5.3 - Getting Information About the FOM

When an HLA DtExerciseConn is constructed, VR-Link reads the appropriate FED file and builds a database of information about the FOM that the application code can query.

This FOM information is stored in a DtFom (defined in fom.h). You can obtain a pointer to a DtExerciseConn's DtFom using the member function fom().

A DtFom contains a list of object class and interaction class descriptors. These descriptors are represented by the VR-Link classes DtObjClassDesc (defined in objClassDesc.h) and DtInterClassDesc (defined in interClassDesc.h) respectively. Class descriptors contain information about FOM classes, including:

• Class handle and name
• A pointer to the base class's descriptor (if any)
• The set of valid attributes or parameters
• Whether the class has been published or subscribed
• The attributes of an object class that have been published or subscribed
• Whether the class and its various attributes are needed by other federates.

The class descriptors also allow you to subscribe and publish, if you want to override the default way these are handled.

DtFom's member functions interClassByName(), interClassByHandle(), objClassByName() and objClassByHandle() allow you to obtain the class descriptor for a particular class. Alternatively, you can iterate through all of the classes in a FOM, using DtFom's firstObjClass() and nextObjClass(), or firstInterClass() and nextInterClass() member functions, for example:

// Print out the names of all object classes in the current FOM.
DtExerciseConn conn(...);
...
DtFom* fom = conn.fom();
for (DtObjClassDesc* desc = fom->firstObjClass(); desc; 

desc = fom->nextObjClass(desc))

{
   std::cout << "ClassName: " << desc->name() << std::endl;
}

The following example shows how to get a class descriptor in RTI 1516. It uses a list provided by the DtFom member functions objClassList() and interClassList(), which return a DtList of object classes to iterate through.

...

// Print out the names of all object classes in the current FOM.
DtExerciseConn conn(...);

DtFom *fom = con.fom();

#if DtHLA_1516
   DtListItem *item;
   for(item = objClassList()->first();
      item;
      item= item->next();
   {
      DtObjectClassDesc *objDesc =
         static_cast<DtObjClassDesc *>item->data();
#else
   for (DtObjClassDesc *objDesc = fom->firstObjClass()
      objDesc;
      objDesc = fom->nextObjClass(objDesc))
   {
#endif

      // Now code is independent
      std::cout << "ClassName: " << objDesc->name() << std::endl;
      ... 

DtFom has a print() member function that is similar to the example, but it prints out more information, such as attribute and parameter names and handles.

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