Both `rehash()`

and `reserve()`

can be used to optimize the performance of a `std::unordered_set`

by preallocating buckets. However, they differ in how they determine the number of buckets toÂ allocate.

`rehash(n)`

Â explicitly sets the number of buckets to at leastÂ`n`

.`reserve(n)`

Â sets the number of buckets based on the container'sÂ`max_load_factor()`

Â and the expected number of elementsÂ`n`

.

Here's an example to illustrate theÂ difference:

```
#include <iostream>
#include <unordered_set>
int main() {
std::unordered_set<int> Set;
Set.max_load_factor(0.5);
Set.rehash(100);
std::cout << "After rehash(100):\n";
std::cout << "Bucket count: "
<< Set.bucket_count() << "\n";
Set.reserve(100);
std::cout << "After reserve(100):\n";
std::cout << "Bucket count: "
<< Set.bucket_count() << "\n";
}
```

```
After rehash(100):
Bucket count: 128
After reserve(100):
Bucket count: 256
```

In thisÂ example:

- We set theÂ
`max_load_factor()`

Â to 0.5 - CallingÂ
`rehash(100)`

Â directly sets the number of buckets to at least 100. The actual number of buckets may be higher to satisfy the container's requirements. In this case, it created 128 buckets. - CallingÂ
`reserve(100)`

Â calculates the number of buckets based on the expected number of elements (100) and theÂ`max_load_factor()`

Â (0.5). In this case, it elects to create 256 buckets to accommodate 100 elements with a load factor of 0.5.

The key difference is that `rehash()`

explicitly sets the minimum number of buckets, while `reserve()`

calculates the number of buckets based on the expected number of elements and the `max_load_factor()`

.

It's generally recommended to use `reserve()`

when you have an estimate of the final size of the container, as it takes into account the load factor. Use `rehash()`

when you need explicit control over the number ofÂ buckets.

Answers to questions are automatically generated and may not have been reviewed.

This Question is from the Lesson:### Hash Sets using

`std::unordered_set`

This lesson provides a thorough understanding of `std::unordered_set`

, from basic initialization to handling custom types and collisions