How do I implement the necessary operators for a custom iterator in C++?

Question

Ryan McCombe · Accepted Answer

To implement the necessary operators for a custom iterator in C++, you need to define at least the following operators:

Dereference Operator *: Allows access to the element the iterator points to.
Increment Operator ++: Moves the iterator to the next element.
Equality Operator ==: Compares two iterators for equality.
Inequality Operator !=: Compares two iterators for inequality.

Let's implement these for a custom iterator that iterates over a simple container of Player objects.

First, define your Player and Party classes:

1#include <iostream>
2#include <string>
3#include <stdexcept>
4
5class Player {
6public:
7  std::string Name;
8};
9
10class Party {
11public:
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    Iterator(Party* ptr, size_t idx)
20      : Party(ptr), idx(idx) {}
21
22    Player& operator*() const {
23      if (idx == 0) return Party->A;
24      if (idx == 1) return Party->B;
25      if (idx == 2) return Party->C;
26      throw std::out_of_range("Invalid index");
27    }
28
29    Iterator& operator++() {
30      ++idx;
31      return *this;
32    }
33
34    bool operator==(const Iterator& other) const {
35      return Party == other.Party && idx == other.idx;
36    }
37
38    bool operator!=(const Iterator& other) const {
39      return !(*this == other);
40    }
41
42  private:
43    size_t idx;
44    Party* Party;
45  };
46
47  Iterator begin() { return Iterator(this, 0); }
48  Iterator end() { return Iterator(this, 3); }
49};
50
51int main() {
52  Party party{Player{"Anna"},
53    Player{"Bob"}, Player{"Cara"}};
54
55  for (Player& p : party) {
56    std::cout << p.Name << ", ";
57  }
58}

1Anna, Bob, Cara,

Explanation

Dereference Operator *: Returns a reference to the Player object the iterator points to. It uses if statements to map the index to the correct Player.
Increment Operator ++: Advances the iterator by incrementing the index.
Equality and Inequality Operators (==, !=): Compare the iterators based on their pointers and indices. The inequality operator is implemented in terms of the equality operator.

By implementing these operators, you enable the basic functionality required for range-based for loops and other iterator-based operations. Make sure to handle edge cases, such as when the iterator points past the end of the container.

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Explanation

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