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;
  }
  bool operator==(maybe_null const&rhs) const { return _ptr == rhs._ptr; }
  bool operator!=(maybe_null const&rhs) const { return _ptr != rhs._ptr; }
  bool operator<=(maybe_null const&rhs) const { return _ptr <= rhs._ptr; }
  bool operator>=(maybe_null const&rhs) const { return _ptr >= rhs._ptr; }
  bool operator< (maybe_null const&rhs) const { return _ptr <  rhs._ptr; }
  bool operator> (maybe_null const&rhs) const { return _ptr >  rhs._ptr; }
private:
  T _ptr;
#if defined( DEBUG )
  mutable bool tested_ = false;
#endif
};