What does null mean?

DeMarcus <demarcus_at_hotmail_com@tellus.orb.dotsrc.org>
Sun, 21 Apr 2013 15:48:31 -0700 (PDT)

If you read this code, what do you think it does?

CameraMan cm( nullptr );

It could mean anything, right?

In a previous post I elaborated with ideas to prevent the explicit usage
of nullptr. Thanks for all the replies there by the way!

I experimented with it and came up with the following solution (if
you're not interested in reading code you can jump directly to the
question in the end).

#include <utility>

template<typename T, class NullT>
class NullNameAdapter
   // Use Perfect Forwarding to pass the parameter (except
   // nullptr) as efficient as possible.
   template<typename U>
   NullNameAdapter( U&& u ) : t_(std::forward<U>(u)) {}

   // Delete the constructor taking nullptr. Doing this
   // is the essense of this adapter.
   NullNameAdapter( std::nullptr_t ) = delete;

   // This constructor is the replacement for the deleted
   // std::nullptr_t constructor above.
   NullNameAdapter( NullT ) : t_(nullptr) {}

   // Conversion operator to be able to return the type
   // easily.
   operator T&() { return t_; }

   T t_;

const struct BadAngleType {} BAD_ANGLE;
typedef NullNameAdapter<int*, BadAngleType> AnglePtrNNA;

class CameraMan
   CameraMan( AnglePtrNNA angle ) : angle_(angle) {}

   int* angle_;

int main()
   int* anglep = new int( 180 );
   CameraMan a( anglep );
   CameraMan b( BAD_ANGLE );
   CameraMan c( nullptr ); // Won't compile. Good!

   return 0;

I almost got it to work on a general basis. The only thing I didn't
manage to solve was the situation if you replace int* with
std::unique_ptr<int>. (If anyone feels tempted to solve it I would be

Even if this could be solved I realize that it's quite a lot of
boilerplate code just to prevent the explicit nullptr, so I'm about to
abandon that strategy. I still think though, that the explicit nullptr
is troublesome so I want at least to come up with some coding guideline.

In C++ Coding Standards by Sutter & Alexandrescu there is the
recommendation Item 17 - Avoid magic numbers. It says that instead of
using 3.1415 explcitly in the code you should make a symbolic constant
out of it, like this.
const double PI = 3.1415;

My question is; would you all agree that nullptr is a magic number that
always should be replaced with a label like the following?
const std::nullptr_t BAD_ANGLE = nullptr;


      [ See http://www.gotw.ca/resources/clcm.htm for info about ]
      [ comp.lang.c++.moderated. First time posters: Do this! ]

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