Custom Comparison for std::equal()

You can customize the comparison behavior for std::ranges::equal() by providing a custom comparison function as an additional argument.

This function should take two arguments (the elements to be compared) and return true if they should be considered equal. Here's a simple example:

1#include <algorithm>
2#include <vector>
3#include <iostream>
4#include <cmath> // Needed for std::abs
5
6int main() {
7  std::vector<int> A{1, -2, 3};
8  std::vector<int> B{-1, 2, -3};
9
10  auto absEqual = [](int x, int y) {
11    return std::abs(x) == std::abs(y);
12  };
13
14  if (std::ranges::equal(A, B, absEqual)) {
15    std::cout << "Ranges are equal";
16  } else {
17    std::cout << "Ranges are not equal";
18  }
19}

1Ranges are equal

In this example, the custom comparison function absEqual() compares the absolute values of the elements from the two ranges. As a result, std::ranges::equal() returns true because the absolute values match.

You can use custom comparison functions to implement more complex logic. For instance, you can compare objects by specific attributes:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5class Person {
6 public:
7  Person(std::string name, int age)
8    : name{name}, age{age} {}
9  std::string getName() const { return name; }
10  int getAge() const { return age; }
11
12 private:
13  std::string name;
14  int age;
15};
16
17int main() {
18  std::vector<Person> A{{"Alice", 30}, {"Bob", 25}};
19  std::vector<Person> B{{"Alice", 30}, {"Bob", 25}};
20
21  auto nameEqual = [](
22    const Person& p1, const Person& p2
23  ) {
24    return p1.getName() == p2.getName();
25  };
26
27  if (std::ranges::equal(A, B, nameEqual)) {
28    std::cout << "Ranges are equal by name";
29  } else {
30    std::cout << "Ranges are not equal by name";
31  }
32}

1Ranges are equal by name

In this example, the custom comparison function nameEqual() compares the name attribute of Person objects. Thus, std::ranges::equal() returns true if the names match, regardless of other attributes like age.

Custom comparison functions allow you to tailor the behavior of std::ranges::equal() to fit specific requirements, providing great flexibility in how you compare elements within collections.