hunter-package updates

[headupinclouds]

• imshow fixes in submodule
• xgboost fixes (WIP)
• beast (submodule + target_include_directories() -- cmake needs work)

Possible:

• xgboostcpp : started moving C++ xgboost interface class to public xgbostcpp lib
• to_string.h ?

Note: Multiple projects need stdlib patches (to_string) to use cereal on Android (pre libc++ (at least)). This has been added via a minimal header, and I'm starting to copy and paste this to different project. Cavet: This involves patching/adding to the std:: namespace for those toolchains. It would be best to push this to cereal, but understandably (per older issues/discussions), cereal prefers to code to a C++11 standard without including local workarounds. Eventually the Android standard library will be complete, but for now it might make sense to include the bare minimum to_string implementation in a single header-only INTERFACE library in the interim.

[ruslo]

it might make sense to include the bare minimum to_string implementation in a single header-only INTERFACE library in the interim.

Sounds dangerous. What is the problem exactly? Is the error in libc++? Do update to latest NDK helps?

[headupinclouds]

For reference, here is the main issue:

/Users/dhirvonen/devel/ruslo/hunter/_Base/xxxxxxx/cda3b20/9201547/Install/include/cereal/cereal.hpp:679:101: error: 'to_string' is not a member of 'std'
           throw Exception("Error while trying to deserialize a smart pointer. Could not find id " + std::to_string(id));

The issue stems from a missing to_string() in the android ndk standard library. I think this might be available in more recent ndk builds w/ libc++. I don't know what options are available for gcc toolchains where lto + MinSizerel still works. Also, newer libc++ ndk libs don't seem to be very stable yet. Adding to std:: is definitely "bad practice", but I think such a patch is a lesser evil here, given that it will eventually be supported, and cereal is a very good solution from cross platform serialization that works elsewhere. I've been relying on this for Android builds using the cereal portable archive files for a while, and have not hit any snags so far. Boost serialization works fine on these platforms, but boost adds another 400K or so in the final library and a fairly long build time.

Background:
USCiLab/cereal#234 (comment)
USCiLab/cereal#307 (comment)

EDIT: Maybe it is preferable to include the local std:: patches in each library as a private header along with the cereal additions. A cmake platform check for availability would be useful to pull in the header as needed.

[headupinclouds]

assimp/assimp#892 (comment)

FWIW, std::to_string is in fact supported by the NDK, but it requires use of the LLVM libc++ runtime (see also https://developer.android.com/ndk/guides/cpp-support.html#cs).

assimp/assimp#892 (comment)

You should be able to use libc++ with both gcc and clang.

[headupinclouds]

But... libc++ doesn't seem to be stable quite yet (from an android-ndk developer):

android/ndk#379 (comment)

libc++ is in much better shape than it was back then, but it's still not quite to the point that I'm comfortable recommending it. For a timeline on that, see our roadmap.

[headupinclouds]

What about a patched cereal in hunter-packages (non-standard version that needs to be added explicitly by the user)? That will avoid a whole bunch of repetition across projects and should support cereal use in Android toolchains via hunter until libc++ stabilizes.

[ruslo]

What about a patched cereal in hunter-packages

I think it's fine. Actually this patch looks good, we just need to update CMakeLists.txt with option and public define.