All C++ class definitions are fixed at compile time. Under normal circumstances, if you write a file translator or an editing command that operates on a number of existing AutoCAD ® classes, you have to redefine all the existing classes to include the new translator or editing functions. And you would have to recompile your library as well as all the applications that use it.
By using the ObjectARX ® protocol extension mechanism described in this section, you can add functionality to existing ObjectARX classes at runtime, without any modification of existing classes and recompilation.