How Do Template Sentinels Work in C++?

Template sentinels in C++ are a way to create sentinels that can work with any iterator type, providing greater flexibility and reusability. By defining a sentinel as a template, you can make it compatible with various containers and iterator types.

Here’s an example of a template sentinel that stops when a negative number is encountered:

1#include <iostream>
2#include <vector>
3
4template <typename T>
5struct Sentinel {
6  bool operator==(T iter) const {
7    return *iter < 0 || iter == end;
8  }
9
10  T end;
11};
12
13int main() {
14  std::vector<int> numbers{1, 4, 3, 8, -2, 5};
15  Sentinel<std::vector<int>::const_iterator> s{
16    numbers.end()};
17
18  for (auto it = numbers.begin(); it != s; ++it) {
19    std::cout << *it << ", ";
20  }
21}

11, 4, 3, 8,

In this example, the Sentinel struct is defined as a template, allowing it to accept any iterator type. The operator==() method checks if the value pointed to by the iterator is negative or if the iterator has reached the end of the container.

Using template sentinels makes your code more flexible and reusable. You can apply the same sentinel logic to different types of containers and iterators without rewriting the sentinel for each specific type. This approach leverages the power of C++ templates to create versatile and efficient code.