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

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

#pragma once

#include <memory>

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

template <typename P>
class not_null<std::weak_ptr<P>>; // A weak_ptr cannot be a not_null

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

  explicit not_null(std::nullptr_t) = delete;
  explicit not_null(int) = delete;
  not_null(P const & p) : _ptr(p) { validate(); }
  not_null(P && p) : _ptr(std::move(p)) { validate(); }
  template <typename Y, typename = typename std::enable_if<
                            std::is_constructible<P, Y>::value>::type>
  not_null(Y const & p) : _ptr(p) {
    validate();
  }
  template <typename Y, typename = typename std::enable_if<
                            std::is_constructible<P, Y>::value>::type>
  not_null(Y && p) : _ptr(std::forward<Y>(p)) {
    validate();
  }
  not_null(not_null const &) noexcept(
      std::is_nothrow_copy_constructible<P>::value) = default;
  not_null(not_null &&) = delete;

  template <typename Y>
  explicit operator maybe_null<Y>() const
      noexcept(std::is_nothrow_constructible<Y, P>::value) {
    return _ptr;
  }
  template <typename Y>
  explicit operator not_null<Y>() const
      noexcept(std::is_nothrow_constructible<Y, P>::value) {
    return _ptr;
  }

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

  operator bool() const noexcept { return true; }

  pointer get() const noexcept { return detail::get_ptr<P>().get(_ptr); }
  reference operator*() const noexcept { return *_ptr; }
  pointer operator->() const noexcept { return get(); }

  void reset(P const & p) { operator=(not_null(p)); }
  void reset(P && p) { operator=(not_null(std::forward(p))); }

private:
  void validate() {
    if (get() == nullptr) {
      throw null_pointer_exception{"not_null<P> cannot be null"};
    }
  }
  P _ptr;
};