C++ .reserve()

In C++, the .reserve() method reserves space for at least the specified number of elements in an unordered map, helping optimize performance by reducing costly rehash operations during bulk insertions.

Syntax

mapName.reserve(count);

Parameters:

• count: The minimum number of elements to reserve space for.

Return value:

This is a void method. It doesn’t return a value but affects the internal capacity of the unordered map.

Example

This example demonstrates the use of the .reserve() method with an unordered map to optimize performance when adding student grades:

#include <iostream>
#include <unordered_map>

int main() {
  // Initializing unordered_map and reserving space for 10 elements
  std::unordered_map<std::string, int> studentGrades;
  studentGrades.reserve(10);

  // Adding student grades
  studentGrades["Alice"] = 95;
  studentGrades["Bob"] = 87;
  studentGrades["Charlie"] = 92;
  studentGrades["Diana"] = 88;

  std::cout << "Student grades after reservation:\n";
  for (const auto& student : studentGrades) {
    std::cout << student.first << ": " << student.second << std::endl;
  }

  std::cout << "Bucket count: " << studentGrades.bucket_count() << std::endl;

  return 0;
}

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Here is the output:

Student grades after reservation:
Charlie: 92
Bob: 87
Diana: 88
Alice: 95
Bucket count: 11

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Codebyte Example

This codebyte example demonstrates the use of the .reserve() method with an unordered map to store employee information efficiently:

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