[vtk-developers] Weak pointers and thread safety

Tue Oct 12 09:21:54 EDT 2010

Hi,

I think we should be carefull about the multithreading improvements. 
They are certainly necessary, but some carefull tests are needed to see if 
multithreading should be supported implicitely, with potential performance 
issues, or explicitely, with some overhead for developers.

I am more in favor of being explicit, at least now that we are only trying 
to find the best programming model. 
Maybe a class vtkThreadedWeakPointer with critical section?

Stephane

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will.schroeder at kitware.com 
Envoyé par : vtk-developers-bounces at vtk.org
12/10/2010 12:32

A
david.gobbi at gmail.com
cc
vtk-developers at vtk.org
Objet
Re: [vtk-developers] Weak pointers and thread safety

How would this impact performance? Would it improve it do you think?
W

On Mon, Oct 11, 2010 at 10:35 PM, David Gobbi <david.gobbi at gmail.com> 
wrote:
Hi All,

The VTK weak pointer implementation is not thread safe because it can
be used just like a regular pointer, even though the object itself
might be deleted by a separate thread at any time.

By comparison, "safe" weak pointer like in boost and Qt cannot be used
in place of ordinary pointers.  Instead, in order to use them their
"GetPointer()"-like methods increment the reference count and then
return a smart pointer, which is guaranteed to be valid until it goes
out of scope:

vtkSmartPointer<SomeClass> smartPtr = weakPtr.GetPointer();
if (smartPtr.GetPointer() != 0)
 {
 // do something with smartPtr
 }
// reference count decremented when smartPtr goes out of scope

Because VTK's vtkWeakPointer::GetPointer() does not return a smart
pointer or do any thread locking, thread-safe use of vtkWeakPointer
requires the following:

vtkSimpleCriticalSection critSec;
critSec.Lock();
vtkSmartPointer<SomeClass> smartPtr = weakPtr.GetPointer();
critSec.Unlock();
if (smartPtr.GetPointer() != 0)
 {
 // do something with smartPtr
 }

If weakPtr.GetPointer() was changed to return a smart pointer, then it
could do the locking internally.  This change would be mostly, but not
completely, backwards compatible.  What do people think?

 David
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