cpp-useful-iterators/include/iterator/proxy.h

#pragma once

#include <iterator/facade.h>
#include <iterator/iterator_fwd.hpp>

namespace iterator {
template <typename Iter, typename Self>
class proxy<Iter, Self, std::input_iterator_tag> : public facade<Self> {
public:
  using single_pass_iterator = void;

private:
  Iter impl_;

public:
  proxy(Iter impl = {}) : impl_(impl) {}
  template <typename... Args>
  proxy(Args &&... args) : impl_(std::forward<Args>(args)...) {}

  decltype(auto) dereference() const { return *impl_; }
  void increment() { ++impl_; }
  bool equal_to(Self const & other) const { return impl_ == other.impl_; }

protected:
  auto & impl() const { return impl_; }
};

template <typename Iter, typename Self>
class proxy<Iter, Self, std::forward_iterator_tag> : public facade<Self> {
private:
  Iter impl_;

public:
  proxy(Iter impl = {}) : impl_(impl) {}
  template <typename... Args>
  proxy(Args &&... args) : impl_(std::forward<Args>(args)...) {}

  decltype(auto) dereference() const { return *impl_; }
  void increment() { ++impl_; }
  bool equal_to(Self const & other) const { return impl_ == other.impl_; }

protected:
  auto & impl() const { return impl_; }
};

template <typename Iter, typename Self>
class proxy<Iter, Self, std::bidirectional_iterator_tag> : public facade<Self> {
private:
  Iter impl_;

public:
  proxy(Iter impl = {}) : impl_(impl) {}
  template <typename... Args>
  proxy(Args &&... args) : impl_(std::forward<Args>(args)...) {}

  decltype(auto) dereference() const { return *impl_; }
  void increment() { ++impl_; }
  void decrement() { --impl_; }
  bool equal_to(Self const & other) const { return impl_ == other.impl_; }

protected:
  auto & impl() const { return impl_; }
};

template <typename Iter, typename Self>
class proxy<Iter, Self, std::random_access_iterator_tag> : public facade<Self> {
public:
  using difference_type = typename std::iterator_traits<Iter>::difference_type;

private:
  Iter impl_;

public:
  proxy(Iter impl = {}) : impl_(impl) {}
  template <typename... Args>
  proxy(Args &&... args) : impl_(std::forward<Args>(args)...) {}

  decltype(auto) dereference() const { return *impl_; }
  void advance(difference_type off) { impl_ += off; }
  // This shouldn't need to be implemented, but for some reason my traits
  // are not correctly deducing here.
  bool equal_to(Self const & other) const { return distance_to(other) == 0; }
  difference_type distance_to(Self const & other) const {
    return other.impl_ - impl_;
  }

protected:
  auto & impl() const { return impl_; }
};
}