[Insight-developers] Explicit Instantiation Proposal

[Brad King]

Kris Thielemans wrote:
> I'm just wondering if this isn't a lot of effort that will become obsolete
> once the 'export' keyword is more widely available. In any case, you should
> make sure that the macros are export-compatible. See also (very briefly)
> http://www.parashift.com/c++-faq-lite/templates.html#faq-35.14

There are three parts to this answer:

1.) The export keyword may never be widely available.  It is so hard to 
implement that there is talk of removing it from the C++ standard.  Even 
if it does we will not be dropping support for most of the current 
non-export compilers for a long time.

2.) The export keyword does not obsolete this design.  Having 
pre-compiled fully instantiated copies of the common template 
instantiations will still be faster/smaller than using export.  All the 
export keword does is allow "separate compilation of templates".  In 
practice this amounts to putting a parse tree in an object file and then 
using it during linking to produce the needed symbols.  This would still 
require the compiler to build these instantiations for user code.  The 
pre-link step used to build the exported templates is hard to 
parallelize too.

3.) The design is export-compatible because explicit instantiations have 
almost nothing to do with export.  If we later add the .txx files into 
the build on an export-supporting compiler it will simply allow the 
non-explicitly-instantiated templates to take advantage of export.  As 
will be seen when I write the section on organizing source files, all 
the .h files include their .txx files optionally inside a #if test which 
is easy to modify later to be false when export is enabled.

-Brad