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{{Short description|Programming construct}}
{{About|the computer programming concept of function objects|functors in mathematics|Functor|the related concept in functional programming|Functor (functional programming)}}
{{
In [[computer programming]], a '''function object'''{{efn|1=In C++, a '''functionoid''' is an object that has one major method, and a '''functor''' is a special case of a functionoid.<ref>[https://isocpp.org/wiki/faq/pointers-to-members#functor-vs-functionoid What's the difference between a functionoid and a functor?]</ref> They are similar to a function object, ''but not the same''.}} is a construct allowing an [[object (computer science)|object]] to be invoked or called as if it were an ordinary [[subroutine|function]], usually with the same syntax (a function parameter that can also be a function).
== Description ==
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| title = C++ Tutorial Part I - Basic: 5.10 Function pointers are mainly used to achieve call back technique, which will be discussed right after.
| author = Silan Liu
| publisher = TRIPOD: Programming Tutorials Copyright © Silan Liu 2002
| quote = Function pointers are mainly used to achieve call back technique, which will be discussed right after.
|
}}</ref> However it can be difficult or awkward to pass a state into or out of the callback function. This restriction also inhibits more dynamic behavior of the function. A function object solves those problems since the function is really a [[facade pattern|façade]] for a full object, carrying its own state.
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| title = C++ Tutorial Part I - Basic: 5.10 Function pointers are mainly used to achieve call back technique, which will be discussed right after.
| author = Paweł Turlejski
| date = 2009-10-02
| publisher = Just a Few Lines
| quote = PHP 5.3, along with many other features, introduced closures. So now we can finally do all the cool stuff that Ruby / Groovy / Scala / any_modern_language guys can do, right? Well, we can, but we probably won’t… Here's why.
|
}}</ref> [[Functional programming]] languages additionally support [[closure (computer science)|closures]], i.e. first-class functions that can 'close over' variables in their surrounding environment at creation time. During compilation, a transformation known as [[lambda lifting]] converts the closures into function objects.
== In C and C++ ==
Consider the example of a sorting routine that uses a callback function to define an ordering relation between a pair of items.
<!-- NOTE: For the compareInts() implementation below, see http://stackoverflow.com/a/10997428/1629102 for an explanation of why the more simple (int) a - (int) b would not work in all cases. -->
<syntaxhighlight lang=
#include <stdlib.h>
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int compareInts(const void* a, const void* b)
{
return ( *(int *)a - *(int *)b
}
...
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In C++, a function object may be used instead of an ordinary function by defining a class that [[operator overloading|overloads]] the [[function call operator]] by defining an <code>operator()</code> member function. In C++, this may appear as follows:
<syntaxhighlight lang=
// comparator predicate: returns true if a < b, false otherwise
struct IntComparator
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Notice that the syntax for providing the callback to the <code>std::sort()</code> function is identical, but an object is passed instead of a function pointer. When invoked, the callback function is executed just as any other member function, and therefore has full access to the other members (data or functions) of the object. Of course, this is just a trivial example. To understand what power a functor provides more than a regular function, consider the common use case of sorting objects by a particular field. In the following example, a functor is used to sort a simple employee database by each employee's ID number.
<syntaxhighlight lang=
struct CompareBy
{
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}
bool operator()(const Employee& a, const Employee& b) const
{
if (SORT_FIELD == "name")
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return 0;
}
</syntaxhighlight>
In [[C++11]], the lambda expression provides a more succinct way to do the same thing.
<syntaxhighlight lang=
int main()
{
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/* code to populate database */
const std::string sort_field = "idnum";
std::sort(emps.begin(), emps.end(), [&sort_field](const Employee& a, const Employee& b) const { /* code to select and compare field */ });
return 0;
}
</syntaxhighlight>
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It is possible to use function objects in situations other than as callback functions. In this case, the shortened term ''functor'' is normally ''not'' used about the function object. Continuing the example,
<syntaxhighlight lang=
</syntaxhighlight>
In addition to class type functors, other kinds of function objects are also possible in C++. They can take advantage of C++'s member-pointer or [[generic programming|template]] facilities. The expressiveness of templates allows some [[functional programming]] techniques to be used, such as defining function objects in terms of other function objects (like [[function composition (computer science)|function composition]]). Much of the C++ [[Standard Template Library]] (STL) makes heavy use of template-based function objects.
Another way to create a function object in C++ is to define a non-explicit conversion function to a function pointer type, a function [[reference (C++)|reference]] type, or a reference to function pointer type. Assuming the conversion does not discard [[Type qualifier|cv-qualifiers]], this allows an object of that type to be used as a function with the same [[function signature|signature]] as the type it is converted to. Modifying an earlier example to use this we obtain the following class, whose instances can be called like function pointers:<ref>{{cite web|url=https://en.cppreference.com/w/cpp/language/overload_resolution#Call_to_a_class_object|title=Overload resolution§Call to a class object|website=cppreference.com}}</ref>
<syntaxhighlight lang="cpp">
// comparator predicate: returns true if a < b, false otherwise
struct IntComparator
{
static bool compare(const int &a, const int &b)
{
return a < b;
}
using T = decltype(compare);
operator T*() const { return compare; }
};
int main()
{
std::sort(items.begin(), items.end(), IntComparator());
return 0;
}
</syntaxhighlight>
=== Maintaining state ===
Another advantage of function objects is their ability to maintain a state that affects <code>operator()</code> between calls. For example, the following code defines a [[generator (computer science)|generator]] counting from 10 upwards and is invoked 11 times.
<syntaxhighlight lang=
#include <algorithm>
#include <iostream>
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class CountFrom {
public:
CountFrom(int
int operator()() { return count_++; }
private:
int
};
int main() {
const int state(10);
std::generate_n(std::ostream_iterator<int>(std::cout, "\n"), 11,
CountFrom(state));
}
</syntaxhighlight>
In C++14 or later, the example above could be rewritten as:
<syntaxhighlight lang="cpp">
#include <algorithm>
#include <iostream>
#include <iterator>
int main() {
std::generate_n(std::ostream_iterator<int>(std::cout, "\n"), 11,
[count=10]() mutable { return count++; });
}
</syntaxhighlight>
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In [[C Sharp (programming language)|C#]], function objects are declared via [[delegate (CLI)|delegate]]s. A delegate can be declared using a named method or a [[Lambda (programming)|lambda expression]]. Here is an example using a named method.
<syntaxhighlight lang=
using System;
using System.Collections.Generic;
public class ComparisonClass1
{ public static int CompareFunction(int x, int y)
return x - y;
}
public static void Main()
▲ List<int> items = new List<int> { 4, 3, 1, 2 };
Comparison<int> del = CompareFunction;
items.Sort(del);
}
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Here is an example using a lambda expression.
<syntaxhighlight lang=
using System;
using System.Collections.Generic;
public class ComparisonClass2
{ public static void Main()
▲ List<int> items = new List<int> { 4, 3, 1, 2 };
var items = new List<int> { 4, 3, 1, 2 };
items.Sort((x, y) => x - y);
}
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[[D (programming language)|D]] provides several ways to declare function objects: Lisp/Python-style via [[closure (computer science)|closures]] or C#-style via [[delegate (CLI)|delegate]]s, respectively:
<syntaxhighlight lang=
bool find(T)(T[] haystack, bool delegate(T) needle_test) {
foreach (straw; haystack) {
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return n == needle;
}
assert(find(haystack, &needleTest));
▲ );
}
</syntaxhighlight>
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The difference between a [[delegate (CLI)|delegate]] and a [[closure (computer science)|closure]] in D is automatically and conservatively determined by the compiler. D also supports function literals, that allow a lambda-style definition:
<syntaxhighlight lang=
void main() {
int[] haystack = [345, 15, 457, 9, 56, 123, 456];
int needle = 123;
▲ find(haystack, (int n) { return n == needle; })
▲ );
}
</syntaxhighlight>
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To allow the compiler to inline the code (see above), function objects can also be specified C++-style via [[operator overloading]]:
<syntaxhighlight lang=
bool find(T, F)(T[] haystack, F needle_test) {
foreach (straw; haystack) {
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}
}
assert(find(haystack, new NeedleTest(needle)));
▲ );
}
</syntaxhighlight>
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Within software text, the language keyword <code>agent</code> allows agents to be constructed in a compact form. In the following example, the goal is to add the action of stepping the gauge forward to the list of actions to be executed in the event that a button is clicked.
<syntaxhighlight lang=
</syntaxhighlight>
The routine <code>extend</code> referenced in the example above is a feature of a class in a [[graphical user interface]] (GUI) library to provide [[event-driven programming]] capabilities.
In other library classes, agents are seen to be used for different purposes. In a library supporting data structures, for example, a class modeling linear structures effects [[universal quantification]] with a function <code>for_all</code> of type <code>BOOLEAN</code> that accepts an agent, an instance of <code>FUNCTION</code>, as an argument. So, in the following example, <code>my_action</code> is executed only if all members of <code>my_list</code> contain the character '!':
<syntaxhighlight lang=
my_list: LINKED_LIST [STRING]
...
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The ability to close or leave open targets and arguments is intended to improve the flexibility of the agent mechanism. Consider a class that contains the following procedure to print a string on standard output after a new line:
<syntaxhighlight lang=
print_on_new_line (s: STRING)
-- Print `s' preceded by a new line
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The following snippet, assumed to be in the same class, uses <code>print_on_new_line</code> to demonstrate the mixing of open arguments and open targets in agents used as arguments to the same routine.
<syntaxhighlight lang=
my_list: LINKED_LIST [STRING]
...
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Open and closed arguments and targets also allow the use of routines that call for more arguments than are required by closing all but the necessary number of arguments:
<syntaxhighlight lang=
</syntaxhighlight>
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For an example from Java's standard library, <code>java.util.Collections.sort()</code> takes a <code>List</code> and a functor whose role is to compare objects in the List. Without first-class functions, the function is part of the Comparator interface. This could be used as follows.
<syntaxhighlight lang=
List<String> list = Arrays.asList("10", "1", "20", "11", "21", "12");
Comparator<String> numStringComparator = new Comparator<String>() {
public int compare(String str1, String str2) {
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In Java 8+, this can be written as:
<syntaxhighlight lang=
List<String> list = Arrays.asList("10", "1", "20", "11", "21", "12");
Comparator<String> numStringComparator = (str1, str2) -> Integer.valueOf(str1).compareTo(Integer.valueOf(str2));
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Compare the following with the subsequent Python example.
<syntaxhighlight lang=
function Accumulator(start) {
var current = start;
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An example of this in use:
<syntaxhighlight lang=
var a = Accumulator(4);
var x = a(5); // x has value 9
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== In Julia ==
In [[
An example is this accumulator mutable struct (based on [[Paul Graham (computer programmer)|Paul Graham's]] study on programming language syntax and clarity):<ref>[http://www.paulgraham.com/accgen.html Accumulator Generator]</ref>
<syntaxhighlight lang=
julia> mutable struct Accumulator
n::Int
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Such an accumulator can also be implemented using closure:
<syntaxhighlight lang=
julia> function Accumulator(n0)
n = n0
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Many uses of functors in languages like C++ are simply emulations of the missing closure constructor. Since the programmer cannot directly construct a closure, they must define a class that has all of the necessary state variables, and also a member function. Then, construct an instance of that class instead, ensuring that all the member variables are initialized through its constructor. The values are derived precisely from those local variables that ought to be captured directly by a closure.
A function-object using the class system in Common Lisp, no use of closures:
<syntaxhighlight lang=
(defclass counter ()
((value :initarg :value :accessor value-of)))
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</syntaxhighlight>
Since there is no standard way to make funcallable objects in Common Lisp, we fake it by defining a [[generic function]] called FUNCTOR-CALL. This can be specialized for any class whatsoever. The standard FUNCALL function is not generic; it only takes function objects.
It is this FUNCTOR-CALL generic function that gives us function objects, which are ''a computer programming construct allowing an object to be invoked or called as if it were an ordinary function, usually with the same syntax.'' We have ''almost'' the same syntax: FUNCTOR-CALL instead of FUNCALL. Some Lisps provide ''funcallable'' objects as a simple extension. Making objects callable using the same syntax as functions is a fairly trivial business. Making a function call operator work with different kinds of ''function things'', whether they be class objects or closures is no more complicated than making a + operator that works with different kinds of numbers, such as integers, reals or complex numbers.
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Now, a counter implemented using a closure. This is much more brief and direct. The INITIAL-VALUE argument of the MAKE-COUNTER [[factory function]] is captured and used directly. It does not have to be copied into some auxiliary class object through a constructor. It ''is'' the counter. An auxiliary object is created, but that happens ''behind the scenes''.
<syntaxhighlight lang=
(defun make-counter (value)
(lambda () (incf value)))
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</syntaxhighlight>
Scheme makes closures even simpler, and Scheme code tends to use such [[higher-order programming]] somewhat more idiomatically.
<syntaxhighlight lang="scheme">
(define (make-counter value)
(lambda () (set! value (+ value 1)) value))
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In [[Objective-C]], a function object can be created from the <code>NSInvocation</code> class. Construction of a function object requires a method signature, the target object, and the target selector. Here is an example for creating an invocation to the current object's <code>myMethod</code>:
<syntaxhighlight lang=
// Construct a function object
SEL sel = @selector(myMethod);
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</syntaxhighlight>
An advantage of <code>NSInvocation</code> is that the target object can be modified after creation. A single <code>NSInvocation</code> can be created and then called for each of any number of targets, for instance from an observable object. An <code>NSInvocation</code> can be created from only a protocol, but it is not straightforward. See {{usurped|1=[https://web.archive.org/web/20110227215311/http://www.a-coding.com/2010/10/making-nsinvocations.html here]}}.
== In Perl ==
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In [[Perl]], a function object can be created either from a class's constructor returning a function closed over the object's instance data, blessed into the class:
<syntaxhighlight lang=
package Acc1;
sub new {
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</syntaxhighlight>
or by overloading the <code>&{}</code> operator so that the object can be used as a function:
<syntaxhighlight lang=
package Acc2;
use overload
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In both cases the function object can be used either using the dereferencing arrow syntax ''$ref->(@arguments)'':
<syntaxhighlight lang=
use Acc1;
my $a = Acc1->new(42);
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</syntaxhighlight>
or using the coderef dereferencing syntax ''&$ref(@arguments)'':
<syntaxhighlight lang=
use Acc2;
my $a = Acc2->new(12);
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== In PHP ==
[[PHP]] 5.3+ has [[first-class function]]s that can be used e.g. as parameter to the {{Code|usort()}} function:
<syntaxhighlight lang=
$a = array(3, 1, 4);
usort($a, function ($x, $y) { return $x - $y; });
</syntaxhighlight>
[[PHP]] 5.3+, supports also lambda functions and closures.
<syntaxhighlight lang="php">
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</syntaxhighlight>
It is also possible in PHP 5.3+ to make objects invokable by adding a magic {{Code|__invoke()}} method to their class:<ref name="phpinvoke">[http://php.net/manual/en/language.oop5.magic.php#object.invoke PHP Documentation on Magic Methods]</ref>
<syntaxhighlight lang="php">
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In the [[Windows PowerShell]] language, a script block is a collection of statements or expressions that can be used as a single unit. A script block can accept arguments and return values. A script block is an instance of a Microsoft [[.NET Framework]] type System.Management.Automation.ScriptBlock.
<syntaxhighlight lang=
Function Get-Accumulator($x) {
{
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<syntaxhighlight lang="python">
class Accumulator
def __init__(self, n) -> None:
self.n = n
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</syntaxhighlight>
Since functions are objects, they can also be defined locally, given attributes, and returned by other functions,
<syntaxhighlight lang="python3">
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In [[Ruby (programming language)|Ruby]], several objects can be considered function objects, in particular Method and Proc objects. Ruby also has two kinds of objects that can be thought of as semi-function objects: UnboundMethod and block. UnboundMethods must first be bound to an object (thus becoming a Method) before they can be used as a function object. Blocks can be called like function objects, but to be used in any other capacity as an object (e.g. passed as an argument) they must first be converted to a Proc. More recently, symbols (accessed via the literal unary indicator <code>:</code>) can also be converted to <code>Proc</code>s. Using Ruby's unary <code>&</code> operator—equivalent to calling <code>to_proc</code> on an object, and [[duck typing|assuming that method exists]]—the [[Ruby Extensions Project]] [https://web.archive.org/web/20060425104650/http://blogs.pragprog.com/cgi-bin/pragdave.cgi/Tech/Ruby/ToProc.rdoc created a simple hack.]
<syntaxhighlight lang=
class Symbol
def to_proc
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</syntaxhighlight>
Now, method <code>foo</code> can be a function object, i.e. a <code>Proc</code>, via <code>&:foo</code> and used via <code>takes_a_functor(&:foo)</code>. <code>Symbol.to_proc</code> was officially added to Ruby on June 11, 2006, during RubyKaigi2006. [https://web.archive.org/web/20060820025032/http://redhanded.hobix.com/cult/symbolTo_procExonerated.html]
Because of the variety of forms, the term Functor is not generally used in Ruby to mean a Function object.
Just a type of dispatch [[delegation (programming)|delegation]] introduced by the [https://web.archive.org/web/20070107205748/http://facets.rubyforge.org/ Ruby Facets] project is named as Functor. The most basic definition of which is:
<syntaxhighlight lang=
class Functor
def initialize(&func)
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The [[ML (programming language)|ML]] family of [[functional programming]] languages uses the term ''functor'' to represent a [[function (mathematics)|mapping]] from modules to modules, or from types to types and is a technique for reusing code. Functors used in this manner are analogous to the original mathematical meaning of [[functor]] in [[category theory]], or to the use of generic programming in C++, Java or [[Ada (programming language)|Ada]].
In [[Haskell (programming language)|Haskell]], the term ''[[Functor (functional programming)|functor]]'' is also used
In [[Prolog]] and related languages, ''functor'' is a synonym for [[function symbol]].
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== External links ==
* [http://c2.com/cgi/wiki?FunctorObject Description from the Portland Pattern Repository]
* [http://www.two-sdg.demon.co.uk/curbralan/papers/AsynchronousC++.pdf C++ Advanced Design Issues - Asynchronous C++] {{Webarchive|url=https://web.archive.org/web/20200922012516/http://www.two-sdg.demon.co.uk/curbralan/papers/AsynchronousC++.pdf |date=2020-09-22 }} by [[Kevlin Henney]]
* [http://www.newty.de/fpt/index.html The Function Pointer Tutorials] by Lars Haendel (2000/2001)
* Article "[https://web.archive.org/web/20041009232434/http://www.cuj.com/documents/s%3D8464/cujcexp0308sutter/ Generalized Function Pointers]" by [[Herb Sutter]]
* [
* [https://web.archive.org/web/20100330073950/http://www.amcgowan.ca/blog/computer-science/php-functors-function-objects-in-php/ PHP Functors - Function Objects in PHP]
* [https://web.archive.org/web/20041013202445/http://www.parashift.com/c++-faq-lite/pointers-to-members.html#faq-33.10 What the heck is a functionoid, and why would I use one?] (C++ FAQ)
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[[Category:Articles with example C code]]
[[Category:Articles with example C++ code]]
[[Category:Articles with example Eiffel code]]
[[Category:Articles with example Java code]]
[[Category:Articles with example JavaScript code]]
[[Category:Articles with example Julia code]]
[[Category:Articles with example Lisp (programming language) code]]
[[Category:Articles with example Objective-C code]]
[[Category:Articles with example Perl code]]
[[Category:Articles with example
[[Category:Articles with example Python (programming language) code]]
[[Category:Articles with example Ruby code]]
▲[[Category:Articles with example Julia code]]
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