Chapter 8: Reusability - TOC - 8.3 ADAPTABILITY

8.3.1 Complete Functionality

guideline

Provide core functionality in a reusable part or set of parts so that the functionality in this abstraction can be meaningfully extended by its reusers.

More specifically, provide initialization and finalization procedures for every data structure that may contain dynamic data.

For data structures needing initialization and finalization, consider deriving them, when possible, from the types Ada.Finalization.Controlled or Ada.Finalization.Limited_Controlled.

example

   Incoming : Queue;
   ...
   Set_Initial (Incoming);     -- initialization operation
   ...
   if Is_Full (Incoming) then  -- query operation
      ...
   end if;
   ...
   Clean_Up (Incoming);        -- finalization operation

rationale

This functionality is particularly important in designing/programming an abstraction. You have to balance the completeness of the abstraction against its extensibility. Completeness ensures that you have configured the abstraction correctly, without built-in assumptions about its execution environment. It also ensures the proper separation of functions so that they are useful to the current application and, in other combinations, to other applications. Extensibility ensures that reusers can add functionality by extension, using tagged type hierarchies (see Guideline 8.4.8 and Chapter 9) or child library packages (see Guidelines 4.1.6, 8.4.1, and 9.4.1).

In designing for reuse, you need to think in terms of clean abstractions. If you provide too little functionality and rely on your reusers to extend the abstraction, they risk having an abstraction that lacks cohesion. This hodgepodge abstraction has inherited many operations, not all of which are necessary or work together.

When a reusable part can be implemented reasonably using dynamic data, then any application that must control memory can use the initialization and finalization routines to guard against memory leakage. Then, if data structures become dynamic, the applications that are sensitive to these concerns can be easily adapted.

The predefined types Ada.Finalization.Controlled or Ada.Finalization.Limited_Controlled provide automatic, user-definable initialization, adjustment, and finalization procedures. When you declare controlled types and objects, you are guaranteed that the compiler will insert the necessary calls to initialization, adjustment, and finalization, making your code less error-prone and more maintainable. When overriding the Initialize and Finalize routines on the controlled types, make sure to call the parent Initialize or Finalize.

notes

The example illustrates end condition functions. An abstraction should be automatically initialized before its user gets a chance to damage it. When that is not possible, it should be supplied with initialization operations. In any case, it needs finalization operations. One way to supply the initialization and finalization operations is to derive the abstraction from the predefined types Ada.Finalization.Controlled or Ada.Finalization.Limited_Controlled. Wherever possible, query operations should be provided to determine when limits are about to be exceeded, so that the user can avoid causing exceptions to be raised.

It is also useful to provide reset operations for many objects. To see that a reset and an initiation can be different, consider the analogous situation of a "warm boot" and a "cold boot" on a personal computer.

Even if all of these operations are not appropriate for the abstraction, the exercise of considering them aids in formulating a complete set of operations, others of which may be used by another application.

Some implementations of the language link all subprograms of a package into the executable file, ignoring whether they are used or not, making unused operations a liability (see Guideline 8.4.5). In such cases, where the overhead is significant, create a copy of the fully functional part and comment out the unused operations with an indication that they are redundant in this application.