Creating Custom Iterators using C++20 Concepts

Making Custom Iterators Default Constructible

Why should I make my custom iterator default constructible and how do I do it?

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Making your custom iterator default constructible is important because it increases flexibility when using the iterator in various contexts. For example, some algorithms may need to declare an iterator without immediately assigning it a value.

To make an iterator default constructible, you need to provide a default constructor. Here’s an example:

1#include <iostream>
2#include <stdexcept>
3#include <string>
4
5class Player {
6 public:
7  std::string Name;
8};
9
10class Party {
11 public:
12  Party(Player A, Player B, Player C)
13    : A{A}, B{B}, C{C} {}
14
15  Player A, B, C;
16
17  class Iterator {
18   public:
19    using iterator_category
20      = std::forward_iterator_tag;
21    using value_type = Player;
22    using difference_type = std::ptrdiff_t;
23    using pointer = Player*;
24    using reference = Player&;
25
26    Iterator() = default;  
27    Iterator(Party* ptr, size_t idx)
28      : Party(ptr), idx(idx) {}
29
30    Player& operator*() const {
31      if (idx == 0) return Party->A;
32      if (idx == 1) return Party->B;
33      if (idx == 2) return Party->C;
34      throw std::out_of_range("Invalid index");
35    }
36
37    Player* operator->() const { return &**this; }
38
39    Iterator& operator++() {
40      ++idx;
41      return *this;
42    }
43
44    Iterator operator++(int) {
45      Iterator tmp = *this;
46      ++(*this);
47      return tmp;
48    }
49
50    bool operator==(const Iterator& other) const {
51      return Party == other.Party && idx == other.idx;
52    }
53
54    bool operator!=(const Iterator& other) const {
55      return !(*this == other); }
56
57   private:
58    size_t idx = 0;
59    Party* Party = nullptr;
60  };
61
62  Iterator begin() { return Iterator(this, 0); }
63  Iterator end() { return Iterator(this, 3); }
64};
65
66int main() {
67  Party P{Player{"Anna"},
68    Player{"Bob"}, Player{"Cara"}};
69
70  Party::Iterator it;  
71
72  for (it = P.begin(); it != P.end(); ++it) {
73    std::cout << it->Name << "\n";
74  }
75}

1Anna
2Bob
3Cara

Explanation

Default Constructor: The Iterator class now has a default constructor (Iterator() = default;). This means you can create an Iterator object without initializing it immediately.
Use in main Function: Demonstrates declaring an Iterator and assigning it a value later. This can be useful in control flow scenarios.

Making your iterator default constructible improves its usability in a variety of contexts and ensures it can be used with more generic algorithms and containers.

This Question is from the Lesson: