cpp-descriptive-pointers/maybe_null.hpp

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

#pragma once

#include <memory>

#include "pointer_fwd.hpp"
#include "ptr_compare.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

#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:
  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(detail::is_nt_cc<P>::value) : _ptr(p) { }
  maybe_null(P && p) noexcept(detail::is_nt_mc<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::move(p)) { }
  maybe_null(maybe_null const&) noexcept(detail::is_nt_cc<P>::value) = default;
  maybe_null(maybe_null &&) noexcept(detail::is_nt_mc<P>::value) = default;
  template <typename Y>
  maybe_null(maybe_null<Y> const&other) noexcept(detail::is_nt_c<P, Y>::value)
  : _ptr(other._ptr) { set_tested(other.tested_); }
  
  maybe_null& operator=(maybe_null const&) noexcept(detail::is_nt_ca<P>::value) = default;
  maybe_null& operator=(maybe_null &&) noexcept(detail::is_nt_ma<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
POINTER_TEMPLATE_COMPARE( maybe_null )