How is std::unordered_map Implemented in C ?

How std::unordered_map is Implemented

Introduction

Understanding the internal workings of data structures is crucial for optimizing performance and understanding their behavior. This article aims to shed light on the implementation details of std::unordered_map, a fundamental component of C 's Standard Library.

Design Overview

Contrary to common assumptions, std::unordered_map does not utilize a purely linked list approach for collision handling. Instead, it employs a hybrid design known as "closed hashing" or "open addressing". This technique allocates an array of buckets, and upon a collision, probes different locations within the array based on a hash function.

Collision Handling

The behavior of std::unordered_map is defined by two parameters: bucket_count and max_load_factor. The bucket_count defines the array size, while the max_load_factor, defaulting to 1.0, specifies the maximum ratio of stored elements to bucket count before the table must be resized.

To ensure the validity of iterators during element insertion or deletion, std::unordered_map must retain the array of buckets. This requirement renders the use of closed hashing, where collisions are resolved by probing different array locations, inescapable.

Rehashing and Resizing

To maintain optimal performance, std::unordered_map redistributes its elements into a new array of buckets whenever the load factor exceeds max_load_factor. This process, known as rehashing, is triggered by an insert operation when the load factor becomes too high. The size of the new array is typically double the size of the previous one.

Performance Implications

While the open hashing approach is a pragmatic compromise for general use, it may not be the most efficient solution for all scenarios. In cases where collisions are infrequent and the data is small, closed addressing with a sentinel value for unused buckets and a robust hash function can significantly improve performance and reduce memory consumption.

Conclusion

Understanding the implementation nuances of std::unordered_map empowers developers to leverage its full potential. By appreciating its hybrid design and collision handling mechanism, it becomes apparent why the choice of hash function and anticipated load characteristics play a pivotal role in optimizing performance and efficiency.

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