cpp-descriptive-pointers/maybe_null.hpp

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

#pragma once

#include <stdexcept>
#include <memory>

#include "pointer_fwd.hpp"

class unchecked_pointer_exception : public std::logic_error {
  using std::logic_error::logic_error;
};

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

template <typename T>
class maybe_null {
public:
  using element_type = typename std::pointer_traits<T>::element_type;
  using pointer = element_type *;
  using reference = element_type &;
  
  maybe_null() : _ptr(nullptr) {}
  maybe_null(T const & p) : _ptr(p) { }
  maybe_null(T && p) : _ptr(std::move(p)) { }
  maybe_null(maybe_null const&) = default;
  
  template <typename Y>
  explicit operator maybe_null<Y>() const {
    return _ptr;
  }
  
  maybe_null& operator=(maybe_null const&) = default;
  template <typename Y> maybe_null& operator=(maybe_null<Y> const&other) {
    if (_ptr != other._ptr) {
      _ptr = other._ptr;
#if defined( DEBUG )
      tested_ = other.tested_;
#endif
    }
    return *this;
  }
  
  operator bool() const {
#if defined( DEBUG )
    tested_ = true;
#endif
    return static_cast<bool>(_ptr);
  }
  
  pointer get() const { return std::addressof(*_ptr); }
  pointer operator->() const {
    return std::addressof(operator*());
  }
  reference operator*() const {
#if defined( DEBUG )
    if ( !tested_ ) {
      throw unchecked_pointer_exception{"did not verify that pointer was non-null"};
    }
#endif
    return *_ptr;
  }
private:
  T _ptr;
#if defined( DEBUG )
  mutable bool tested_ = false;
#endif
};

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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