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It looks like you're trying to use cloud_out as an output parameter. This isn't a good idea.

All of the parameters to your callback function should be inputs. They can be messages or static inputs, but since the arguments (other than the message) are passed in once when you create the publisher, you shouldn't pass in something that the callback will attempt to modify.

Depending on what you're trying to do, you could write your callback to modify a global variable or a class member, or it could publish the output cloud on a new topic.