World Library  
Flag as Inappropriate
Email this Article

Void type

Article Id: WHEBN0002056815
Reproduction Date:

Title: Void type  
Author: World Heritage Encyclopedia
Language: English
Subject: Data types, Comparison of programming languages (basic instructions), C (programming language), Void, Expression (computer science)
Collection:
Publisher: World Heritage Encyclopedia
Publication
Date:
 

Void type

The void type, in several programming languages derived from C and Algol68, is the type for the result of a function that returns normally, but does not provide a result value to its caller. Usually such functions are called for their side effects, such as performing some task or writing to their output parameters. The usage of the void type in such context is comparable to that of the syntactic constructs which define subroutines in Visual Basic and procedures in Pascal. It is also similar to the unit type used in functional programming languages and type theory; however, there are some differences in allowable usage, in that the void type is taken to be an empty type with no values. See Unit type#In programming languages for a comparison.

C and C++ also support the pointer to void type (specified as void *), but this is an unrelated notion. Variables of this type are pointers to data of an unspecified type, so in this context (but not the others) void acts as a universal or top type. A program can probably convert a pointer to any type of data (except a function pointer) to a pointer to void and back to the original type without losing information, which makes these pointers useful for polymorphic functions. The C language standard does not guarantee that the different pointer types have the same size.

In C and C++

A function with void result type ends either by reaching the end of the function or by executing a return statement with no returned value. The void type may also appear as the sole argument of a function prototype to indicate that the function takes no arguments. Note that despite the name, in all of these situations, the void type serves as a unit type, not as a zero or bottom type, even though unlike a real unit type which is a singleton, the void type is said to comprise an empty set of values, and the language does not provide any way to declare an object or represent a value with type void.

In the earliest versions of C, functions with no specific result defaulted to a return type of int and functions with no arguments simply had empty argument lists. Pointers to untyped data were declared as integers or pointers to char. Some early C compilers had the feature, now seen as an annoyance, of generating a warning on any function call that did not use the function's returned value. Old code sometimes casts such function calls to void to suppress this warning. By the time Bjarne Stroustrup began his work on C++ in 1979–1980, void and void pointers were part of the C language dialect supported by AT&T-derived compilers.[1]

The explicit use of void vs. giving no arguments in a function prototype has different semantics in C and C++, as detailed in the following table:[2]

C C++ equivalent
void f(void); void f(); (preferred)
void f(void); void f(void);
void f(); (accepts a constant but unknown number of arguments) no equivalent

A C prototype taking no arguments, e.g. void f() above, has been deprecated however in C99.[3]

References

  1. ^ http://cm.bell-labs.com/cm/cs/who/dmr/chist.html "Standardisation."
  2. ^ Stroustrup, Bjarne (2009). Programming: Principles and Practice Using C++. Boston: Addison-Wesley. p. 996.  
  3. ^ Bjarne Stroustrup, C and C++: Case Studies in Compatibility. Reconcilable differences? You decide, Dr. Dobb's, September 01, 2002; print version
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.