it's the specializations that the linker wants
template <>
memory_region_allocator<unsigned char>::mempool_type*
    memory_region_allocator<unsigned char>::mempool_ptr = nullptr;
template <>
memory_region_allocator<double>::mempool_type*
    memory_region_allocator<double>::mempool_ptr = nullptr;
template <>
memory_region_allocator<float>::mempool_type*
    memory_region_allocator<float>::mempool_ptr = nullptr;