cpp-descriptive-pointers/include/pointers/maybe_null.hpp

//
//  maybe_null.hpp
//  pointer
//
//  Created by Sam Jaffe on 9/24/15.
//
//

#pragma once

#include <memory>

#include "detail/compare.hpp"
#include "detail/get_ptr.hpp"
#include "exception.hpp"
#include "pointer_fwd.hpp"

template <typename P> class maybe_null<not_null<P>>; // not permitted

#if defined(DEBUG)
#define set_tested(value) tested_ = value
#else
#define set_tested(_)
#endif

template <typename P>
class maybe_null : private detail::get_ptr<P>,
                   public detail::pointer_compare<maybe_null<P>> {
public:
  using element_type = typename std::pointer_traits<P>::element_type;
  using pointer = element_type *;
  using reference = element_type &;

  maybe_null() noexcept : _ptr(nullptr) {}
  maybe_null(P const & p) noexcept(std::is_nothrow_copy_constructible<P>::value)
      : _ptr(p) {}
  maybe_null(P && p) noexcept(std::is_nothrow_move_constructible<P>::value)
      : _ptr(std::move(p)) {}
  template <typename Y, typename = typename std::enable_if<
                            std::is_constructible<P, Y>::value>::type>
  maybe_null(Y const & p) : _ptr(p) {}
  template <typename Y, typename = typename std::enable_if<
                            std::is_constructible<P, Y>::value>::type>
  maybe_null(Y && p) : _ptr(std::forward<Y>(p)) {}
  maybe_null(maybe_null const &) noexcept(
      std::is_nothrow_copy_constructible<P>::value) = default;
  maybe_null(maybe_null &&) noexcept(
      std::is_nothrow_move_constructible<P>::value) = default;
  template <typename Y>
  maybe_null(maybe_null<Y> const & other) noexcept(
      std::is_nothrow_constructible<P, Y>::value)
      : _ptr(other._ptr) {
    set_tested(other.tested_);
  }

  maybe_null & operator=(maybe_null const &) noexcept(
      std::is_nothrow_copy_assignable<P>::value) = default;
  maybe_null & operator=(maybe_null &&) noexcept(
      std::is_nothrow_move_assignable<P>::value) = default;

  operator bool() const noexcept {
    set_tested(true);
    return static_cast<bool>(_ptr);
  }

  pointer get() const noexcept(noexcept(detail::get_ptr<P>::get(_ptr))) {
    return detail::get_ptr<P>::get(_ptr);
  }
  pointer operator->() const /*throw(unchecked_pointer_exception)*/ {
    return std::addressof(operator*());
  }
  reference operator*() const /*throw(unchecked_pointer_exception)*/ {
#if defined(DEBUG)
    if (!tested_) {
      throw unchecked_pointer_exception{
          "did not verify that pointer was non-null"};
    }
#endif
    if (!_ptr) {
      throw null_pointer_exception{"dereferencing maybe_null in null state"};
    }
    return *_ptr;
  }

  void reset(P const & p) { operator=(maybe_null(p)); }
  void reset(P && p = P()) { operator=(maybe_null(std::move(p))); }

private:
  template <typename Y> friend class maybe_null;
  P _ptr;
#if defined(DEBUG)
  mutable bool tested_ = false;
#endif
};

#undef set_tested