Void-safety: how Eiffel removes null-pointer dereferencing

This white paper (see document as pdf) presents the Eiffel void-safety mechanism, fully implemented in EiffelStudio 6.4.

In almost every program running today there is a ticking time bomb: the risk of a "void call". A void call is possible in programs written in almost any programming language; its effect is usually to crash the program. Many unexplained program failures and other abnormal behaviors result from void calls.

While extensive testing can decrease the likelihood of a void call, it cannot remove the possibility. The solution has to come from the programming language.

"The invention of the null reference in 1965" [the source of void calls] "has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years."

(Citation at: http://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare )

The Eiffel solution relies on a combination of language mechanisms:

  • "Certified Attachment Patterns" are code schemes that the EiffelStudio compiler guarantees to be void-safe.
  • "Attached types" are types that are guaranteed to have non-void values.
  • The "Object Test" instruction lets programmers treat void values in a special way.

The White Paper (see the link below) describes the theoretical and practical challenges of ensuring void-safety and presents the Eiffel mechanism.

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