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ROS C++ dependencies missconception

Hey,

I've created a package with only a roscpp dependencies using :

catkin_create_pkg my_package roscpp

On my script, I'm using a Twist message, so I've added :

#include <geometry_msgs/Twist.h>

It works fine and that's the problem... Why does it work since I've not added the std_msgs nor the geometry_msgs dependency? Is this just optional ?

Asked by risbo6 on 2022-03-28 04:38:51 UTC

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Which dependency entries are you asking about: CMakeLists.txt or package.xml? They have different reasons to exist.

Asked by Mike Scheutzow on 2022-03-28 15:24:35 UTC

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This likely "works" because geometry_msgs will be installed with a regular ROS 1 installation.

So even if you don't add geometry_msgs as a (build) dependency to your CMakeLists.txt and package.xml, /opt/ros/$ROS_DISTRO/include will end up on the include path of your compiler/preprocessor (as part of the include paths exported by some other package you (transitively) depend on), and it will be able to resolve geometry_msgs/Twist.h to /opt/ros/$ROS_DISTRO/include/geometry_msgs/Twist.h.

Now you could ask: "if this works, then why bother adding (build) dependencies to packages at all?"

The answer would be: because you cannot rely on packages like geometry_msgs to always already have been installed.

A ROS 1 installation without geometry_msgs would be perfectly possible, and in that case, when someone tries to build your package, it would fail, as you've not made the dependency on geometry_msgs explicit, and it will not be detected to be missing before the build is started.

Asked by gvdhoorn on 2022-03-29 03:17:05 UTC

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