[vtk-developers] AddObserver taking C++11 lambda?

[David Gobbi]

Hi Steve,

I don't think we have to worry about overhead in VTK's observers.  Our main
concerns should be safety and usability.  The VTK observer implementation
is heavy by design, it is meant to used for overall application control
(e.g. for the "controller" part of the MVC pattern).  It is generally not
meant to be used within the VTK computational algorithms themselves.

 - David


On Thu, Mar 23, 2017 at 9:14 AM, Hahn, Steven E. <hahnse at ornl.gov> wrote:

> Would it be more efficient to pass the lambda via a template parameter,
> like what is done in vtkClassMemberCallback? std::function<> adds a lot of
> overhead (see https://vittorioromeo.info/index/blog/passing_functions_
> to_functions.html ) and may be overkill.
>
> Steven
>
> On 3/23/17, 10:31 AM, "vtk-developers on behalf of Robert Maynard" <
> vtk-developers-bounces at vtk.org on behalf of robert.maynard at kitware.com>
> wrote:
>
>     While allowing lambda's to be held by std::function is really nice, it
>     becomes very easy to mistakenly captures items whose lifetime is
>     shorter than the Observer you are binding too.
>
>     I am currently writing up the first draft of c++11 guidelines for VTK
>     and for lambda functions and these are the guidelines I currently
>     have:
>
>     Usage of lambda expressions are allowed with the following guidelines.
>
>     - Use default capture by value ([=]) only as a means of binding a few
>     variables for a short lambda, where the set of captured variables is
>     obvious at a glance. Prefer not to write long or complex lambdas with
>     default capture by value.
>     - Except for the above, all capture arguments must be explicitly
>     captured. Using the default capture by reference ([&]) is not allowed.
>     This is to done so that it is easier to evaluate lifespan and
>     reference ownership.
>     - Keep unnamed lambdas short. If a lambda body is more than maybe five
>     lines long, prefer using a named function instead of a lambda.
>     - Specify the return type of the lambda explicitly if that will make
>     it more obvious to readers.
>
>     On-top of these guidelines I have also stated that lambda functions
>     should be kept to implementation files in VTK, which is fine as we go
>     with an approach like David proposed.
>
>     On Wed, Mar 22, 2017 at 2:00 PM, Elvis Stansvik
>     <elvis.stansvik at orexplore.com> wrote:
>     > Den 22 mars 2017 6:48 em skrev "David Gobbi" <david.gobbi at gmail.com
> >:
>     >>
>     >> It doesn't look hard.  New AddObserver methods would be needed that
> take
>     >> std::function<> as a parameter, and they'd have to store the
> "function"
>     >> object and call it.  Overall, it would probably be simpler than the
> code
>     >> that currently exists in vtkObject.h for adding observers.
>     >
>     > Good to hear, I might have a go at it then.
>     >
>     > Elvis
>     >
>     >>
>     >> On Wed, Mar 22, 2017 at 12:53 AM, Elvis Stansvik
>     >> <elvis.stansvik at orexplore.com> wrote:
>     >>>
>     >>> Hi all,
>     >>>
>     >>> With C++11 now required, would there be interest in an AddObserver
>     >>> version that takes a C++11 lambda? I've found myself wishing there
> was
>     >>> one a couple of times.
>     >>>
>     >>> Anyone know off the top of their head how easy/hard that would be
> to add?
>     >>>
>     >>> Cheers,
>     >>> Elvis
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>     >>
>     >
>     >
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