[vtk-developers] [coding tip] no call to Set in constructors

[Moreland, Kenneth]

A slight qualification: it is OK to call the Set function in the constructors _so long as you initialize the underlying variable first_.  I find I do this frequently with the string and object ivars to ensure that the memory management happens correctly.  For example:

this->FileName = NULL;
this->SetFileName("/tmp/log");

or

this->Controller = NULL;
this->SetController(vtkMultiProcessController::GetGlobalController());

Of course, setting the ivar to NULL first is vital, or things could potentially die a really horrible death.

-Ken


On 4/29/09 7:31 AM, "Francois Bertel" <francois.bertel at kitware.com> wrote:

Hello,

I've seen that more often in recent additions to VTK: calls to some
Set methods in the default constructor.

I would like to stress that Set methods defined with vtkSetMacro (it
might be OK for other Set methods) cannot be used in the default
constructor to initialize ivars because the first line of a Set method
is to compare the current value of the ivar with the value in
argument. As at this point (in the constructor), the ivar is not
initialized yet, the comparison happens against an uninitialized
value.

valgrind will detect this kind of fault.

For this reason, in the constructor, the value of an ivar has to be
initialized directly with the assignment operator not through a Set
method defined with vtkSetMacro.

Example:

class vtkFoo : public vtkObject
{
 public:
...
  vtkGetMacro(X,int);
  vtkSetMacro(X,int);
...
 protected:
  vtkFoo();
  int X;
...
};

vtkFoo::vtkFoo
{
 this->SetX(12); // neh
}

vtkFoo::vtkFoo
{
 this->X=12; // good
}

The issue looks pretty obvious in an isolated example like the one
shown above. Sometimes it can also happen with an indirect call:

void vtkFoo::SetXToZero()
{
 this->SetX(0);
}

vtkFoo::vtkFoo
{
 this->SetXToZero(); // neh
}

I added a section addressing this issue in the VTK Coding Standards:
http://www.vtk.org/Wiki/VTK_Coding_Standards#Set_method_in_constructor



Here is a real case in VTK, detected by valgrind:

http://www.cdash.org/CDash/viewDynamicAnalysisFile.php?id=325444

vtkRenderView::vtkRenderView() calls SetInteractionModeTo3D(), which
calls SetInteractionMode(), but ivar InteractionMode has not been
initialized yet.



--
François Bertel, PhD  | Kitware Inc. Suite 204
1 (518) 371 3971 x113 | 28 Corporate Drive
                      | Clifton Park NY 12065, USA
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   ****      Kenneth Moreland
    ***      Sandia National Laboratories
***********
*** *** ***  email: kmorel at sandia.gov
**  ***  **  phone: (505) 844-8919
    ***      web:   http://www.cs.unm.edu/~kmorel

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