What is a Sentinel in C++?

A sentinel in C++ is an object used to signal the end of a range, allowing algorithms to know when to stop processing. Unlike traditional iterators, which point to specific memory addresses, sentinels provide a more flexible way to define the end of a range.

Sentinels can be compared to iterators using the == and != operators. When the comparison returns true, the algorithm stops.

Sentinels are particularly useful when the endpoint of a range is not known in advance or is determined dynamically during the execution of an algorithm.

For instance, you might use a sentinel to end a range at the first negative number in a list, which is a condition discovered during iteration.

Here is a simple example:

1#include <iostream>
2#include <vector>
3
4struct Sentinel {
5  bool operator==(
6    std::vector<int>::const_iterator iter) const {
7    return *iter < 0;
8  }
9};
10
11int main() {
12  std::vector<int> numbers{1, 4, 3, 8, -2, 5};
13  Sentinel s;
14
15  for (auto it = numbers.begin(); it != s; ++it) {
16    std::cout << *it << ", ";
17  }
18}

11, 4, 3, 8,

In this example, the loop stops when it encounters the first negative number, thanks to the custom sentinel.