`rosidl_adapter` does not emit include guards

[Muon]

Converting a .msg to ROS IDL using rosidl_adapter does not add include guards. This means that, after preprocessing (using GCC for example), definitions may appear multiple times in the source. This is a problem for using OMG IDL tooling with ROS IDL. According to OMG IDL v4.2:

• Section 7.5.2 "Scoping Rules and Name Resolution" states "An identifier can only be defined once in a scope."
• Section 7.5.3 "Special Scoping Rules for Type Names" states "Once a type has been defined anywhere within the scope of a module, interface or value type, it may not be redefined except within the scope of a nested module, interface or value type, or within the scope of a derived interface or value type."

I'm not sure whether this is a deliberate difference, but if it is, it should be documented.

[sloretz]

Thank you for the report @Muon ! Are you saying the generated OMG IDL lacks include guards? I didn't realize they could have those. Would you be willing to give an example OMG IDL with include guides to show how that works? That would show what needs to be added to rosidl_adapter's OMG IDL templates

Maybe related, in today's 🧇 meeting someone mentioned Fast-DDS explicitly de-duplicates includes https://github.com/eProsima/Fast-DDS-Gen/blob/564b23175c3ac9a3adc7f45847c7f152b1cd5c0e/src/main/java/com/eprosima/fastdds/fastddsgen.java#L568-L591 .

[Muon]

OMG IDL just uses the C++ preprocessor. The include guard pattern is like this:

#ifndef FOO
#define FOO

/* my unique definitions go here */

#endif

Say you #include the above file twice. Since FOO is defined after the first #include, the second #include will expand to nothing.

For OMG IDL it might look like this:

#ifndef SOME_UNIQUE_NAME_FOR_MY_THING
#define SOME_UNIQUE_NAME_FOR_MY_THING

module Package {
    module msg {
        struct MyThing {
            uint8 x;
        };
    };
};

#endif

If Fast-DDS generally accepts current ROS IDL files as-is, then it must be doing some kind of deduplication. (Though the linked code is about deduplicating include search directories, not deduplicating the included files.)