Variadic function: Difference between revisions

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In [[mathematics]] and in [[computer programming]], a '''variadic function''' is a [[function (programming)|function]] of indefinite [[arity]], i.e., one which accepts a variable number of [[argument (computer science)|argument]]s. Support for variadic functions differs widely among [[programming language]]s.

The term ''variadic'' is a [[neologism]], dating back to 1936–19371936/1937.<ref>Henry S. Leonard and H. N. Goodman, ''A calculus of individuals''. Abstract of a talk given at the Second Meeting of the Association for Symbolic Logic, held in Cambridge MA on December 28–30, 1936, [https://www.jstor.org/stable/2268861], ''Journal of Symbolic Logic'' '''2'''(1) 1937, 63.</ref> The term was not widely used until the 1970s.

==Overview==

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Variadic functions can expose [[Type safety|type-safety]] problems in some languages. For instance, C's {{code|printf}}, if used incautiously, can give rise to a class of security holes known as [[format string attack]]s. The attack is possible because the language support for variadic functions is not type-safe: it permits the function to attempt to pop more arguments off the [[Stack (abstract data type)#Hardware stacks|stack]] than were placed there, corrupting the stack and leading to unexpected behavior. As a consequence of this, the [[CERT Coordination Center]] considers variadic functions in C to be a high-severity security risk.<ref>{{cite book|last=Klemens|first=Ben|title=21st Century C: C Tips from the New School|publisher=O'Reilly Media, Inc.|date=2014|pages=224|isbn=978-1491904442}}</ref>

In [[functional programming]] languages, variadics can be considered complementary to the [[apply]] function, which takes a function and a list/sequence/array as arguments, and calls the function with the arguments supplied in that list, thus passing a variable number of arguments to the function.{{citation needed|date=February 2018}} In the functional language [[Haskell (programming language)|Haskell]], variadic functions can be implemented by returning a value of a [[type class]] {{code|T}}; if instances of {{code|T}} are a final return value {{code|r}} and a function {{code|1=(T t) => x -> t}}, this allows for any number of additional arguments {{code|x}}.{{Explain|reason=Either this should be rephrased in a simpler way, or better explained. It is not immediately understandable, especially for readers not knowing Haskell, which is likelymost thereaders, vasteven majorityfor ofthis readersarticle.|date=May 2018}}

A related subject in [[term rewriting]] research is called '''hedges''', or '''hedge variables'''.<ref>[https://arxiv.org/abs/1503.00336 CLP (H): Constraint Logic Programming for Hedges]</ref> Unlike variadics, which are functions with arguments, hedges are sequences of arguments themselves. They also can have constraints ('take no more than 4 arguments', for example) to the point where they are not variable-length (such as 'take exactly 4 arguments') - thus calling them ''variadics'' can be misleading. However they are referring to the same phenomenon, and sometimes the phrasing is mixed, resulting in names such as ''variadic variable'' (synonymous to hedge). Note the double meaning of the word ''variable'' and the difference between arguments and variables in functional programming and term rewriting. For example, a term (function) can have three variables, one of them a hedge, thus allowing the term to take three or more arguments (or two or more if the hedge is allowed to be empty).

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===In C===

To portably implement variadic functions in the [[C (programming language)|C programming language]], the standard [[stdarg.h|{{code|stdarg.h}}]] header file is used. The older [[varargs.h|{{code|varargs.h}}]] header has been [[Deprecation|deprecated]] in favor of {{code|stdarg.h}}. In C++, the header file {{code|cstdarg}} is used.<ref>{{cite web|url=http://www.cplusplus.com/reference/clibrary/cstdarg/|title=<cstdarg> (stdarg.h) - C++ Reference|website=www.cplusplus.com|access-date=2007-10-02|archive-date=2012-10-31|archive-url=https://web.archive.org/web/20121031160547/http://www.cplusplus.com/reference/clibrary/cstdarg/|url-status=dead}}</ref>

<syntaxhighlight lang="C">

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</syntaxhighlight>

This will compute the average of an arbitrary number of arguments. Note that the function does not know the number of arguments or their types. The above function expects that the types will be {{code|int}}, and that the number of arguments is passed in the first argument (this is a frequent usage but by no means enforced by the language or compiler). In some other cases, for example [[printf]], the number and types of arguments are figured out from a format string. In both cases, this depends on the programmer to supply the correct information. (Alternatively, a [[sentinel value]] like {{code|NULL}} or {{code|nullptr}} may be used to indicate the numberend of the parameter list.) If fewer arguments are passed in than the function believes, or the types of arguments are incorrect, this could cause it to read into invalid areas of memory and can lead to vulnerabilities like the [[format string attack]]. Depending on the system, even using {{code|NULL}} as a sentinel may encounter such problems; {{code|nullptr}} or a dedicated null pointer of the correct target type may be used to avoid them.

{{code|stdarg.h}} declares a type, {{code|va_list}}, and defines four macros: [[va start|{{code|va_start}}]], [[va arg|{{code|va_arg}}]], [[va copy|{{code|va_copy}}]], and [[va end|{{code|va_end}}]]. Each invocation of {{code|va_start}} and {{code|va_copy}} must be matched by a corresponding invocation of {{code|va_end}}. When working with variable arguments, a function normally declares a variable of type {{code|va_list}} ({{code|ap}} in the example) that will be manipulated by the macros.

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===In C++===

The basic variadic facility in C++ is largely identical to that in C. The only difference is in the syntax, where the comma before the ellipsis can be omitted. C++ allows variadic functions without [[named parametersparameter]]s but provides no way to access those arguments since <code>va_start</code> requires the name of the last fixed argument of the function. <!-- When C23 is released the text should be updated to reflect that this is a difference between the languages. When C++26 ports the one argument va_start to C++ (https://wg21.link/p2537) this should be updated again. See also the C section of the article. -->

<syntaxhighlight lang="c++">

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The [[CERT Coding Standards]] for C++ strongly prefers the use of [[variadic templates]] (parameter pack) in C++ over the C-style variadic function due to a lower risk of misuse.<ref>{{cite web |title=DCL50-CPP. Do not define a C-style variadic function |url=https://wiki.sei.cmu.edu/confluence/display/cplusplus/DCL50-CPP}}</ref>

=== In Fortran ===

Since the Fortran 90 revision, [[Fortran]] functions or subroutines can accept optional arguments:<ref>{{Cite web |title=Optional Arguments |url=https://www.intel.com/content/www/us/en/docs/fortran-compiler/developer-guide-reference/2023-0/optional-arguments.html |access-date=2025-03-18 |website=Intel |language=en}}</ref> the argument list is still fixed, but the ones that have the {{code|optional}} attribute can be omitted in the function/subroutine call. The intrinsic function {{code|present()}} can be used to detect the presence of an optional argument. The optional arguments can appear anywhere in the argument list.

<syntaxhighlight lang="fortran">program test

implicit none

real :: x

!> all arguments are passed:

call foo( 1, 2, 3.0, 4, x )

!< outputs 1 \ 2 \ 3.0 \ 4 \ 6.0 (the "\" denotes a newline)

!> the last 2 arguments are omitted:

call foo( 1, 2, 3.0 )

!< outputs 1 \ 2 \ 3.0

!> the 2nd and 4th arguments are omitted: the arguments that are positioned after

!> an omitted argument must be passed with a keyword:

call foo( 1, c=3.0, e=x )

!< outputs 1 \ 3.0 \ 6.0

!> alternatively, the Fortran 2023 revision has introduced the .NIL. pseudo constant

!> to denote an omitted argument

call foo( 1, .NIL., 3.0, .NIL., x )

!< outputs 1 \ 3.0 \ 6.0

contains

!> the subroutine foo() has 2 mandatory and 3 optional arguments

subroutine foo( a, b, c, d, e )

integer, intent(in) :: a

integer, intent(in), optional :: b

real, intent(in) :: c

integer, intent(in), optional :: d

real, intent(out), optional :: e

print*, a

if (present(b)) print*, b

print*, c

if (present(d)) print*, d

if (present(e)) then

e = 2*c

print*, c

end if

end subroutine

end program</syntaxhighlight>

'''Output:'''

<pre>

The sum of [1 2] is 3

The sum of [1 2 3] is 6

The sum of [1 2 3 4] is 10

</pre>

===In Go===

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This requirement is circumvented by utilizing a variant record.

The [[GNU Pascal]] defines a real variadic formal parameter specification using an ellipsis ({{code|...|pascal}}), but as of 2022 no portable mechanism to use such has been defined.<ref name="gpc">{{cite web|url=https://www.gnu-pascal.de/gpc/Special-Parameters.html|title=The GNU Pascal Manual|access-date=2023-08-28}}</ref>

Both GNU Pascal and FreePascal allow externally declared functions to use a variadic formal parameter specification using an ellipsis ({{code|...|pascal}}).

===In PHP===

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==External links==

* [http://rosettacode.org/wiki/Variadic_function Variadic function]. [[Rosetta Code]] task showing the implementation of variadic functions in over 120 programming languages.

* [https://web.archive.org/web/20070927215504/http://www.codeproject.com/cpp/argfunctions.asp?df=100&forumid=15556&exp=0&select=503481 Variable Argument Functions] — A tutorial on Variable Argument Functions for C++

* [https://www.gnu.org/software/hello/manual/libc/Variadic-Functions.html GNU libc manual]

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