request to add new message type, StringMultiArray.msg

[k-okada]

Hi,

we uses array of string data type internally and would be nice if this type is included in official std_msg type.

we're using this type of message, for example obtaion speech recongnition result candidates.

https://svn.code.sf.net/p/jsk-ros-pkg/code/trunk/jsk_smart_apps/jsk_gui_msgs/msg/VoiceMessage.msg

[tfoote]

The message you linked to is a single array. Why do you want the multi array? It requires fixed length sub elements for strings. This is basically serializing a bunch of fixed length strings into a single string which then must be broken apart on the other end. Can you explain a little bit more about the use case?

The original MultiArray datatypes were originally thought to be the basis for image and point cloud and other structured data. However they were found to be inadaquate quickly and were dropped. They are mostly there now for backwards compatibility reasons.

Part of the reason the super generic MultiArray datatypes were not adaquate was that they do not distinguish the semantics differences. Things like you can't debayer an point cloud effectively, but you can an image. If they have the same datatype (because they have the same structure) you could accidentally try to debayer a point cloud.

In general this seems like a message with a specific semantic use case and thus should be named and typed to represent the scope of the semantics. I think we could put a more specific message through a review process and promote it to be part of the common_msgs metapackage, but I would recommend against adding this to std_msgs which is more about primative datatypes to be used as building blocks for other semantically specific messages.

[k-okada]

Thank you for detailed explanation on the background and philosophy. I used multi array just because all array type message in std_msg package uses multi array. StringArray is ok for me.

I opened new issue in common_msgs, ros/common_msgs#21