[vtk-developers] vtkNew<> (was: Smart pointer declaration macro?)

[David Gobbi]

Bill,

Yup, that's exactly how it would be used.  It's essentially Marcus'
original proposal, but re-named and no longer derived from
vtkSmartPointer in order to avoid certain gotcha's that Brad explained
but which I didn't fully understand.

   David


On Thu, Jan 28, 2010 at 2:56 PM, Bill Lorensen <bill.lorensen at gmail.com> wrote:
> I admit I don't understand the "how", but the "what" looks great.
>
> So,
> instead of:
>
> vtkSmartPointer<vtkImageRectilinearWipe> wipe =
>  vtkSmartPointer<vtkImageRectilinearWipe>::New();
>
> it would be what?
> vtkNew<vtkImageRectilinearWipe> wipe;
>
>
> On Thu, Jan 28, 2010 at 4:41 PM, Brad King <brad.king at kitware.com> wrote:
>> David Gobbi wrote:
>>> This is reply mainly to Brad about vtkLocalPointer<vtkClass>.
>>>
>>> I'm not quite sure why having a smart pointer declaration do a
>>> allocation is such a bad thing, as long as it is explicit that an
>>> object is being allocated.  How about if we make the allocation
>>> explicit in the name of the template, e.g.
>>>
>>> 4. vtkNew<vtkClass> object;
>>>
>>> I know that it appears that the object is being created on the stack,
>>> but that shouldn't be an issue because the stl container classes use
>>> heap allocation, too.
>>
>> Nice!  It's okay if two conditions are satisfied:
>>
>>  - the allocation is explicit in the name
>>  - the type is not a smart pointer
>>
>> If the name "Pointer" is not present then it takes away expectations
>> of pointer-like operations (e.g. assignment).
>>
>> I propose a vtkNew<> that is not a subclass of vtkSmartPointer and
>> has very few operations:
>>
>>  - default constructor     (allocate & take-reference)
>>  - arrow operator ->       (access)
>>  - operator T*             (conversion & raw pointer access)
>>  - destructor              (deletes reference)
>>
>> We would specifically disable other operations:
>>
>>  - no copy constructor
>>  - no assignment operator=
>>
>> We could teach vtkSmartPointer<> to construct from vtkNew<>.
>>
>> A local variable declared with vtkNew<> would create and own one
>> object for its whole lifetime.  The object will go away on its
>> destruction unless ownership has been shared by assigning the
>> object to a real smart pointer.  The exact same class could be
>> used in place of the vtkNew<> function template I proposed too:
>>
>>  vtkSmartPointer<vtkBase> o = vtkNew<vtkDerived>();
>>
>> In this example the RHS expression creates a temporary instance
>> of the vtkNew<> template.  The vtkSmartPointer<> initialization
>> takes its own reference to the object before the vtkNew<> template
>> goes away.
>>
>> A basic implementation of this vtkNew<> appears below.  It would
>> just need some tweaking in case the object allocation fails.
>> This class template is so simple we could consider defining it in
>> a top-level VTK header so that no sources need to include it.
>> Alternatively it could be provided in vtkSmartPointer.h.
>>
>> -Brad
>>
>>
>> template <class T> class vtkNew
>> {
>> public:
>>  vtkNew(): Object(T::New()) {}
>>  ~vtkNew() { this->Object->Delete(); }
>>  T* operator->() const { return this->Object; }
>>  operator T*() const   { return this->Object; }
>> private:
>>  vtkNew(vtkNew<T> const&);         // Not implemented.
>>  void operator=(vtkNew<T> const&); // Not implemented.
>>  T* Object;
>> };
>>
>> template <class T>
>> class vtkSmartPointer: public vtkSmartPointerBase
>> {
>>  static T* CheckType(T* t) { return t; }
>> public:
>>  ...
>>  template <class U>
>>  vtkSmartPointer(const vtkNew<U>& r): vtkSmartPointerBase(CheckType(r)) {}
>>  ...
>> };
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>