Re: What does null mean?

DeMarcus <>
Thu, 25 Apr 2013 16:42:50 -0700 (PDT)

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

CameraMan cm( nullptr );

I would guess that CameraMan is a pointer or PointerLike (whatever the
latter means in detail).

It could mean anything, right?

In theory: Yes. In practice: I hope not.

This is a good example of what different people expects from vague code.
You make a clear assumption that this will be a smart pointer. How
annoyed won't you be when you discover it's something completely
different. At least I tend to become slightly uncomfortable when I have
check the documentation for every function call I go through in a
less-than-perfect code base.

Had they at least written

CameraMan cm( ANGLE_MISSING );

then I would at least have avoided the worst surprises.

I cannot prevent any programmer to assign a special funny/obscure
meaning to what it means that some (user-defined) type CameraMan is
initialized by the std::nullptr_t value. I would *hope* that this type
is indeed something that is related to (smart-)pointers. I would still
not know yet whether it means that CameraMan *is* a pointer-like type or
whether it has to be constructed with a pointer-like type (for whatever

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

It depends what you mean with "manage to solve". Since std::unique_ptr
is movable but not copyable, it means that you have to provide rvalues
instead of lvalues in situations of assignment or construction. And
obviously the container type (such as CameraMan) would be move-only as
well. If you mean, that your adapted code using your NullNameAdapter
template still does not compile, when you have replaced all occurrences
of int* by std::unique_ptr<int> *plus* the necessary additions of
std::move at the appropriate places, you may consider to replace your

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


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

to provide move-only support (in regard to the hosted type T) to

Impressive! I hadn't even seen that operator syntax before. I will take
a look at it. Thanks!

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;

I wouldn't recommend this as a general guide-line. Instead I would
recommend to think twice if you consider to give values of type
std::nullptr_t a very different meaning from initializing a pointer-like
thingee. It is unclear to me, why your NullNameAdapter gives
std::nullptr_t values such a special meaning. The hidden assumption is,
that all values *different* from std::nullptr_t would be "reasonable"
initialization values. I see no fundamental reason for this assumption.
So what about wrapping boost::optional in your adapter and reflecting
upon whether boost::none_t should be allowed as an initializer? Why
would this be "good" or "bad"?

As I wrote in another post in this thread; my own mistake proves my
point that unclear code drains energy. Of course BAD_ANGLE should have

boost::optional is a good tool, but used as a parameter in a constructor
it doesn't stop you from providing nullptr as an argument which will
force you to check the documentation to understand /what/ is missing.


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