[vtk-developers] Removed windows.h from VTK headers.

[Brad King]

Sebastien BARRE wrote:
> I definitely like less compilation time, but what is the extent of that 
> change, I mean, how much *hacking* is expected on a decent size project 
> to make it work correctly with the new change ?

If the project defines VTK_INCLUDE_WINDOWS_H then it should not need any 
changes except where it overrides virtual methods defined by VTK.  Only 
if the project wants to achieve the same compile-time benefit does it 
need hacking.  To put this in perspective it took me less than an hour 
to fix all of ParaView.

There are more benefits than just compile time.  This change also avoids 
forcing the windows.h mangling on all user code.  Not all projects that 
use VTK include VTK headers in all their source files.  These projects 
then have problems when VTK headers include windows.h and mangle their 
own method names in only some of their sources.

> When I look at Widgets/vtkKWRenderWidget, I'm a little scared:
> 
> Instead of a single virtual RemoveProp (which I *know* is overriden in 
> subclasses)

The VTK/Utilities/Upgrading/FindWindowsMangledMethods.cmake script will 
quickly reveal any classes that override methods that may have been renamed.

>         virtual RemoveProp(vtkProp *prop)
> 
> We end up with "that" in the .h (and as you said, the RemoveProp is 
> non-virtual, thus breaking the subclass):
> 
> #ifdef VTK_WORKAROUND_WINDOWS_MANGLE
>   // Avoid windows name mangling.
> # define RemovePropA RemoveProp
> # define RemovePropW RemoveProp
> #endif
> 
>   // Description:
>   // Manage props inside this widget renderer(s). Add, remove, query.
>   void RemoveProp(vtkProp *prop);
> 
> #ifdef VTK_WORKAROUND_WINDOWS_MANGLE
> # undef RemovePropA
> # undef RemovePropW
>   //BTX
>   // Define possible mangled names.
>   void RemovePropA(vtkProp*);
>   void RemovePropW(vtkProp*);
>   //ETX
> #endif
> 
> [...]
>   virtual void RemovePropInternal(vtkProp* prop);

Only the top-most class has to do this.  Subclasses can now just 
override RemovePropInternal.  The other option is to just rename the 
method completely to something that does not need this hack but that 
will break application code that calls it.

> It's interesting because I remember 4 or 5 years ago we had a lot of 
> discussons on how to speed up compilation time in the users list, on 
> Borland and MSVC, and many people suggested solutions. At that time, I 
> was not at Kitware already, but I wrote a Perl script that would 
> significantly reduce compile time by detecting the headers that were 
> most used, in what order, and would create a unique .h that could be 
> pre-compiled/processed, yadi yada, a hack too which I'm glad did not 
> make it through actually, but I think it was rejected because it 
> introduced like... 2 or 3 extra-lines of #pragma in some headers, and 
> also a global feeling that it was not too good to introduce that much 
> compiler/system specific code in the source tree...

It seems feelings have changed as VTK has gotten bigger and compilers 
have gotten slower.  If you can dig that up again to help us get rid of 
extra headers in the .cxx files then that would be great.

-Brad