How to Count Elements in a Custom Container Using std::ranges::count()

To count elements in a custom container using std::ranges::count(), you need to ensure your container meets the requirements for a range. This means your container should provide begin() and end() functions that return iterators.

Here's an example of how to count elements in a custom container:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
CustomContainer Class|Show
5class CustomContainer {/*...*/};
19
20int main() {
21  CustomContainer<int> Numbers;
22  Numbers.add(1);
23  Numbers.add(2);
24  Numbers.add(3);
25  Numbers.add(4);
26  Numbers.add(4);
27  Numbers.add(5);
28
29  auto Fours{std::ranges::count(Numbers, 4)};  
30  std::cout << "Count of fours: " << Fours;
31}

1Count of fours: 2

In this example:

• CustomContainer is a template class that stores elements in a std::vector.
• The add() method adds elements to the container.
• The begin() and end() methods return iterators to the beginning and end of the container, respectively.

By ensuring CustomContainer provides begin() and end() methods, it meets the requirements of a range, allowing us to use std::ranges::count() to count elements.

This approach works for any type of custom container, as long as it provides the necessary iterator access functions. This makes std::ranges::count() a flexible tool for counting elements in various types of containers in C++20 and later.