Combining Multiple Projection Functions in C++

Combining multiple projection functions for a single algorithm in C++ is possible but requires careful design.

While standard algorithms typically accept a single projection function, you can chain projections or compose them to achieve the desired result.

Method 1: Chaining Projections

You can chain projections by having one projection call another. Here's an example where we sort Player objects first by Level and then by Name:

1#include <vector>
2#include <iostream>
3#include <algorithm>
4
5struct Player {
6  std::string Name;
7  int Level;
8};
9
10int main() {
11  std::vector<Player> Party {
12    {"Legolas", 49},
13    {"Gimli", 47},
14    {"Gandalf", 49},
15    {"Aragorn", 47}
16  };
17
18  std::ranges::sort(Party, {}, [](const Player& P) {
19    return std::tie(P.Level, P.Name);
20  });
21
22  for (const auto& P : Party) {
23    std::cout << "[" << P.Level << "] "
24      << P.Name << "\n";
25  }
26}

1[47] Aragorn
2[47] Gimli
3[49] Gandalf
4[49] Legolas

In this example, the projection function returns a std::tie of Level and Name, allowing std::ranges::sort() to sort by both criteria.

Method 2: Composition of Projections

You can create higher-order functions that compose multiple projection functions. Here’s an example:

1#include <vector>
2#include <iostream>
3#include <algorithm>
4#include <tuple>
5
6struct Player {
7  std::string Name;
8  int Level;
9};
10
11template<typename F1, typename F2>
12auto compose(F1 f1, F2 f2) {
13  return [f1, f2](const auto& x) {
14    return std::make_tuple(f1(x), f2(x));
15  };
16}
17
18int main() {
19  std::vector<Player> Party {
20    {"Legolas", 49},
21    {"Gimli", 47},
22    {"Gandalf", 49},
23    {"Aragorn", 47}
24  };
25
26  auto projection = compose(
27    [](const Player& P) { return P.Level; },
28    [](const Player& P) { return P.Name; }
29  );
30
31  std::ranges::sort(Party, {}, projection);
32
33  for (const auto& P : Party) {
34    std::cout << "[" << P.Level << "] "
35      << P.Name << "\n";
36  }
37}

1[47] Aragorn
2[47] Gimli
3[49] Gandalf
4[49] Legolas

Key Points:

• Chaining Projections: Use std::tie or similar constructs to chain projections within a single function.
• Composition: Create higher-order functions to compose multiple projections into one.

Combining projections can provide powerful and flexible sorting and transformation capabilities in your algorithms, allowing you to handle complex sorting criteria effectively.