[Insight-developers] Explicit instantiation

[Gaetan Lehmann]

I'm not sure to understand what the explicit instantiation does.
It build some template specialization for a restricted set of template  
arguments, and put this code in the ITKCommon lib ?
What is the benefit ? Compilation time ? executable size ?


On Fri, 21 Apr 2006 15:46:06 +0200, Brad King <brad.king at kitware.com>  
wrote:

> Miller, James V (GE, Research) wrote:
>> We've noticed code being checked into ITK to support explicit  
>> instantiation.
>>  Can someone put a page on the ITK proposals wiki page that details the  
>> proposed methodology?
>
> Sure.  I was planning to do this next week.
>
>> It looks like there are a collection of macros and nested include files  
>> to indicate what needs to be instantiated.
>>  Looking at the macros being introduced, I fear that our developers  
>> will not know how to write the necessary macro for a new class they are  
>> introducing.  Do we need to use macros at all?  Is it possible to just  
>> type out the instantiations we want?  I assume the macros are meant to  
>> simply the export/import syntax.
>
> Users need only write one macro per template for which explicit  
> instantiations are to be used.  I plan to include extensive examples in  
> the documentation to help users write them.  The macros are necessary  
> for making sure that everything exported is imported properly, and to do  
> imports in a cross-platform way.
>
>> There seems to be a lot of files. Very nested. I fear it will be hard  
>> for people to navigate.
>
> There is simply one .cxx and one .h per instantiation.  This is just as  
> if each instantiation were its own class, which from the compiler's  
> point of view is the case.
>
> There is also one extra .h file per template, but its purpose is  
> actually to simplify the code.  We could just have itkImage.h include  
> all the Templates/itkImage*.h instantiation import headers, but I  
> created the extra Templates/itkImage+-.h as an intermediate step so that  
> we can switch to generating the instantiations later without modifying  
> itkImage.h.
>
>> Also, it appears that we are using a one-file-per-instantiation model.  
>> Is this the best approach? Or would it be better to lump a number of  
>> related instantiations into one file (as long as it wouldn't get too  
>> big to compile)?
>
> See Bill Hoffman's answer for this.  This is the same reason VXL uses  
> one-per-file.
>
> -Brad
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-- 
Gaëtan Lehmann
Biologie du Développement et de la Reproduction
INRA de Jouy-en-Josas (France)
tel: +33 1 34 65 29 66    fax: 01 34 65 29 09
http://voxel.jouy.inra.fr