Unique Copy with Predicate

Yes, you can use std::ranges::unique_copy() with a predicate that depends on multiple object properties by providing a custom comparison function.

This allows you to define what it means for two objects to be "unique" based on your specific criteria.

Here’s an example using a custom comparison function with a Player class, where two Player objects are considered the same if they have the same Name and Level:

1#include <algorithm>
2#include <iostream>
3#include <string>
4#include <vector>
5
6class Player {
7 public:
8  Player() = default;
9  Player(std::string name, int level)
10    : name(name), level(level) {}
11
12  std::string getName() const { return name; }
13  int getLevel() const { return level; }
14
15 private:
16  std::string name;
17  int level;
18};
19
20bool customCompare(const Player& a, const Player& b) {
21  return a.getName() == b.getName()
22    && a.getLevel() == b.getLevel();
23}
24
25int main() {
26  std::vector<Player> Source{
27    {"Anna", 10}, {"Anna", 10},
28    {"Bob", 20}, {"Bob", 25},
29    {"Anna", 10}, {"Anna", 20}
30  };
31  std::vector<Player> Dest;
32  Dest.resize(Source.size());
33
34  auto [in, out] = std::ranges::unique_copy(
35    Source.begin(), Source.end(),
36    Dest.begin(), customCompare
37  );  
38
39  for (auto it = Dest.begin(); it != out; ++it) {
40    std::cout << it->getName() << " (Level "
41      << it->getLevel() << ")\n";
42  }
43}

1Anna (Level 10)
2Bob (Level 20)
3Bob (Level 25)
4Anna (Level 10)
5Anna (Level 20)

In this example:

• The Player class has Name and Level properties.
• The customCompare() function returns true if both Name and Level are equal.
• std::ranges::unique_copy() uses this custom comparison to determine uniqueness.

You can modify the customCompare() function to include more properties or different logic based on your requirements.

Steps to Use Custom Comparison:

1. Define the Custom Comparison Function: Create a function that takes two objects and returns true if they should be considered equal.
2. Call the std::ranges::unique_copy() algorithm: Pass the custom comparison function as the third argument to std::ranges::unique_copy().

This allows for flexible and powerful control over what constitutes a unique object, enabling complex comparisons based on multiple properties.