Using Custom Comparators

To use std::ranges::includes() with a custom comparator, you need to define a comparator function that specifies the order of elements in your sets.

The comparator should be a callable object (like a lambda function) that takes two elements as parameters and returns true if the first element is considered less than the second.

Here’s a step-by-step guide:

1. Define your sets and the comparator function: The sets can be any type of container that supports forward iterators, and the comparator function determines the order.
2. Call std::ranges::includes() with the comparator: Pass the sets and the comparator to the function.

Here’s an example using sets of integers, where the comparator defines a descending order:

1#include <algorithm>
2#include <iostream>
3#include <set>
4
5int main() {
6  std::set<int> A{3, 2, 1};
7  std::set<int> B{2, 1};
8
9  // Custom comparator for descending order
10  auto Comparer = [](const int& A, const int& B) {
11    return A > B;
12  };
13
14  if (std::ranges::includes(A, B, Comparer)) {
15    std::cout << "A includes B";
16  } else {
17    std::cout << "A does not include B";
18  }
19}

1A includes B

In this example, the Comparer() lambda function is passed to std::ranges::includes(). The function checks if all elements in B are present in A considering the custom order defined by the comparator.

Custom Comparators for Complex Types

For more complex types, like a custom class, you need to adjust the comparator accordingly. Here's an example using a Character class, comparing based on a Name attribute:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5class Character {
6 public:
7  Character(std::string Name) : Name{Name} {}
8  std::string Name;
9};
10
11int main() {
12  std::vector<Character> A{
13    {"Aragorn"}, {"Frodo"}, {"Gimli"}};
14  std::vector<Character> B{
15    {"Frodo"}, {"Gimli"}};
16
17  // Comparator for ascending order by Name
18  auto Comparer = [](
19    const Character& A,
20    const Character& B
21  ) {
22    return A.Name < B.Name;
23  };
24
25  if (std::ranges::includes(A, B, Comparer)) {
26    std::cout << "A includes B";
27  } else {
28    std::cout << "A does not include B";
29  }
30}

1A includes B

Here, the Comparer() lambda compares Character objects based on their Name attribute. The algorithm works similarly, ensuring all elements in B are in A according to the custom comparison.

Using custom comparators allows std::ranges::includes() to be flexible and work with various data types and ordering schemes.