In this article, we will discuss how to implement Custom Hash Functions for User-Defined Types in Std::unorderedmap in C++. Before discussing the implementation of the custom hash function, we must know about the std::unorderedmap in C++.
What is the std::unordered_map?
The std::unorderedmap 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::unorderedmap 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.
- Ensure Deterministic Behaviour: Hash functions should yield the same value for identical items.
- Balance Both Complexity and Performance: Aim for a hash function that balances hashing quality and computational efficiency.
Considerations of Hashing Functions
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++.
#include<iostream>
#include<unordered_map>
#include<string>
// Define your custom type
struct MyType {
std::string key;
int value;
// Equality operator for MyType
bool operator==(const MyType& other) const {
return key == other.key && value == other.value;
}
};
// Define a hash function for MyType
struct MyTypeHash {
std::size_t operator()(const MyType& obj) const {
// Combine hash values of member variables
std::hash<std::string> hasher;
std::size_t hash = hasher(obj.key);
hash ^= std::hash<int>{}(obj.value) + 0x9e3779b9 + (hash << 6) + (hash >> 2); // Combine with some bit magic
return hash;
}
};
int main() {
// Provide a custom hash function to unordered_map
std::unordered_map<MyType, int , MyTypeHash> myMap;
// Dynamic inputs
std::string keyInput;
int valueInput;
std::cout << "Enter key1 (string): ";
std::cin >> keyInput;
std::cout << "Enter value for key1 (int): ";
std::cin >> valueInput;
MyType key1 = {keyInput, valueInput};
std::cout << "Enter key2 (string): ";
std::cin >> keyInput;
std::cout << "Enter value for key2 (int): ";
std::cin >> valueInput;
MyType key2 = {keyInput, valueInput};
// Inserting elements into unordered_map
myMap[key1] = 100;
myMap[key2] = 200;
// Accessing elements
std::cout<<"Value for key1:"<<myMap[key1]<<std::endl;
std::cout<<"Value for key2:"<<myMap[key2]<<std::endl;
return 0;
}
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 .
- 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.
- Regardless of the time or location at which an object is hashed, custom hash functions ensure deterministic behavior by producing the same hash value.
- 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.