Inherit cpp class from external file in two packages

edit

My file structure is as follows:

catkin_workspace/src/
                     package/
                             subpackage_1/src/subpackage_1.cpp
                             subpackage_2/src/subpackage_2.cpp
                             package (Metapackage)

I define a class in each of these .cpp files, and each one contains an instance of the same base class. Where should I save this base class to conform with the ROS C++ Style Guide? And what would I need to add to each subpackage_#'s CMakeLists.txt? Thanks.

EDIT: It's working. I didn't modify the structure of the packages at all. Here are the contents of catkin_workspace/src/package/subpackage_3/CMakeLists.txt:

cmake_minimum_required(VERSION 2.8.3)
project(subpackage_3)
SET(CMAKE_BUILD_TYPE RELEASE)

find_package(catkin REQUIRED COMPONENTS
  roscpp)
find_package( PCL REQUIRED )

catkin_package(CATKIN_DEPENDS roscpp
           INCLUDE_DIRS include
           LIBRARIES subpackage_3
           )

include_directories(
  include 
  ${catkin_INCLUDE_DIRS}
  ${PCL_INCLUDE_DIRS}
)
set(PKG3_COM_LIB_SOURCES
    src/subpackage_3.cpp)
add_library (subpackage_3 ${PKG3_COM_LIB_SOURCES})

The only modification I needed to make to subpackage_<1, 2>/CMakeLists.txt was to add find_package(subpackage_3). I also added < depend>subpackage_3< /depend> to subpackage_<1, 2>/package.xml (without the spaces).