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# 9. Extensible and Dynamic Topic Types for DDS annotation support

The specification defines an annotation language as extension to IDL. Even though this specification has not been implemented in Vortex OpenSplice, its IDL pre-processor is already able to parse these extensions even though it does not generate anything special for them yet. This allows users to write future-proof IDL definitions with annotations already in them.

The specification describes two notations for defining annotations in IDL, a prefix and a suffix notation. An annotation type is defined by prefixing a local interface definition with the new token `@Annotation`. The members of these types shall be represented using IDL attributes, as shown in the following example using the prefix notation:

    1  @Annotation
    2  local interface MyAnnotation {
    3     attribute long my_annotation_member_1;
    4     attribute double my_annotation_member_2;
    5  };

Alternately and equivalently, an annotation can be defined by suffixing the interface with the new annotation token using slash-slash-at (`//@Annotation`) instead, like this:

    1  local interface MyAnnotation {
    2     attribute long my_annotation_member_1;
    3     attribute double my_annotation_member_2;
    4  }; //@Annotation

An annotation can be applied to a type or type member by prefixing it with an at sign (`@`) and the name of the annotation type to apply. To specify the values of any members of the annotation type, include them in name=value syntax between parentheses; for example:

    1  @MyTypeAnnotation
    2  struct Gadget {
    3     @MyAnnotation(my_annotation_member_1=5,
       my_annotation_member_2=3.4) long my_integer;
    4  };

Alternately and equivalently, an annotation can also be applied to a type or type member by suffixing it with an annotation type name using slash-slash-at (`//@`) instead of the at sign by itself; for example:

    1  struct Gadget {
    2  long my_integer;//@MyAnnotation(my_annotation_member_1=5,
       my_annotation_member_2=3.4)
    3  }; //@MyTypeAnnotation

> Please note that the IDL Pre-Processor only supports the suffix notation when selecting C++ (`-l cpp`, `-l c++`, `-l isocpp`, `-l isoc++`, `-l isocpp2` or `-l isoc++2`) or CORBA-cohabitated java (`-l java -C`) as language.

For other languages both prefix and suffix notations are supported. The Extensible and Dynamic Topic Types for DDS specification also defines a number of annotations for use by applications. These types do not appear as annotations at runtime; they exist at runtime only in order to extend the capabilities of IDL. The following annotations have been defined and are accepted by the Vortex OpenSplice IDL pre-processor as well:

- ID
- Optional
- Key
- Shared
- BitBound
- Value
- BitSet
- Nested
- Extensibility
- MustUnderstand
- Verbatim

For more details on built-in annotations and annotations in general please refer to section 7.3.1 of the OMG Extensible and Dynamic Topic Types for DDS specification.

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