Customizing Predicates for std::ranges::count_if()

Customizing the behavior of std::ranges::count_if() for specific data types involves writing specialized predicate functions.

These predicates can leverage the properties and methods of the data types to determine the count criteria.

Example with Custom Class

Let's consider a custom class Player and use std::ranges::count_if() to count players with specific attributes.

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
Player Class|Show
5class Player {/*...*/};
20
21int main() {
22  std::vector<Player> players {
23    {"Roderick", 100, 10},
24    {"Anna", 50, 5},
25    {"Robert", 150, 12}
26  };
27
28  int threshold = 8;
29  auto highLevelPredicate = [&](const Player& p) {
30    return p.isHighLevel(threshold); 
31  };
32
33  int highLevelCount = std::ranges::count_if(
34    players, highLevelPredicate); 
35
36  std::cout << "Number of high-level players: "
37    << highLevelCount;
38}

1Number of high-level players: 2

Custom Predicate Function

In this example:

• The Player class has an isHighLevel() method to check if a player's level exceeds a given threshold.
• In main(), we create a lambda highLevelPredicate that binds the threshold and uses isHighLevel().
• We pass this lambda to std::ranges::count_if() to count players above the threshold level.

General Tips

• Use Lambdas: Lambdas are great for creating custom predicates on-the-fly.
• Member Functions: Leverage member functions within your predicates for readability and encapsulation.
• Capture Lists: Use lambda capture lists to bind external variables to your predicates.

By writing custom predicates, you can tailor std::ranges::count_if() to fit various complex criteria, making your code more expressive and powerful.