Make certain that your program declares variables of all possible RWCollectable objects that you might restore. For an example of this practice, see Example Two: Restoring Polymorphically, above.

These declarations are of particular concern when you save an RWCollectable in a collection, then attempt to take advantage of polymorphic persistence by restoring the collection in a different program, without using the RWCollectable that you saved. If you don't declare the appropriate variables, during the restore attempt the RWFactory will throw an exception for some RWCollectable class ID that you know exists. The RWFactory won't throw an exception when you declare variables of all the RWCollectables that could be polymorphically restored.

The problem occurs because your compiler's linker only links the code that RWFactory needs to create the missing RWCollectable when that RWCollectable is specifically mentioned in your code. Declaring the missing RWCollectables gives the linker the information it needs to link the appropriate code needed by RWFactory.