new_contents: # Partially stolen from https://bitbucket.org/mblum/libgp/src/2537ea7329ef/.ycm_extra_conf.py
  import os
  import ycm_core
  
  # These are the compilation flags that will be used in case there's no
  # compilation database set (by default, one is not set).
  # CHANGE THIS LIST OF FLAGS. YES, THIS IS THE DROID YOU HAVE BEEN LOOKING FOR.
  flags = [
      '-Wall',
      '-Wextra',
      '-Werror',
      '-Wc++98-compat',
      '-Wno-long-long',
      '-Wno-variadic-macros',
      '-fexceptions',
      # THIS IS IMPORTANT! Without a "-std=<something>" flag, clang won't know which
      # language to use when compiling headers. So it will guess. Badly. So C++
      # headers will be compiled as C headers. You don't want that so ALWAYS specify
      # a "-std=<something>".
      # For a C project, you would set this to something like 'c99' instead of
      # 'c++11'.
      '-std=c++11',
      # ...and the same thing goes for the magic -x option which specifies the
      # language that the files to be compiled are written in. This is mostly
      # relevant for c++ headers.
      # For a C project, you would set this to 'c' instead of 'c++'.
      '-x', 'c++',
      # This path will only work on OS X, but extra paths that don't exist are not
      # harmful
      '-isystem', '/System/Library/Frameworks/Python.framework/Headers',
      '-isystem', '/usr/local/include',
      '-isystem', '/usr/local/include/eigen3',
      '-I', 'include',
  ]
  
  # Set this to the absolute path to the folder (NOT the file!) containing the
  # compile_commands.json file to use that instead of 'flags'. See here for
  # more details: http://clang.llvm.org/docs/JSONCompilationDatabase.html
  #
  # Most projects will NOT need to set this to anything; you can just change the
  # 'flags' list of compilation flags. Notice that YCM itself uses that approach.
  compilation_database_folder = ''
  
  if compilation_database_folder:
    database = ycm_core.CompilationDatabase( compilation_database_folder )
  else:
    database = None
  
  
  def DirectoryOfThisScript():
    return os.path.dirname( os.path.abspath( __file__ ) )
  
  
  def MakeRelativePathsInFlagsAbsolute( flags, working_directory ):
    if not working_directory:
      return list( flags )
    new_flags = []
    make_next_absolute = False
    path_flags = [ '-isystem', '-I', '-iquote', '--sysroot=' ]
    for flag in flags:
      new_flag = flag
  
      if make_next_absolute:
        make_next_absolute = False
        if not flag.startswith( '/' ):
          new_flag = os.path.join( working_directory, flag )
  
      for path_flag in path_flags:
        if flag == path_flag:
          make_next_absolute = True
          break
  
        if flag.startswith( path_flag ):
          path = flag[ len( path_flag ): ]
          new_flag = path_flag + os.path.join( working_directory, path )
          break
  
      if new_flag:
        new_flags.append( new_flag )
    return new_flags
  
  
  def FlagsForFile( filename ):
    if filename.endswith(".h"):
        filename = filename.replace(".h", ".cxx")
    if database:
      # Bear in mind that compilation_info.compiler_flags_ does NOT return a
      # python list, but a "list-like" StringVec object
      compilation_info = database.GetCompilationInfoForFile( filename )
      final_flags = MakeRelativePathsInFlagsAbsolute(
        compilation_info.compiler_flags_,
        compilation_info.compiler_working_dir_ )
    else:
      relative_to = DirectoryOfThisScript()
      final_flags = MakeRelativePathsInFlagsAbsolute( flags, relative_to )
  
    return {
      'flags': final_flags,
      'do_cache': True
    }
  
subject: Add Youcompleteme extra config