Thanks for the PR.

I guess we could also change the type of ConnectivityMap for all platforms, no?

@NikolausDemmel updated. Not sure is it will compile normally on Linux.

Thanks for the update.

I think removing the a aligning allocator is problematic. It doesn't result in a compile time error, but it might result in runtime alignment errors (which can sometimes happen and sometimes not, which makes them hard to track down).

What is the compilation error with the ConnectivityMap as is?

Also, which version of OS X and which compiler?

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/map:820:5: error: 
      static_assert failed "Allocator::value_type must be same type as
      value_type"
    static_assert((is_same<typename allocator_type::value_type, value_type>::value),
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/alex/Documents/roadar/dso/src/OptimizationBackend/EnergyFunctional.h:121:6: note: 
      in instantiation of template class 'std::__1::map<unsigned long long,
      Eigen::Matrix<int, 2, 1, 0, 2, 1>, std::__1::less<unsigned long long>,
      Eigen::aligned_allocator<std::__1::pair<unsigned long long,
      Eigen::Matrix<int, 2, 1, 0, 2, 1> > > >' requested here
          > connectivityMap;

AppleClang 9.0.0.9000037, Mac OS Siera 10.12.6

After downgrade to AppleClang 8.1.0.8020042 it compiles without errors. With previous definition of connectivityMap works without crashes now.

Hm... still curious about why more recent clang chokes on the allocator. But I'll merge for now. Thanks for iterating on this!