Re: What does null mean?

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <>
Mon, 22 Apr 2013 15:48:09 CST
On 2013-04-22 00:48, DeMarcus wrote:

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.

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 member

  // 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

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;

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"?

HTH & Greetings from Bremen,

Daniel Kr?gler

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