Introduction

Eiffel Software recommends that any new development efforts be implemented using Eiffel's void-safe approach, thus eliminating one more common type of runtime failure. It is also recommended that existing software be converted to void-safety at the earliest opportunity.

Under some circumstances it is possible and even helpful to mix void-safe and void-unsafe libraries. During conversion to void-safety, for example, it can be helpful to compile and test a void-unsafe system with void-safe versions of the libraries it depends upon.

Overview

During the adoption of void-safety, the software libraries provided by Eiffel Software have been converted to be void-safe. The bulk of the changes made to these libraries will have little or no adverse effect on your existing software as you go through the process of void-safe conversion. However, there are a few changes to the library that we consider "breaking" changes, that is, important changes that might cause problems in existing systems that use certain library classes.

If you have been using Eiffel for a while, you may be maintaining systems which were developed before Eiffel became void-safe. If that's the case, then you will probably want to make those systems void-safe.

In this section we will use the experience of converting a set of simple (but not too simple) legacy Eiffel classes to show the types of issues that you may encounter, and how to deal with them.

So in the discussion below, you will see references to these classes:

Background

The primary focus of Eiffel is on software quality. Void-safety, like static typing, is another facility for improving software quality. Void-safe software is protected from run time errors caused by calls to void references, and therefore will be more reliable than software in which calls to void targets can occur. The analogy to static typing is a useful one.

About void-safe software development using Eiffel

When you develop software in Eiffel, you can be assured (at compile time) that your system will not attempt (at run time) to apply a feature to a void reference. That is, Eiffel prevents situations in which systems fail at run time with the error: "Feature call on void target".

Throughout the history of Eiffel, a small number of important new capabilities, agents for example, have been added.