iterator_facade

iterator_facade for C++20

• Free software: CC0
• Documentation: https://iterator-facade.readthedocs.io.
• Workaround for Clang bug

Features

• constexpr support
• automatic deduction of iterator category
• specialized std::iterator_traits for iterators subclassing iterator_facade
• no need to specify any other types unlike boost::iterator_facade
• propagated noexcept
• works with sentinels
• possible to wrap contiguous iterators and preserve contiguousness

Install

Copy paste the single header into your project

Usage

#include <iterator>

#include <iterator_facade/iterator_facade.hpp>

namespace iterf = iterator_facade;

template <std::input_or_output_iterator Iter>
class my_iterator : public iterf:iterator_facade<my_iterator<Iter>, std::contiguous_iterator<Iter>> {
    template <std::input_or_output_iterator>
    friend class my_iterator;

public:
    using value_type = std::iter_value_t<Iter>;
    using reference = std::iter_reference_t<Iter>;
    using difference_type = std::iter_difference_t<Iter>;

    constexpr my_iterator() noexcept(std::is_nothrow_constructible_v<Iter>) = default;
    constexpr my_iterator(Iter iter) noexcept(std::is_nothrow_move_constructible_v<Iter>) : iter_(std::move(iter)) {}

    template <class U>
        requires(std::constructible_from<Iter, U> && !std::same_as<Iter, U>)
    constexpr my_iterator(my_iterator<U> const& other) noexcept(std::is_nothrow_constructible_v<Iter, U>)
        : iter_(other.iter_) {}

    // Required:
    [[nodiscard]] constexpr auto dereference() const noexcept(iterf::nothrow_dereference<Iter>) -> reference { return *iter_; }
    constexpr void increment() noexcept(iterf::nothrow_increment<Iter>) { ++iter_; }

    // For forward iterators:
    template <std::sentinel_for<Iter> S>
    [[nodiscard]] constexpr auto equals(my_iterator<S> const& sentinel) const noexcept(iterf::nothrow_equals<Iter, S>)
        -> bool requires std::forward_iterator<Iter> {
        return iter_ == sentinel.iter_;
    }

    // For bidirectional iterators:
    constexpr void decrement() noexcept(iterf::nothrow_decrement<Iter>) requires std::bidirectional_iterator<Iter> { --iter_; }

    // For random access iterators:
    constexpr void advance(difference_type n) noexcept(iterf::nothrow_advance<Iter>) requires std::random_access_iterator<Iter> {
        iter_ += n;
    }

    template <std::sized_sentinel_for<Iter> S>
    [[nodiscard]] constexpr auto distance_to(my_iterator<S> const& sentinel) const noexcept(iterf::nothrow_distance_to<Iter, S>)
        -> difference_type requires std::random_access_iterator<Iter> {
        return sentinel.iter_ - iter_;
    }

private:
    Iter iter_;
};

Namespace can be customized by setting ITERATOR_FACADE_NS before including the header

my_iterator::dereference() is not required to return lvalue references. However, the value returned should not be a reference to a value owned by the iterator itself as it can result in dangling references.

my_iterator::advance(difference_type) will be used in place of my_iterator::increment() or my_iterator::decrement() if any of them are not defined.

my_iterator::distance_to(T) will be used in place of my_iterator::equals(T) if it is not defined.

iterator_facade::iterator_facade<T> will provide operators based on defined subclass methods:

• dereference will enable

  • constexpr auto T::operator*() const noexcept(...) -> reference
  • constexpr auto T::operator->() const noexcept(...) -> pointer (a proxy object if dereference returns a temporary)

• increment or advance will enable

  • constexpr auto T::operator++() noexcept(...) -> T&
  • constexpr auto T::operator++(int) noexcept(...) -> T

• equals or distance_to will enable

  • constexpr friend auto operator==(T const&, sentinel const&) noexcept(...) -> bool

• decrement or advance will enable

  • constexpr auto T::operator--() noexcept(...) -> T&
  • constexpr auto T::operator--(int) noexcept(...) -> T

• distance_to will additionally enable

  • constexpr friend auto T::operator-(T const&, sized_sentinel const&) noexcept(...) -> difference_type
  • constexpr friend auto T::operator-(sized_sentinel const&, T const&) noexcept(...) -> difference_type
  • constexpr friend auto T::operator<=>(T const&, sentinel const&) noexcept(...)`

• advance will additionally enable

  • constexpr friend auto T::operator+(T, difference_type) noexcept(...) -> T
  • constexpr friend auto T::operator+(difference_type, T) noexcept(...) -> T
  • constexpr friend auto T::operator+=(T&, difference_type) noexcept(...) -> T&
  • constexpr friend auto T::operator-(T, difference_type) noexcept(...) -> T
  • constexpr friend auto T::operator-=(T&, difference_type) noexcept(...) -> T&
  • constexpr auto T::operator[](difference_type) const noexcept(...) -> reference (will leave a dangling reference if it points to a value owned by the iterator)

Credits

This package was created with cookietemple using Cookiecutter.