[vtk-developers] Which compilers to use "extern template" with

[David Lonie]

On Thu, Sep 1, 2016 at 1:18 PM, David Gobbi <david.gobbi at gmail.com> wrote:
> Hi David,
>
> You've been playing around with the "extern template" stuff for longer than
> I have, so I'm wondering if you can share some experience.

The extent of my understanding of this syntax is that means a
particular template instantiation will be available at link time, and
thus doesn't need to be compiled into the current translation unit.

> It seems that restricting "extern template" to MSVC 2015 and up would reduce
> or eliminate the need for "#pragma warning".  Would it be a bad thing if we
> stopped using "extern template" with MSVC 2008, 2010, etc?  The reason that
> I ask is that, as I expand the use of "extern template" to other parts of
> VTK, I don't want to have to add "#pragma warning" every time.

I think you're talking about the oddities of the MSVC-specific blocks
in AOS/SOADataArrayTemplate, correct? The only practical difference I
notice is that these are also explicitly exported. I'm not entirely
sure the warning pragmas are still necessary on the versions of MSVC
we support.

Brad (cc'd) might know more, IIRC he originally wrote that block and
tracked down the issues surrounding this.

> For clang and gcc, I was only planning to use "extern template" when
> -std=c++11 because otherwise clang generates a warning about this being a
> "c++11 feature".

This sounds reasonable, and might allow us to eliminate the test for
_MINGW_, since I suspect those problems are actually an issue with
older GCC, not necessarily MinGW itself.

> On the other hand I could bite the bullet and also use "extern template" for
> pre-c++11 builds, and insert pragmas into a whole bunch of source files
> (probably 20 or so) to silence the warnings.

I'd vote for limiting these to C++11 builds rather than adding pragmas
all over, as long as we can get away with it. I don't think non-C++11
builds would complain about them missing (with the possible exception
of MSVC), and the worst case scenario is a slightly longer compile
time as the templates are emitted in each translation unit (and
subsequently stripped by the linker).

Dave