On Windows, the yaml-cpp is built as shared libraries. In such case, YAML_CPP_DLL is required to define to correctly consume the yaml-cpp (otherwise, visibility decoration won't be defined and it results in linkage time error).

And currently every project consuming yaml-cpp will need to define it in their CMake project to enable Windows build. It would be nice to see this exported directly from this package and all the downstream project can get the definition for free.

UPDATED:
For example,

• https://github.com/ros-perception/image_common/blob/ros2/camera_calibration_parsers/src/parse_yml.cpp#L49
• https://github.com/ros2/rosbag2/blob/master/rosbag2_transport/src/rosbag2_transport/recorder.cpp#L36
• https://github.com/ros2/rviz/blob/ros2/rviz_common/src/rviz_common/yaml_config_reader.cpp#L42

And I discovered this issue when I try to enable navigation2 on Windows.