Boost C++ Libraries

boost/histogram/detail/iterator_adaptor.hpp

// Copyright 2019 Hans Dembinski
//
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt
// or copy at http://www.boost.org/LICENSE_1_0.txt)
//
// Uses code segments from boost/iterator/iterator_adaptor.hpp
// and boost/iterator/iterator_fascade.hpp

#ifndef BOOST_HISTOGRAM_DETAIL_ITERATOR_ADAPTOR_HPP
#define BOOST_HISTOGRAM_DETAIL_ITERATOR_ADAPTOR_HPP

#include <cstddef>
#include <iterator>
#include <memory>
#include <type_traits>
#include <utility>

namespace boost {
namespace histogram {
namespace detail {

// operator->() needs special support for input iterators to strictly meet the
// standard's requirements. If *i is not a reference type, we must still
// produce an lvalue to which a pointer can be formed.  We do that by
// returning a proxy object containing an instance of the reference object.
template <class Reference>
struct operator_arrow_dispatch_t {
  struct pointer {
    explicit pointer(Reference const& x) noexcept : m_ref(x) {}
    Reference* operator->() noexcept { return std::addressof(m_ref); }
    Reference m_ref;
  };

  using result_type = pointer;
  static result_type apply(Reference const& x) noexcept { return pointer(x); }
};

// specialization for "real" references
template <class T>
struct operator_arrow_dispatch_t<T&> {
  using result_type = T*;
  static result_type apply(T& x) noexcept { return std::addressof(x); }
};

// it is ok if void_t is already defined in another header
template <class...>
using void_t = void;

template <class T, class = void>
struct get_difference_type_impl : std::make_signed<T> {};

template <class T>
struct get_difference_type_impl<
    T, void_t<typename std::iterator_traits<T>::difference_type>> {
  using type = typename std::iterator_traits<T>::difference_type;
};

template <class T>
using get_difference_type = typename get_difference_type_impl<T>::type;

// adaptor supports only random access Base
// Base: underlying base type of the iterator; can be iterator, pointer, integer
// Reference: type returned when pointer is dereferenced
template <class Derived, class Base, class Reference = std::remove_pointer_t<Base>&,
          class Value = std::decay_t<Reference>>
class iterator_adaptor {
  using operator_arrow_dispatch = operator_arrow_dispatch_t<Reference>;

public:
  using base_type = Base;

  using reference = Reference;
  using value_type = Value;
  using pointer = typename operator_arrow_dispatch::result_type;
  using difference_type = get_difference_type<base_type>;
  using iterator_category = std::random_access_iterator_tag;

  iterator_adaptor() = default;

  explicit iterator_adaptor(base_type const& iter) : iter_(iter) {}

  // you can override this in derived
  decltype(auto) operator*() const noexcept { return *iter_; }

  // you can override this in derived
  Derived& operator+=(difference_type n) {
    iter_ += n;
    return this->derived();
  }

  // you should override this in derived if there is an override for operator+=
  template <class... Ts>
  difference_type operator-(const iterator_adaptor<Ts...>& x) const noexcept {
    return iter_ - x.iter_;
  }

  // you can override this in derived
  template <class... Ts>
  bool operator==(const iterator_adaptor<Ts...>& x) const noexcept {
    return iter_ == x.iter_;
  }

  reference operator[](difference_type n) const { return *(this->derived() + n); }
  pointer operator->() const noexcept {
    return operator_arrow_dispatch::apply(this->derived().operator*());
  }

  Derived& operator-=(difference_type n) { return this->derived().operator+=(-n); }
  Derived& operator++() { return this->derived().operator+=(1); }
  Derived& operator--() { return this->derived().operator+=(-1); }

  Derived operator++(int) {
    Derived tmp(this->derived());
    operator++();
    return tmp;
  }

  Derived operator--(int) {
    Derived tmp(this->derived());
    operator--();
    return tmp;
  }

  Derived operator+(difference_type n) const { return Derived(this->derived()) += n; }
  Derived operator-(difference_type n) const { return Derived(this->derived()) -= n; }

  template <class... Ts>
  bool operator!=(const iterator_adaptor<Ts...>& x) const noexcept {
    return !this->derived().operator==(x);
  }

  template <class... Ts>
  bool operator<(const iterator_adaptor<Ts...>& x) const noexcept {
    return iter_ < x.iter_;
  }

  template <class... Ts>
  bool operator>=(const iterator_adaptor<Ts...>& x) const noexcept {
    return iter_ >= x.iter_;
  }

  template <class... Ts>
  bool operator>(const iterator_adaptor<Ts...>& x) const noexcept {
    return iter_ > x.iter_;
  }

  template <class... Ts>
  bool operator<=(const iterator_adaptor<Ts...>& x) const noexcept {
    return iter_ <= x.iter_;
  }

  friend Derived operator+(difference_type n, const Derived& x) { return x + n; }

  Base const& base() const noexcept { return iter_; }

protected:
  // for convenience: refer to base class in derived class
  using iterator_adaptor_ = iterator_adaptor;

private:
  Derived& derived() noexcept { return *static_cast<Derived*>(this); }
  const Derived& derived() const noexcept { return *static_cast<Derived const*>(this); }

  base_type iter_;

  template <class, class, class, class>
  friend class iterator_adaptor;
};

} // namespace detail
} // namespace histogram
} // namespace boost

#endif