cpp-descriptive-pointers/not_null.hpp

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

#pragma once

#include <stdexcept>
#include <memory>

#include "pointer_fwd.hpp"
#include "maybe_null.hpp"

class null_pointer_exception : public std::invalid_argument {
  using std::invalid_argument::invalid_argument;
};

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

template <typename T>
class not_null {
public:
  using element_type = typename std::pointer_traits<T>::element_type;
  using pointer = element_type *;
  using reference = element_type &;
  
  explicit not_null(std::nullptr_t) = delete;
  explicit not_null(int) = delete;
  not_null(T const & p) : _ptr(p) { validate(); }
  not_null(T && p) : _ptr(std::move(p)) { validate(); }
  not_null(not_null const&) = default;

  template <typename Y>
  explicit operator maybe_null<Y>() const {
    return _ptr;
  }
  template <typename Y>
  explicit operator not_null<Y>() const {
    return _ptr;
  }

  not_null& operator=(not_null const&) = default;
  template <typename Y> not_null& operator=(not_null<Y> const&other) {
    _ptr = other._ptr;
    return *this;
  }
  
  explicit operator maybe_null<T>() const;
  operator bool() const { return true; }
  
  pointer get() const { return std::addressof(operator*()); }
  reference operator*() const { return *_ptr; }
  pointer operator->() const { return get(); }
  
private:
  void validate() {
    if (get() == nullptr) {
      throw null_pointer_exception{"not_null<T> cannot be null"};
    }
  }
  T _ptr;
};

template <typename T, typename U>
bool operator==(not_null<T> const&lhs, not_null<U> const&rhs) {
  return lhs.get() == rhs.get();
}

template <typename T, typename U>
bool operator!=(not_null<T> const&lhs, not_null<U> const&rhs) {
  return !(lhs == rhs);
}

template <typename T, typename U>
bool operator< (not_null<T> const&lhs, not_null<U> const&rhs) {
  typedef typename std::common_type<
  typename not_null<T>::pointer,
  typename not_null<U>::pointer>::type V;
  return std::less<V>(lhs.get(), rhs.get());
}

template <typename T, typename U>
bool operator> (not_null<T> const&lhs, not_null<U> const&rhs) {
  return rhs < lhs;
}

template <typename T, typename U>
bool operator<=(not_null<T> const&lhs, not_null<U> const&rhs) {
  return !(rhs < lhs);
}

template <typename T, typename U>
bool operator>=(not_null<T> const&lhs, not_null<U> const&rhs) {
  return !(lhs < rhs);
}

template <typename T>
bool operator==(not_null<T> const&lhs, std::nullptr_t) {
  return !lhs;
}

template <typename T>
bool operator==(std::nullptr_t, not_null<T> const&rhs) {
  return !rhs;
}

template <typename T>
bool operator!=(not_null<T> const&lhs, std::nullptr_t) {
  return static_cast<bool>(lhs);
}

template <typename T>
bool operator!=(std::nullptr_t, not_null<T> const&rhs) {
  return static_cast<bool>(rhs);
}

template <typename T>
bool operator< (not_null<T> const&lhs, std::nullptr_t) {
  typedef typename not_null<T>::pointer V;
  return std::less<V>(lhs.get(), nullptr);
}

template <typename T>
bool operator< (std::nullptr_t, not_null<T> const&rhs) {
  typedef typename not_null<T>::pointer V;
  return std::less<V>(nullptr, rhs.get());
}

template <typename T>
bool operator> (not_null<T> const&lhs, std::nullptr_t) {
  return nullptr < lhs;
}

template <typename T>
bool operator> (std::nullptr_t, not_null<T> const&rhs) {
  return rhs < nullptr;
}

template <typename T>
bool operator<=(not_null<T> const&lhs, std::nullptr_t) {
  return !(nullptr < lhs);
}

template <typename T>
bool operator<=(std::nullptr_t, not_null<T> const&rhs) {
  return !(rhs < nullptr);
}

template <typename T>
bool operator>=(not_null<T> const&lhs, std::nullptr_t) {
  return !(lhs < nullptr);
}

template <typename T>
bool operator>=(std::nullptr_t, not_null<T> const&rhs) {
  return !(nullptr < rhs);
}