[vtk-developers] New way of doing wrapper hints

[David Gobbi]

On Mon, Jun 6, 2016 at 9:12 AM, David Thompson <david.thompson at kitware.com>
wrote:

>
> 1. I assume that attributes are allowed prior to function parameters...
> from the spec it was not immediately obvious. For example, what about this
> method:
>
> VTK_SIZEHINT(sz) double* findOrAddEntries(
>     int key, int sz, VTK_SIZEHINT(sz) double* vals);
>
> that both accepts and returns a variable-length array (of length sz, one
> of the other parameters)?
>

Yes, the hints can be used in either location.  The hint attaches to the
type.


> 2. Would you put them in the declaration or implementation? I have a mild
> preference for the implementation so that the declaration is short and easy
> to skim.
>

The wrappers only see the declaration.  They never see the .cxx file.


On Mon, Jun 6, 2016 at 9:13 AM, Brad King <brad.king at kitware.com> wrote:
>
>
> Neat.  I suggest using a prefix like `VTK_HINT_` for all such macros,
> and corresponding attribute names with a prefix like `vtk::hint::`.
> That will reserve space for other attributes to be added later for
> reasons other than hints.


My preference is for "VTK_SIZEHINT" because it's easier to say... I can
ask Joe in the cubicle next me me "Hey Joe, what's the size hint for
that method?"  In comparison, "VTK_HINT_SIZE" just doesn't roll off
the tongue.  In all seriousness, though, I'll go with the latter if people
feel it's the better choice.

Regarding "vtk::hint::", in C++ Section 7.6.1 "Attribute syntax and
semantics", the grammar does not allow more than one "::".  Apparently
attribute namespaces cannot be nested.



On Mon, Jun 6, 2016 at 10:03 AM, David Lonie <david.lonie at kitware.com>
 wrote:

> +1 to the idea and Brad's suggestion!
>

Grumble.


> Does this sort of pattern occur in VTK a lot? AIUI, the current hints file
> only allows fixed-length arrays and is not dependent on other variables to
> get the size.
>

We want to allow more flexibility than what the current hints file offers.
For example, sometimes a size hint should refer to one of the object's
methods or even to one of the other parameters:

    void InsertNextCell(int npts, VTK_SIZEHINT(npts) vtkIdType* conn)

    void SetTuple(VTK_SIZEHINT(GetNumberOfComponents()) double* tuple);

Ideally, the size hint could be an expression that would be evaluated to
compute the size.  There would be some limitations, of course, but
there are tons of possibilities.

 - David
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