How to Determine the Size of a Range Defined by a Sentinel in C++

To determine the size of a range defined by a sentinel in C++, you typically use the iterator returned by the algorithm that processes the range. Most range-based standard library algorithms return an object that includes an iterator pointing to where the sentinel was triggered.

For example, std::ranges::for_each() returns an object with an in member, which is the iterator to where the algorithm stopped. You can use this iterator to calculate the distance from the beginning of the range, giving you the size of the range.

Here’s an example:

1#include <iostream>
2#include <vector>
3#include <algorithm>
4
5template <typename T>
6struct Sentinel {
7  bool operator==(T iter) const {
8    return *iter < 0 || iter == end;
9  }
10
11  T end;
12};
13
14void Log(int x) {
15  std::cout << x << ", ";
16}
17
18int main() {
19  std::vector<int> numbers {1, 4, 3, 8, -2, 5};
20  Sentinel<std::vector<int>::const_iterator> s{
21    numbers.end()};
22
23  auto [in, fun] = std::ranges::for_each(
24    numbers.begin(), s, Log
25  );
26
27  std::cout << "\nObject at sentinel: " << *in;
28  std::cout << "\nSize of range: "
29    << std::distance(numbers.begin(), in);
30  std::cout << "\nLast in range: " << *(in - 1);
31}

11, 4, 3, 8,
2Object at sentinel: -2
3Size of range: 4
4Last in range: 8

In this example, the std::ranges::for_each() algorithm processes the range and returns an object with the in member pointing to where the sentinel was triggered. By calculating the distance from the beginning of the range to this iterator, you can determine the size of the range.

This approach ensures that you accurately and efficiently determine the size of ranges defined by sentinels without needing to run additional algorithms.