How Does `std::unordered_map` Handle Collisions, Resizing, and Rehashing While Maintaining C Standard Compliance?

Internal Workings of std::unordered_map

Introduction

std::unordered_map is an invaluable data structure in the C arsenal for storing key-value pairs. However, its implementation can sometimes be shrouded in confusion. This article delves into the internal workings of std::unordered_map, shedding light on how it addresses collisions, resizes, and rehashes while adhering to C standard requirements.

Collision Handling

std::unordered_map uses open hashing or separate chaining to handle collisions. Each element in the underlying array serves as the head of a linked list, with each node representing a key-value pair. This approach ensures that iterators remain valid even during insertions or deletions.

Resizing and Rehashing

To prevent excessive collisions and maintain performance, std::unordered_map resizes and rehashes when the load factor (the ratio of elements to buckets) exceeds a threshold value. Resizing involves doubling the number of buckets, effectively distributing the elements more evenly. Rehashing involves recalculating the hash codes for all elements and assigning them to the new buckets.

Compliance with C Standard

The implementation of std::unordered_map aligns with the C standard in several key aspects:

• Iterators remain valid even when elements are inserted or deleted, ensuring referential stability.
• The initial maximum load factor is set to 1.0, which triggers resizing before the table becomes overly dense.
• Rehashing occurs only when resizing occurs beyond the specified load factor.

Performance Considerations

While open hashing guarantees stability, it can lead to linked lists with many elements, potentially impacting performance. However, std::unordered_map employs optimizations like linear probing and bucket lists to mitigate this issue.

Alternative Implementation Options

Closed hashing, or open addressing, is another hashing technique that doesn't use linked lists. However, it presents challenges in handling collisions and maintaining iterator validity, making it less suitable for general-purpose use in std::unordered_map.

Conclusion

std::unordered_map's implementation strikes a balance between performance, flexibility, and C standard requirements. Its use of open hashing ensures iterator stability while resizing and rehashing help maintain efficiency. While alternative implementation options exist, open hashing remains the appropriate choice for providing the general-purpose functionality of std::unordered_map.

The above is the detailed content of How Does `std::unordered_map` Handle Collisions, Resizing, and Rehashing While Maintaining C Standard Compliance?. For more information, please follow other related articles on the PHP Chinese website!