How to +use a polymorphic interface DLL

The following code can use any implementation of the interface defined +by the example class, CMessenger, to issue a simple greeting.

The name of the DLL, which contains the implementation to be used, is +passed in the descriptor aLibraryName.

Notes:

the RLibrary class +is the interface to dynamically loaded DLLs. It encapsulates a handle to the +DLL.
The DLL has a TUidType which +is assigned when the DLL is built. This is ensures that the DLL that is loaded +complies with the CMessenger protocol.
If a specific implementation +of the DLL is to be used, then specify a third UID when declaring the TUidType. +Each specific implementation of the DLL is uniquely identified by the third +UID.
A pointer to the ordinal +1 function in the DLL is fetched by calling the Lookup() member +function of RLibrary and passing it a parameter value of +1. The returned function has type TLibraryFunction; such +a function takes no parameters and returns a TAny* type.
The function pointer +is assigned to the variable called entry and the function +itself is called to construct a CMessenger object and return +a pointer to that object. The implementation uses the new (ELeave) operator +to allocate the new CMessenger object and invoke the C++ +default constructor.
As no arguments are +passed to the function, a second-phase constructor, ContructL(), +is called to finish off construction.
The CMessenger object +can then be used like any other C++ object. In this example, the ShowMessage() function +is used to emit a greeting in whatever way the implementation chooses. In +more complex cases, the object could have a long lifetime, and create many +dependent objects.
When the CMessenger object +is no longer needed, the DLL is unloaded using RLibrary::Close(). +Do not call any code in the DLL after the library has been closed. It is important +to note, therefore, that the library must not be closed until the object constructed +by the DLL, and all its dependent objects, have been destroyed.