C++ reserve()

The reserve() method requests a capacity change for an std::unordered_set. It sets the number of buckets to the amount needed to accommodate at least n elements without exceeding the container’s max_load_factor(). Calling reserve(n) may trigger a rehash; if it does, all iterators are invalidated, but references and pointers to elements remain valid. This operation does not sort elements or impose any ordering.

Syntax

unordered_set_name.reserve(n);

Parameters:

• n (size_type): Minimum number of elements the container should be able to accommodate without exceeding max_load_factor().

Return value:

This method doesn’t return anything (void).

Example

This example shows bucket capacity before and after reserve(), and demonstrates that elements remain accessible with the same unordered iteration semantics:

#include <iostream>
#include <unordered_set>
using namespace std;

int main() {
  unordered_set<int> numbers = {10, 20, 30};

  cout << "Buckets before: " << numbers.bucket_count() << "\n";

  numbers.reserve(10);

  cout << "Buckets after: " << numbers.bucket_count() << "\n";
  cout << "Max load factor: " << numbers.max_load_factor() << "\n";
  cout << "Load factor: " << numbers.load_factor() << "\n";

  for (int i = 1; i <= 7; ++i) {
    numbers.insert(i * 5);
  }

  cout << "Size: " << numbers.size() << "\n";
  cout << "Buckets final: " << numbers.bucket_count() << "\n";

  return 0;
}

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The output of this code is:

Buckets before: 3
Buckets after: 11
Max load factor: 1
Load factor: 0.272727
Size: 7
Buckets final: 11

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Codebyte Example

This example demonstrates reserve() with strings, and prints capacity-related information:

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