use extern template in generated c++ files

barcode

hi.
the files generated by slice2cpp contain a lot of templates.
template instantiations slow down compilation.
a way to circumvent this problem is to use a explicit instantiation declaration for the template instance in the header and add the explicit instantiation declaration to the cpp (see here for more information).

i think it would be nice if slice2cpp would use this feature (this can speed up the compilation of large code-bases).
since explicit instantiation declarations are a c++11 feature, this probably should be enabled via a flag.

will you add such a flag to slice2cpp in future?

best regards.
raphael

bernard

Hi Raphael,

We're always looking for ways to improve Ice, so thanks for the suggestion!

slice2cpp does indeed rely on template instantiations, even more so with the new C++11 mapping.

Did you update some of the generated .h and .cpp for your Slice files with explicit template instantiations to verify it this does indeed reduce your compilation time? Which Ice version and C++ compiler are you working with?

Explicit template instantiations have some downsides as well:

• our template have quite a bit of inline code--explicit instantiations would not / should not help for those
• explicitly instantiating every template class and member function in a generated .h, even those you never use, could lead to code bloat, which in turn can create other problems, see for example: https://forums.zeroc.com/discussion/comment/46600
• the interaction with "dll export" (and similar non-hidden symbols with gcc/clang) can be tricky

We actually used some explicit template instantiations in the generated code with Ice 3.6 (Windows only), but removed it for Ice 3.7 to reduce code size: https://github.com/zeroc-ice/ice/commit/c474b08073c63f6b1fcffe33a5d8172702c4d1f9

Best regards,
Bernard