docs(coding-guidelines): add note for test file with private headers (#…

…402)

Signed-off-by: Takagi, Isamu <[email protected]>

docs/contributing/coding-guidelines/ros-nodes/directory-structure.md

@@ -68,7 +68,7 @@ Place source files and private header files.
 
 #### `test`
 
-Place source files for testing.
+Place source files for testing. See [unit testing](../../testing-guidelines/unit-testing.md) for details.
 
 ## Python package
 

docs/contributing/testing-guidelines/unit-testing.md

@@ -41,12 +41,14 @@ if(BUILD_TESTING)
 
   ament_add_ros_isolated_gtest(test_my_cool_pkg test/test_my_cool_pkg.cpp)
   target_link_libraries(test_my_cool_pkg ${PROJECT_NAME})
+  target_include_directories(test_my_cool_pkg PRIVATE src)  # For private headers.
 ...
 endif()
 ```
 
 This automatically links the test with the default main function provided by `gtest`.
 The code under test is usually in a different CMake target (`${PROJECT_NAME}` in the example) and its shared object for linking needs to be added.
+If the test source files include private headers from the `src` directory, the directory needs to be added to the include path using `target_include_directories()` function.
 
 To register a new `gtest` item, wrap the test code with the macro `TEST ()`.
 `TEST ()` is a predefined macro that helps generate the final test code,