suggest generic message generation target to depend on with add_dependencies

[tkruse]

The current design of genmsg requires users to depend on the xyz_gencpp target to build c++ libraries against messages in the same package. This requires users to know about gencpp and causes an ugly reference to gencpp internals in userland CMakeLists.txt.

See:
ros/catkin_tutorials#4

I suggest that instead genmsg should if possible provide a CMake target that users can depend on generically. If changed, this also affects

ros-infrastructure/catkin_pkg#26

[dirk-thomas]

Could you please clarify what you mean. Perhaps by giving an example or sketching how you imagine the better approach.

[tkruse]

If I understand cmake correctly, then currently the user has to write:

add_dependencies(some_target packagename_gencpp)

And instead of that, I would like users to be able to state:

add_dependencies(some_target packagename_messages)

or something similar, a target that is not generated by gencpp, but by genmsg. Not sure whether this also has technical benefits. I guess it also makes build a little slower, since fist all message generators would build messages, rather than only the c++ ones. But I hope the abstraction of message generation will make it easier for users to understand what is going on.

[dirk-thomas]

A generic target could easily be added which itself depends on all generators. But I would rather teach the users to depend on the specific message generator target than on a generic one. As you already pointed out that makes definitely a difference because it adds significant useless dependencies. And since the user has to anyway specify a dep it should be directly the right one.

For the generic target I would use a more descriptive suffix like _generate_messages resulting in i.e. std_msgs_generate_messages. Else the meaning of that target std_msgs_messages would be difficult to understand again.

[tkruse]

Maybe a compromise could be to give the current targets a more meaningfull
name, like packagename_generate_messages_cpp or something. We know what
gencpp means, but users seeing only message_generation and message_runtime
dependencies cannot guess that well.

On Sun, Jan 27, 2013 at 12:36 AM, Dirk Thomas notifications@github.comwrote:

A generic target could easily be added which itself depends on all
generators. But I would rather teach the users to depend on the specific
message generator target than on a generic one. As you already pointed out
that makes definitely a difference because it adds significant useless
dependencies. And since the user has to anyway specify a dep it should be
directly the right one.

For the generic target I would use a more descriptive suffix like
_generate_messages resulting in i.e. std_msgs_generate_messages. Else the
meaning of that target std_msgs_messages would be difficult to understand
again.

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