map
When using custom objects in an unordered_map, it is important to avoid data redundancy and memory leaks. Additionally, when designing custom objects, efficiency and performance factors should be considered to minimize unnecessary complex operations. Reasonably using iterators can provide convenience for traversing and operating c