How to Implement Custom Hash Functions for User-Defined Types in std::unordered_map in C++

In this article, we will discuss how to implement Custom Hash Functions for User-Defined Types in Std::unordered_map in C++. Before discussing the implementation of the custom hash function, we must know about the std::unordered_map in C++.

What is the std::unordered_map?

The std::unordered_map container in contemporary C++ programming provides an effective method of managing collections of key-value pairs. Although it works well with built-in types, adding user-defined types to a std::unordered_map can be difficult, especially when it comes to creating the right hash functions.

Recognizing the Value of Custom Hash Functions:

A hashing method that enables constant-time access to elements based on their keys is at the core of std::unordered_map. However, the success of this hashing procedure mostly depends on carefully designed hash algorithms. The built-in C++ default hash function might not be appropriate when working with user-defined types because it might need to adequately capture the distinctive features of the custom types. Creating a unique hash function for the particular classes becomes crucial in these situations.

Challenges of Default Hash Functions:

• By default, std::unordered_map depends on the std::hash specialization for your custom type if you don't provide your custom hash function.
• It often proves inadequate for handling advanced user-defined types, even if it might work for basic types like texts or integers.
• Using an unordered map can have speed benefits, but these can be offset by higher collision rates caused by inadequate hashing.

Implementing a Custom Hash Function:

The procedures below can be used to create a custom hash function for user-defined types in C++:

Step 1: Create Your Custom Type

• Start by encapsulating the data structure we want to store in the std::unordered_map in the custom type definition.
• Any user-defined type could be used here, including classes and structs.

Step 2: Hash Function Definition

• Next, specify the custom type's hash function.
• This function should return a size_t hash value after receiving an object of your custom type as input.
• In order to reduce collisions, the hash function should try to distribute hash values equally throughout the hash table.

Considerations of Hashing Functions

• Ensure Deterministic Behaviour: Hash functions should yield the same value for identical items.
equally as possible over the whole size_t range.
• Balance Both Complexity and Performance: Aim for a hash function that balances hashing quality and computational efficiency.

Step 3: Allow std::unordered_map to use the hash function

• It would help if we used the custom hash function as the third template parameter when declaring the std::unordered_map.
• This instructs the unordered map to hash keys of the custom type using the custom hash function.

Example:

Let us take an example to illustrate the custom hash function for user-defined in std::unordered_map in C++.

Output:

Explanation:

• This example uses MyTypeHash, a functor that gives MyType a hash function. For each member variable in MyType, the operator() function uses the std::hash methods to get a hash value, which it combines to get the final hash value.
• MyTypeHash is the hash function to be used for keys of type MyType, and it is provided as the third template argument when declaring the std::unordered_map.

Advantages of implementing custom hash functions:

• Performance:
  Within the unordered map, insertion, deletion, and lookup operations are accelerated by custom hash functions that optimize the distribution of hash values.
• Collision Reduction:
  Custom hash functions preserve the effectiveness of hash-based data structures by reducing collisions, guaranteeing that operations continue to be quick and predictable even when dealing with big datasets.
• Complexity Handling:
  In order to efficiently hash a wide range of complicated and varied data, custom hash functions can handle nested or sophisticated data structures within user-defined types.
• Type Specificity:
  Developers may ensure precise and effective hashing by customizing the hashing process to the unique properties and traits of their user-defined types.

Deterministic Behaviour:

• Regardless of the time or location at which an object is hashed, custom hash functions ensure deterministic behavior by producing the same hash value.

Control:

• Developers can obtain more control over the behavior and performance of hash-based data structures by implementing custom hash functions.
• It allows for optimization and fine-tuning based on particular application requirements.