DESC Overview

The DESC mechanism enables you to construct objects representing external routines determined at execution time through their name and libraries, and call these routines with the appropriate arguments.

Two classes, DLL_32 and DLL_32_ROUTINE , supported by an auxiliary class SHARED_LIBRARY_CONSTANTS provide the basis for DESC under Windows:

• An instance of class DLL_32 describes a 32-bit Dynamically Linked Library. This class is a descendant of the deferred class SHARED_LIBRARY, covering the platform-independent notion of shared library.
• An instance of class DLL_32_ROUTINE describes a routine from a 32-bit DLL. Predictably, this class has an attribute of type DLL_32 describing the library to which the routine belongs. It has a deferred ancestor SHARED_LIBRARY_ROUTINE capturing the platform-independent notion of shared library routine.
• SHARED_LIBRARY_CONSTANTS introduces a few declarations useful for dealing with shared libraries and routines, in particular a few integer constants describing error codes and type codes. It is an ancestor to both of the preceding classes; application classes using DESC can also inherit from it to gain access to its facilities.

The basic sequence of instructions to use the DESC mechanism is the following:

1. Create a library object (an instance of DLL_32, providing the library's name as argument to the creation procedure.
2. Create a routine object (an instance of DLL_32_ROUTINE ), providing the library object, the routine's name or index in the library, and the routine's signature - number of arguments, types of arguments, type of result if any - as arguments to the creation procedure.
3. Apply procedure call to the routine object, passing an array that contains the actual arguments required by the external routine.

Each of these steps may be repeated as often as necessary to use multiple libraries, multiple routines in a library, or multiple calls to a given routine.