What are iterator category tags and how do I use them in C++?

Question

Ryan McCombe · Accepted Answer

Iterator category tags in C++ are used to specify the capabilities of an iterator. They help algorithms determine what operations can be performed with the iterator. The main categories are:

std::input_iterator_tag
std::output_iterator_tag
std::forward_iterator_tag
std::bidirectional_iterator_tag
std::random_access_iterator_tag
std::contiguous_iterator_tag

For example, to create a forward iterator for a custom container, you can use std::forward_iterator_tag. Here's how to implement it:

1#include <iostream>
2#include <string>
3#include <iterator>
4#include <stdexcept>
5
6class Player {
7public:
8  std::string Name;
9};
10
11class Party {
12public:
13  Party(Player A, Player B, Player C)
14    : A{A}, B{B}, C{C} {}
15
16  Player A, B, C;
17
18  class Iterator {
19  public:
20    using iterator_category = 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(Party* ptr, size_t idx)
27      : Party(ptr), idx(idx) {}
28
29    Player& operator*() const {
30      if (idx == 0) return Party->A;
31      if (idx == 1) return Party->B;
32      if (idx == 2) return Party->C;
33      throw std::out_of_range("Invalid index");
34    }
35
36    Iterator& operator++() {
37      ++idx;
38      return *this;
39    }
40
41    bool operator==(const Iterator& other) const {
42      return Party == other.Party && idx == other.idx;
43    }
44
45    bool operator!=(const Iterator& other) const {
46      return !(*this == other);
47    }
48
49  private:
50    size_t idx;
51    Party* Party;
52  };
53
54  Iterator begin() { return Iterator(this, 0); }
55  Iterator end() { return Iterator(this, 3); }
56};
57
58int main() {
59  Party party{Player{"Anna"},
60    Player{"Bob"}, Player{"Cara"}};
61
62  for (Player& p : party) {
63    std::cout << p.Name << ", ";
64  }
65}

1Anna, Bob, Cara,

Explanation

iterator_category: Specifies that the Iterator is a forward iterator.
value_type, difference_type, pointer, reference: These typedefs provide information about the types used by the iterator.

Using these tags and typedefs makes your iterator more compatible with the standard library algorithms and allows the compiler to optimize code based on the iterator's capabilities.

Creating Custom Iterators using C++20 Concepts

Using Iterator Category Tags in C++

Explanation

Creating Custom Iterators using C++20 Concepts

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