Void safety is a guarantee within an object-oriented programming language that that no object references

Reference (computer science)

In computer science, a reference is a value that enables a program to indirectly access a particular data item, such as a variable or a record, in the computer's memory or in some other storage device. The reference is said to refer to the data item, and accessing those data is called...

 will have null or void values.

In object-oriented languages, access to objects is achieved through references

Reference (computer science)

In computer science, a reference is a value that enables a program to indirectly access a particular data item, such as a variable or a record, in the computer's memory or in some other storage device. The reference is said to refer to the data item, and accessing those data is called...

 (or, equivalently, pointers). A typical call is of the form:

x.f(a, ...)

where f denotes an operation and x denotes a reference to some object. At execution time, however, a reference can be void (or null). In such cases, the call above will be a void call, leading to a run-time exception

Exception handling

Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....

, often resulting in abnormal termination

Crash (computing)

A crash in computing is a condition where a computer or a program, either an application or part of the operating system, ceases to function properly, often exiting after encountering errors. Often the offending program may appear to freeze or hang until a crash reporting service documents...

 of the program.

Void safety is a static (compile-time) guarantee that no void calls will ever arise.

History

In a 2009 talk, Tony Hoare traced the invention of the null pointer to his design of the Algol W

ALGOL W

ALGOL W is a programming language. It was based on a proposal for ALGOL X by Niklaus Wirth and C. A. R. Hoare as a successor to ALGOL 60 in IFIP Working Group 2.1. When the committee decided that the proposal was not a sufficient advance over ALGOL 60, the proposal was published as A contribution...

 language (although the Lisp language had the NIL value as early as 1959) and called it a "mistake":
Bertrand Meyer introduced the term "void safety".

In programming languages

An early attempt to guarantee void safety was the design of the Self programming language.

The Spec# language

Spec sharp

Spec# is a programming language with specification language features that extends the capabilities of the C# programming language with Eiffel-like contracts, including object invariants, preconditions and postconditions. Like ESC/Java, it includes a static checking tool based on a theorem prover...

, a research language from Microsoft Research, has a notion of "non-nullable type" addressing void safety.

The Eiffel language

Eiffel (programming language)

Eiffel is an ISO-standardized, object-oriented programming language designed by Bertrand Meyer and Eiffel Software. The design of the language is closely connected with the Eiffel programming method...

 is void-safe according to its ISO-ECMA

Ecma International

Ecma International is an international, private non-profit standards organization for information and communication systems. It acquired its name in 1994, when the European Computer Manufacturers Association changed its name to reflect the organization's global reach and activities...

 standard; the void-safety mechanism is implemented in EiffelStudio

EiffelStudio

EiffelStudio is a development environment for the Eiffel programming language developed and distributed by Eiffel Software.EiffelStudio includes a combination of tools integrated under a single user interface: compiler, interpreter, debugger, browser, metrics tool, profiler, diagram tool...

starting with version 6.1 and continuing to mature through version 6.4.