Recursive message definition in ROS2

asked 2023-01-09 15:00:19 -0500

jbarry
280 ●8 ●16 ●21

Hi,

Is it possible to create a recursive message type in ROS2? E.g.: message Node.msg int64 id my_msgs/Node neighbors

This is currently causing compiler errors in ROS2 Humble for me but I was wondering if there was a flag or something I could use?

For more context, I'm working on writing a protobuf to ROS converter (which has generally been straightforward). However protobuf allows recursive messages and message loops (e.g. message A references message B which references message A). Figuring out a ROS equivalent to this if ROS doesn't allow recursive messages is going to be tricky!

Thanks! -Jenny

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answered 2023-01-10 12:40:15 -0500

fmrico
85 ●4 ●10 ●10

updated 2023-01-10 12:41:48 -0500

No. It is not possible, sorry. I made the same question here:

https://github.com/ros2/rosidl_typesu...

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Ah thanks! I googled a bunch but didn't find that. I wonder if pointing out that that solution isn't good for message loops would increase its likelihood of being added someday...

jbarry ( 2023-01-10 12:50:48 -0500 )edit

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answered 2023-01-10 02:52:47 -0500

Olivier Kermorgant

280 ●9 ●24 ●20 http://pagesperso.ls2n...

Hi,

To my understanding message definitions are used to generate POD structures. A recursive definition would generate an infinite structure, e.g. in C++ :

struct Node
{
  Node neighbor;
};

Note that an array of Node should do the trick but it seems it does not compile either, though the C++ version is fine:

struct Node
{
  std::vector<Node> neighbors;
};

In your case one could suggest an array of id's to represent the neighbors of the node.

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Unfortunately this doesn't help in my case. I'm writing a tool to automatically convert protobuf messages to ROS messages. I don't know the content of the messages ahead of time and can't change it. Protobuf allows recursive message definitions (presumably their implemention involves pointers for C++) but ROS messages with recursive definitions don't compile because they appear not to be able to forward declare. I can work around this using byte strings but if there's a way to tell the compiler to forward declare the message type that would be easier.

jbarry ( 2023-01-10 08:03:46 -0500 )edit

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