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Function object

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</style><div role="note" class="hatnote navigation-not-searchable">This article is about the computer programming concept of function objects. For functors in mathematics, see <a href="Functor" title="Functor">Functor</a>. For the related concept in functional programming, see <a href="Functor_(functional_programming)" title="Functor (functional programming)">Functor (functional programming)</a>.</div>

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In <a href="Computer_programming" title="Computer programming">computer programming</a>, a function object<a href="#cite_note-2">[a]</a> is a construct allowing an <a href="Object_(computer_science)" title="Object (computer science)">object</a> to be invoked or called as if it were an ordinary <a href="Subroutine" class="mw-redirect" title="Subroutine">function</a>, usually with the same syntax (a function parameter that can also be a function). In some languages, particularly C++, function objects are often called functors (not related to <a href="Functor_(functional_programming)" title="Functor (functional programming)">the functional programming concept</a>).

<div class="mw-heading mw-heading2"><h2 id="Description">Description</h2></div>

A typical use of a function object is in writing <a href="Callback_(computer_science)" class="mw-redirect" title="Callback (computer science)">callback</a> functions. A callback in <a href="Procedural_programming" title="Procedural programming">procedural languages</a>, such as <a href="C_(programming_language)" title="C (programming language)">C</a>, may be performed by using <a href="Function_pointer" title="Function pointer">function pointers</a>.<a href="#cite_note-3">[2]</a> 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 <a href="Facade_pattern" title="Facade pattern">façade</a> for a full object, carrying its own state.

Many modern (and some older) languages, e.g. <a href="C%2B%2B" title="C++">C++</a>, <a href="Eiffel_(programming_language)" title="Eiffel (programming language)">Eiffel</a>, <a href="Groovy_(programming_language)" class="mw-redirect" title="Groovy (programming language)">Groovy</a>, <a href="Lisp_(programming_language)" title="Lisp (programming language)">Lisp</a>, <a href="Smalltalk" title="Smalltalk">Smalltalk</a>, <a href="Perl" title="Perl">Perl</a>, <a href="PHP" title="PHP">PHP</a>, <a href="Python_(programming_language)" title="Python (programming language)">Python</a>, <a href="Ruby_(programming_language)" title="Ruby (programming language)">Ruby</a>, <a href="Scala_(programming_language)" title="Scala (programming language)">Scala</a>, and many others, support <a href="First-class_function" title="First-class function">first-class function</a> objects and may even make significant use of them.<a href="#cite_note-4">[3]</a> <a href="Functional_programming" title="Functional programming">Functional programming</a> languages additionally support <a href="Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closures</a>, i.e. first-class functions that can 'close over' variables in their surrounding environment at creation time. During compilation, a transformation known as <a href="Lambda_lifting" title="Lambda lifting">lambda lifting</a> converts the closures into function objects.

Consider the example of a sorting routine that uses a callback function to define an ordering relation between a pair of items. The following C/C++ program uses function pointers:

<div class="mw-highlight mw-highlight-lang-c mw-content-ltr" dir="ltr"><pre>#include <stdlib.h>

/* qsort() callback function, returns < 0 if a < b, > 0 if a > b, 0 if a == b */

int compareInts(const void* a, const void* b)

{

return ( *(int *)a - *(int *)b );

}

...

// prototype of qsort is

// void qsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));

...

int main(void)

{

int items[] = { 4, 3, 1, 2 };

qsort(items, sizeof(items) / sizeof(items[0]), sizeof(items[0]), compareInts);

return 0;

}

</pre></div>

In C++, a function object may be used instead of an ordinary function by defining a class that <a href="Operator_overloading" title="Operator overloading">overloads</a> the <a href="Function_call_operator" class="mw-redirect" title="Function call operator">function call operator</a> by defining an <code>operator()</code> member function. In C++, this may appear as follows:

<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>// comparator predicate: returns true if a < b, false otherwise

struct IntComparator

{

bool operator()(const int &a, const int &b) const

{

return a < b;

}

};

int main()

{

std::vector<int> items { 4, 3, 1, 2 };

std::sort(items.begin(), items.end(), IntComparator());

return 0;

}

</pre></div>

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.

<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>struct CompareBy

{

const std::string SORT_FIELD;

CompareBy(const std::string& sort_field="name")

: SORT_FIELD(sort_field)

{

/* validate sort_field */

}

bool operator()(const Employee& a, const Employee& b) const

{

if (SORT_FIELD == "name")

return a.name < b.name;

else if (SORT_FIELD == "age")

return a.age < b.age;

else if (SORT_FIELD == "idnum")

return a.idnum < b.idnum;

else

/* throw exception or something */

}

};

int main()

{

std::vector<Employee> emps;

/* code to populate database */

// Sort the database by employee ID number

std::sort(emps.begin(), emps.end(), CompareBy("idnum"));

return 0;

}

</pre></div>

In <a href="C%2B%2B11" title="C++11">C++11</a>, the lambda expression provides a more succinct way to do the same thing.

<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>int main()

{

std::vector<Employee> emps;

/* 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;

}

</pre></div>

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,

<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>IntComparator cpm;

bool result = cpm(a, b);

</pre></div>

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 <a href="Generic_programming" title="Generic programming">template</a> facilities. The expressiveness of templates allows some <a href="Functional_programming" title="Functional programming">functional programming</a> techniques to be used, such as defining function objects in terms of other function objects (like <a href="Function_composition_(computer_science)" title="Function composition (computer science)">function composition</a>). Much of the C++ <a href="Standard_Template_Library" title="Standard Template Library">Standard Template Library</a> (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 <a href="Reference_(C%2B%2B)" title="Reference (C++)">reference</a> type, or a reference to function pointer type. Assuming the conversion does not discard <a href="Type_qualifier" title="Type qualifier">cv-qualifiers</a>, this allows an object of that type to be used as a function with the same <a href="Function_signature" class="mw-redirect" title="Function signature">signature</a> 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:<a href="#cite_note-5">[4]</a>

<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>// 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::vector<int> items { 4, 3, 1, 2 };

std::sort(items.begin(), items.end(), IntComparator());

return 0;

}

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Maintaining_state">Maintaining state</h3></div>

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 <a href="Generator_(computer_science)" class="mw-redirect" title="Generator (computer science)">generator</a> counting from 10 upwards and is invoked 11 times.

<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>#include <algorithm>

#include <iostream>

#include <iterator>

class CountFrom {

public:

CountFrom(int count) : count_(count) {}

int operator()() { return count_++; }

private:

int count_;

};

int main() {

const int state(10);

std::generate_n(std::ostream_iterator<int>(std::cout, "\n"), 11,

CountFrom(state));

}

</pre></div>

In C++14 or later, the example above could be rewritten as:

<div class="mw-highlight mw-highlight-lang-cpp mw-content-ltr" dir="ltr"><pre>#include <algorithm>

#include <iostream>

#include <iterator>

int main() {

std::generate_n(std::ostream_iterator<int>(std::cout, "\n"), 11,

[count=10]() mutable { return count++; });

}

</pre></div>

In <a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a>, function objects are declared via <a href="Delegate_(CLI)" title="Delegate (CLI)">delegates</a>. A delegate can be declared using a named method or a <a href="Lambda_(programming)" class="mw-redirect" title="Lambda (programming)">lambda expression</a>. Here is an example using a named method.

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>using System;

using System.Collections.Generic;

public class ComparisonClass1

{

public static int CompareFunction(int x, int y)

{

return x - y;

}

public static void Main()

{

var items = new List<int> { 4, 3, 1, 2 };

Comparison<int> del = CompareFunction;

items.Sort(del);

}

}

</pre></div>

Here is an example using a lambda expression.

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre>using System;

using System.Collections.Generic;

public class ComparisonClass2

{

public static void Main()

{

var items = new List<int> { 4, 3, 1, 2 };

items.Sort((x, y) => x - y);

}

}

</pre></div>

<a href="D_(programming_language)" title="D (programming language)">D</a> provides several ways to declare function objects: Lisp/Python-style via <a href="Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closures</a> or C#-style via <a href="Delegate_(CLI)" title="Delegate (CLI)">delegates</a>, respectively:

<div class="mw-highlight mw-highlight-lang-d mw-content-ltr" dir="ltr"><pre>bool find(T)(T[] haystack, bool delegate(T) needle_test) {

foreach (straw; haystack) {

if (needle_test(straw))

return true;

}

return false;

}

void main() {

int[] haystack = [345, 15, 457, 9, 56, 123, 456];

int needle = 123;

bool needleTest(int n) {

return n == needle;

}

assert(find(haystack, &needleTest));

}

</pre></div>

The difference between a <a href="Delegate_(CLI)" title="Delegate (CLI)">delegate</a> and a <a href="Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closure</a> in D is automatically and conservatively determined by the compiler. D also supports function literals, that allow a lambda-style definition:

<div class="mw-highlight mw-highlight-lang-d mw-content-ltr" dir="ltr"><pre>void main() {

int[] haystack = [345, 15, 457, 9, 56, 123, 456];

int needle = 123;

assert(find(haystack, (int n) { return n == needle; }));

}

</pre></div>

To allow the compiler to inline the code (see above), function objects can also be specified C++-style via <a href="Operator_overloading" title="Operator overloading">operator overloading</a>:

<div class="mw-highlight mw-highlight-lang-d mw-content-ltr" dir="ltr"><pre>bool find(T, F)(T[] haystack, F needle_test) {

foreach (straw; haystack) {

if (needle_test(straw))

return true;

}

return false;

}

void main() {

int[] haystack = [345, 15, 457, 9, 56, 123, 456];

int needle = 123;

class NeedleTest {

int needle;

this(int n) { needle = n; }

bool opCall(int n) {

return n == needle;

}

assert(find(haystack, new NeedleTest(needle)));

}

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="In_Eiffel">In Eiffel</h2></div>

In the <a href="Eiffel_(programming_language)" title="Eiffel (programming language)">Eiffel</a> software development method and language, operations and objects are seen always as separate concepts. However, the <a href="Eiffel_(programming_language)#Agents" title="Eiffel (programming language)">agent</a> mechanism facilitates the modeling of operations as runtime objects. Agents satisfy the range of application attributed to function objects, such as being passed as arguments in procedural calls or specified as callback routines. The design of the agent mechanism in Eiffel attempts to reflect the object-oriented nature of the method and language. An agent is an object that generally is a direct instance of one of the two library classes, which model the two types of routines in Eiffel: <code>PROCEDURE</code> and <code>FUNCTION</code>. These two classes descend from the more abstract <code>ROUTINE</code>.

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.

<div class="mw-highlight mw-highlight-lang-eiffel mw-content-ltr" dir="ltr"><pre>my_button.select_actions.extend (agent my_gauge.step_forward)

</pre></div>

The routine <code>extend</code> referenced in the example above is a feature of a class in a <a href="Graphical_user_interface" title="Graphical user interface">graphical user interface</a> (GUI) library to provide <a href="Event-driven_programming" title="Event-driven programming">event-driven programming</a> 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 <a href="Universal_quantification" title="Universal quantification">universal quantification</a> 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 '!':

<div class="mw-highlight mw-highlight-lang-eiffel mw-content-ltr" dir="ltr"><pre> my_list: LINKED_LIST [STRING]

...

if my_list.for_all (agent {STRING}.has ('!')) then

my_action

end

...

</pre></div>

When agents are created, the arguments to the routines they model and even the target object to which they are applied can be either closed or left open. Closed arguments and targets are given values at agent creation time. The assignment of values for open arguments and targets is deferred until some point after the agent is created. The routine <code>for_all</code> expects as an argument an agent representing a function with one open argument or target that conforms to actual generic parameter for the structure (<code>STRING</code> in this example.)

When the target of an agent is left open, the class name of the expected target, enclosed in braces, is substituted for an object reference as shown in the text <code>agent {STRING}.has ('!')</code> in the example above. When an argument is left open, the question mark character ('?') is coded as a placeholder for the open argument.

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:

<div class="mw-highlight mw-highlight-lang-eiffel mw-content-ltr" dir="ltr"><pre> print_on_new_line (s: STRING)

-- Print `s' preceded by a new line

do

print ("%N" + s)

end

</pre></div>

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.

<div class="mw-highlight mw-highlight-lang-eiffel mw-content-ltr" dir="ltr"><pre> my_list: LINKED_LIST [STRING]

...

my_list.do_all (agent print_on_new_line (?))

my_list.do_all (agent {STRING}.to_lower)

my_list.do_all (agent print_on_new_line (?))

...

</pre></div>

This example uses the procedure <code>do_all</code> for linear structures, which executes the routine modeled by an agent for each item in the structure.

The sequence of three instructions prints the strings in <code>my_list</code>, converts the strings to lowercase, and then prints them again.

Procedure <code>do_all</code> iterates across the structure executing the routine substituting the current item for either the open argument (in the case of the agents based on <code>print_on_new_line</code>), or the open target (in the case of the agent based on <code>to_lower</code>).

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:

<div class="mw-highlight mw-highlight-lang-eiffel mw-content-ltr" dir="ltr"><pre>my_list.do_all (agent my_multi_arg_procedure (closed_arg_1, ?, closed_arg_2, closed_arg_3)

</pre></div>

The Eiffel agent mechanism is detailed in the <a rel="nofollow" class="external text" href="http://www.ecma-international.org/publications/standards/Ecma-367.htm">Eiffel ISO/ECMA standard document</a>.

<a href="Java_(programming_language)" title="Java (programming language)">Java</a> has no <a href="First-class_function" title="First-class function">first-class functions</a>, so function objects are usually expressed by an interface with a single method (most commonly the <code>Callable</code> interface), typically with the implementation being an anonymous <a href="Inner_class" title="Inner class">inner class</a>, or, starting in Java 8, a <a href="Anonymous_function" title="Anonymous function">lambda</a>.

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.

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>List<String> list = Arrays.asList("10", "1", "20", "11", "21", "12");

Comparator<String> numStringComparator = new Comparator<String>() {

public int compare(String str1, String str2) {

return Integer.valueOf(str1).compareTo(Integer.valueOf(str2));

}

};

Collections.sort(list, numStringComparator);

</pre></div>

In Java 8+, this can be written as:

<div class="mw-highlight mw-highlight-lang-java mw-content-ltr" dir="ltr"><pre>List<String> list = Arrays.asList("10", "1", "20", "11", "21", "12");

Comparator<String> numStringComparator = (str1, str2) -> Integer.valueOf(str1).compareTo(Integer.valueOf(str2));

Collections.sort(list, numStringComparator);

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="In_JavaScript">In JavaScript</h2></div>

In <a href="JavaScript" title="JavaScript">JavaScript</a>, functions are first class objects. JavaScript also supports closures.

Compare the following with the subsequent Python example.

<div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function Accumulator(start) {

var current = start;

return function (x) {

return current += x;

};

}

</pre></div>

An example of this in use:

<div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>var a = Accumulator(4);

var x = a(5); // x has value 9

x = a(2); // x has value 11

var b = Accumulator(42);

x = b(7); // x has value 49 (current = 49 in closure b)

x = a(7); // x has value 18 (current = 18 in closure a)

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="In_Julia">In Julia</h2></div>

In <a href="Julia_(programming_language)" title="Julia (programming language)">Julia</a>, methods are associated with types, so it is possible to make any arbitrary Julia object "callable" by adding methods to its type. (Such "callable" objects are sometimes called "functors.")

An example is this accumulator mutable struct (based on <a href="Paul_Graham_(computer_programmer)" class="mw-redirect" title="Paul Graham (computer programmer)">Paul Graham's</a> study on programming language syntax and clarity):<a href="#cite_note-6">[5]</a>

<div class="mw-highlight mw-highlight-lang-julia-repl mw-content-ltr" dir="ltr"><pre>julia> mutable struct Accumulator

n::Int

end

julia> function (acc::Accumulator)(n2)

acc.n += n2

end

julia> a = Accumulator(4)

Accumulator(4)

julia> a(5)

9

julia> a(2)

11

julia> b = Accumulator(42)

Accumulator(42)

julia> b(7)

49

</pre></div>

Such an accumulator can also be implemented using closure:

<div class="mw-highlight mw-highlight-lang-julia-repl mw-content-ltr" dir="ltr"><pre>julia> function Accumulator(n0)

n = n0

function(n2)

n += n2

end

Accumulator (generic function with 1 method)

julia> a = Accumulator(4)

(::#1) (generic function with 1 method)

julia> a(5)

9

julia> a(2)

11

julia> b = Accumulator(42)

(::#1) (generic function with 1 method)

julia> b(7)

49

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="In_Lisp_and_Scheme">In Lisp and Scheme</h2></div>

In Lisp family languages such as <a href="Common_Lisp" title="Common Lisp">Common Lisp</a>, <a href="Scheme_(programming_language)" title="Scheme (programming language)">Scheme</a>, and others, functions are objects, just like strings, vectors, lists, and numbers. A closure-constructing operator creates a function object from a part of the program: the part of code given as an argument to the operator is part of the function, and so is the lexical environment: the bindings of the lexically visible variables are captured and stored in the function object, which is more commonly called a <a href="Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closure</a>. The captured bindings play the role of member variables, and the code part of the closure plays the role of the anonymous member function, just like operator () in C++.

The closure constructor has the syntax <code>(lambda (parameters ...) code ...)</code>. The <code>(parameters ...)</code> part allows an interface to be declared, so that the function takes the declared parameters. The <code>code ...</code> part consists of expressions that are evaluated when the functor is called.

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:

<div class="mw-highlight mw-highlight-lang-lisp mw-content-ltr" dir="ltr"><pre>(defclass counter ()

((value :initarg :value :accessor value-of)))

(defmethod functor-call ((c counter))

(incf (value-of c)))

(defun make-counter (initial-value)

(make-instance 'counter :value initial-value))

;;; use the counter:

(defvar *c* (make-counter 10))

(functor-call *c*) --> 11

(functor-call *c*) --> 12

</pre></div>

Since there is no standard way to make funcallable objects in Common Lisp, we fake it by defining a <a href="Generic_function" title="Generic function">generic function</a> 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.

Now, a counter implemented using a closure. This is much more brief and direct. The INITIAL-VALUE argument of the MAKE-COUNTER <a href="Factory_function" class="mw-redirect" title="Factory function">factory function</a> 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.

<div class="mw-highlight mw-highlight-lang-lisp mw-content-ltr" dir="ltr"><pre>(defun make-counter (value)

(lambda () (incf value)))

;;; use the counter

(defvar *c* (make-counter 10))

(funcall *c*) ; --> 11

(funcall *c*) ; --> 12

</pre></div>

Scheme makes closures even simpler, and Scheme code tends to use such <a href="Higher-order_programming" title="Higher-order programming">higher-order programming</a> somewhat more idiomatically.

<div class="mw-highlight mw-highlight-lang-scheme mw-content-ltr" dir="ltr"><pre>(define (make-counter value)

(lambda () (set! value (+ value 1)) value))

;;; use the counter

(define c (make-counter 10))

(c) ; --> 11

(c) ; --> 12

</pre></div>

More than one closure can be created in the same lexical environment. A vector of closures, each implementing a specific kind of operation, can quite faithfully emulate an object that has a set of virtual operations. That type of <a href="Single_dispatch" class="mw-redirect" title="Single dispatch">single dispatch</a> object-oriented programming can be done fully with closures.

Thus there exists a kind of tunnel being dug from both sides of the proverbial mountain. Programmers in OOP languages discover function objects by restricting objects to have one main function to do that object's functional purpose, and even eliminate its name so that it looks like the object is being called! While programmers who use closures are not surprised that an object is called like a function, they discover that multiple closures sharing the same environment can provide a complete set of abstract operations like a virtual table for <a href="Single_dispatch" class="mw-redirect" title="Single dispatch">single dispatch</a> type OOP.

<div class="mw-heading mw-heading2"><h2 id="In_Objective-C">In Objective-C</h2></div>

In <a href="Objective-C" title="Objective-C">Objective-C</a>, 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>:

<div class="mw-highlight mw-highlight-lang-objc mw-content-ltr" dir="ltr"><pre>// Construct a function object

SEL sel = @selector(myMethod);

NSInvocation* inv = [NSInvocation invocationWithMethodSignature:

[self methodSignatureForSelector:sel]];

[inv setTarget:self];

[inv setSelector:sel];

// Do the actual invocation

[inv invoke];

</pre></div>

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 <a rel="nofollow" class="external text" href="https://web.archive.org/web/20110227215311/http://www.a-coding.com/2010/10/making-nsinvocations.html">here</a>.

In <a href="Perl" title="Perl">Perl</a>, 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:

<div class="mw-highlight mw-highlight-lang-perl mw-content-ltr" dir="ltr"><pre>package Acc1;

sub new {

my $class = shift;

my $arg = shift;

my $obj = sub {

my $num = shift;

$arg += $num;

};

bless $obj, $class;

}

1;

</pre></div>

or by overloading the <code>&{}</code> operator so that the object can be used as a function:

<div class="mw-highlight mw-highlight-lang-perl mw-content-ltr" dir="ltr"><pre>package Acc2;

use overload

'&{}' =>

sub {

my $self = shift;

sub {

my $num = shift;

$self->{arg} += $num;

}

};

sub new {

my $class = shift;

my $arg = shift;

my $obj = { arg => $arg };

bless $obj, $class;

}

1;

</pre></div>

In both cases the function object can be used either using the dereferencing arrow syntax $ref->(@arguments):

<div class="mw-highlight mw-highlight-lang-perl mw-content-ltr" dir="ltr"><pre>use Acc1;

my $a = Acc1->new(42);

print $a->(10), "\n"; # prints 52

print $a->(8), "\n"; # prints 60

</pre></div>

or using the coderef dereferencing syntax &$ref(@arguments):

<div class="mw-highlight mw-highlight-lang-perl mw-content-ltr" dir="ltr"><pre>use Acc2;

my $a = Acc2->new(12);

print &$a(10), "\n"; # prints 22

print &$a(8), "\n"; # prints 30

</pre></div>

<a href="PHP" title="PHP">PHP</a> 5.3+ has <a href="First-class_function" title="First-class function">first-class functions</a> that can be used e.g. as parameter to the <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">usort()</code> function:

<div class="mw-highlight mw-highlight-lang-php mw-content-ltr" dir="ltr"><pre>$a = array(3, 1, 4);

usort($a, function ($x, $y) { return $x - $y; });

</pre></div>

<a href="PHP" title="PHP">PHP</a> 5.3+, supports also lambda functions and closures.

<div class="mw-highlight mw-highlight-lang-php mw-content-ltr" dir="ltr"><pre>function Accumulator($start)

{

$current = $start;

return function($x) use(&$current)

{

return $current += $x;

};

}

</pre></div>

An example of this in use:

<div class="mw-highlight mw-highlight-lang-php mw-content-ltr" dir="ltr"><pre>$a = Accumulator(4);

$x = $a(5);

echo "x = $x "; // x = 9

$x = $a(2);

echo "x = $x "; // x = 11

</pre></div>

It is also possible in PHP 5.3+ to make objects invokable by adding a magic <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">__invoke()</code> method to their class:<a href="#cite_note-phpinvoke-7">[6]</a>

<div class="mw-highlight mw-highlight-lang-php mw-content-ltr" dir="ltr"><pre>class Minus

{

public function __invoke($x, $y)

{

return $x - $y;

}

$a = array(3, 1, 4);

usort($a, new Minus());

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="In_PowerShell">In PowerShell</h2></div>

In the <a href="Windows_PowerShell" class="mw-redirect" title="Windows PowerShell">Windows PowerShell</a> 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 <a href=".NET_Framework" title=".NET Framework">.NET Framework</a> type System.Management.Automation.ScriptBlock.

<div class="mw-highlight mw-highlight-lang-powershell mw-content-ltr" dir="ltr"><pre>Function Get-Accumulator($x) {

{

param($y)

return $x += $y

}.GetNewClosure()

}

</pre></div>

<div class="mw-highlight mw-highlight-lang-ps1con mw-content-ltr" dir="ltr"><pre>PS C:\> $a = Get-Accumulator 4

PS C:\> & $a 5

9

PS C:\> & $a 2

11

PS C:\> $b = Get-Accumulator 32

PS C:\> & $b 10

42

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="In_Python">In Python</h2></div>

In <a href="Python_(programming_language)" title="Python (programming language)">Python</a>, functions are first-class objects, just like strings, numbers, lists etc. This feature eliminates the need to write a function object in many cases. Any object with a <code>__call__()</code> method can be called using function-call syntax.

An example is this accumulator class (based on <a href="Paul_Graham_(computer_programmer)" class="mw-redirect" title="Paul Graham (computer programmer)">Paul Graham's</a> study on programming language syntax and clarity):<a href="#cite_note-8">[7]</a>

<div class="mw-highlight mw-highlight-lang-python mw-content-ltr" dir="ltr"><pre>class Accumulator:

def __init__(self, n) -> None:

self.n = n

def __call__(self, x):

self.n += x

return self.n

</pre></div>

An example of this in use (using the interactive interpreter):

<div class="mw-highlight mw-highlight-lang-pycon mw-content-ltr" dir="ltr"><pre>>>> a = Accumulator(4)

>>> a(5)

9

>>> a(2)

11

>>> b = Accumulator(42)

>>> b(7)

49

</pre></div>

Since functions are objects, they can also be defined locally, given attributes, and returned by other functions,<a href="#cite_note-9">[8]</a> as demonstrated in the following example:

<div class="mw-highlight mw-highlight-lang-python3 mw-content-ltr" dir="ltr"><pre>def Accumulator(n):

def inc(x):

nonlocal n

n += x

return n

return inc

</pre></div>

In <a href="Ruby_(programming_language)" title="Ruby (programming language)">Ruby</a>, 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 <a href="Duck_typing" title="Duck typing">assuming that method exists</a>—the Ruby Extensions Project <a rel="nofollow" class="external text" href="https://web.archive.org/web/20060425104650/http://blogs.pragprog.com/cgi-bin/pragdave.cgi/Tech/Ruby/ToProc.rdoc">created a simple hack.</a>

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>class Symbol

def to_proc

proc { |obj, *args| obj.send(self, *args) }

end

end

</pre></div>

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. <a rel="nofollow" class="external autonumber" href="https://web.archive.org/web/20060820025032/http://redhanded.hobix.com/cult/symbolTo_procExonerated.html">[1]</a>

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 <a href="Delegation_(programming)" class="mw-redirect" title="Delegation (programming)">delegation</a> introduced by the <a rel="nofollow" class="external text" href="https://web.archive.org/web/20070107205748/http://facets.rubyforge.org/">Ruby Facets</a> project is named as Functor. The most basic definition of which is:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>class Functor

def initialize(&func)

@func = func

end

def method_missing(op, *args, &blk)

@func.call(op, *args, &blk)

end

end

</pre></div>

This usage is more akin to that used by functional programming languages, like <a href="ML_(programming_language)" title="ML (programming language)">ML</a>, and the original mathematical terminology.

<div class="mw-heading mw-heading2"><h2 id="Other_meanings">Other meanings</h2></div>

In a more theoretical context a function object may be considered to be any instance of the class of functions, especially in languages such as <a href="Common_Lisp" title="Common Lisp">Common Lisp</a> in which functions are <a href="First-class_object" class="mw-redirect" title="First-class object">first-class objects</a>.

The <a href="ML_(programming_language)" title="ML (programming language)">ML</a> family of <a href="Functional_programming" title="Functional programming">functional programming</a> languages uses the term functor to represent a <a href="Function_(mathematics)" title="Function (mathematics)">mapping</a> 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 <a href="Functor" title="Functor">functor</a> in <a href="Category_theory" title="Category theory">category theory</a>, or to the use of generic programming in C++, Java or <a href="Ada_(programming_language)" title="Ada (programming language)">Ada</a>.

In <a href="Haskell_(programming_language)" class="mw-redirect" title="Haskell (programming language)">Haskell</a>, the term <a href="Functor_(functional_programming)" title="Functor (functional programming)">functor</a> is also used for a concept related to the meaning of functor in category theory.

In <a href="Prolog" title="Prolog">Prolog</a> and related languages, functor is a synonym for <a href="Function_symbol" class="mw-redirect" title="Function symbol">function symbol</a>.

<ul><li><a href="Callback_(computer_science)" class="mw-redirect" title="Callback (computer science)">Callback (computer science)</a></li>

<li><a href="Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">Closure (computer science)</a></li>

<li><a href="Function_pointer" title="Function pointer">Function pointer</a></li>

<li><a href="Higher-order_function" title="Higher-order function">Higher-order function</a></li>

<li><a href="Command_pattern" title="Command pattern">Command pattern</a></li>

<li><a href="Currying" title="Currying">Currying</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="Notes">Notes</h2></div>

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</style><div class="reflist reflist-lower-alpha">

<li id="cite_note-2"><a href="#cite_ref-2">^</a> In C++, a functionoid is an object that has one major method, and a functor is a special case of a functionoid.<a href="#cite_note-1">[1]</a> They are similar to a function object, but not the same.

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

<li id="cite_note-1"><a href="#cite_ref-1">^</a> <a rel="nofollow" class="external text" href="https://isocpp.org/wiki/faq/pointers-to-members#functor-vs-functionoid">What's the difference between a functionoid and a functor?</a>

</li>

/* start https://en.wikipedia.org/ */

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/* end https://en.wikipedia.org/ */

</style><cite id="CITEREFSilan_Liu" class="citation web cs1">Silan Liu. <a rel="nofollow" class="external text" href="http://progtutorials.tripod.com/cpp1.htm#_Toc50820124">"C++ Tutorial Part I - Basic: 5.10 Function pointers are mainly used to achieve call back technique, which will be discussed right after"</a>. TRIPOD: Programming Tutorials Copyright © Silan Liu 2002. Retrieved 2012-09-07. <q>Function pointers are mainly used to achieve call back technique, which will be discussed right after.</q></cite>

</li>

<li id="cite_note-4"><a href="#cite_ref-4">^</a> <cite id="CITEREFPaweł_Turlejski2009" class="citation web cs1">Paweł Turlejski (2009-10-02). <a rel="nofollow" class="external text" href="http://progtutorials.tripod.com/cpp1.htm#_Toc50820124">"C++ Tutorial Part I - Basic: 5.10 Function pointers are mainly used to achieve call back technique, which will be discussed right after"</a>. Just a Few Lines. Retrieved 2012-09-07. <q>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.</q></cite>

</li>

<li id="cite_note-5"><a href="#cite_ref-5">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://en.cppreference.com/w/cpp/language/overload_resolution#Call_to_a_class_object">"Overload resolution§Call to a class object"</a>. cppreference.com.</cite>

</li>

<li id="cite_note-6"><a href="#cite_ref-6">^</a> <a rel="nofollow" class="external text" href="http://www.paulgraham.com/accgen.html">Accumulator Generator</a>

</li>

<li id="cite_note-phpinvoke-7"><a href="#cite_ref-phpinvoke_7-0">^</a> <a rel="nofollow" class="external text" href="http://php.net/manual/en/language.oop5.magic.php#object.invoke">PHP Documentation on Magic Methods</a>

</li>

<li id="cite_note-8"><a href="#cite_ref-8">^</a> <a rel="nofollow" class="external text" href="http://www.paulgraham.com/accgen.html">Accumulator Generator</a>

</li>

<li id="cite_note-9"><a href="#cite_ref-9">^</a> <a rel="nofollow" class="external text" href="https://docs.python.org/3/reference/compound_stmts.html#function-definitions">Python reference manual - Function definitions</a>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="Further_reading">Further reading</h2></div>

<ul><li>David Vandevoorde & Nicolai M Josuttis (2006). C++ Templates: The Complete Guide, <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-201-73484-2</bdi>: Specifically, chapter 22 is devoted to function objects.</li></ul>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="http://c2.com/cgi/wiki?FunctorObject">Description from the Portland Pattern Repository</a></li>

<li><a rel="nofollow" class="external text" href="http://www.two-sdg.demon.co.uk/curbralan/papers/AsynchronousC++.pdf">C++ Advanced Design Issues - Asynchronous C++</a> <a rel="nofollow" class="external text" href="https://web.archive.org/web/20200922012516/http://www.two-sdg.demon.co.uk/curbralan/papers/AsynchronousC++.pdf">Archived</a> 2020-09-22 at the <a href="Wayback_Machine" title="Wayback Machine">Wayback Machine</a> by <a href="Kevlin_Henney" title="Kevlin Henney">Kevlin Henney</a></li>

<li><a rel="nofollow" class="external text" href="http://www.newty.de/fpt/index.html">The Function Pointer Tutorials</a> by Lars Haendel (2000/2001)</li>

<li>Article "<a rel="nofollow" class="external text" href="https://web.archive.org/web/20041009232434/http://www.cuj.com/documents/s%3D8464/cujcexp0308sutter/">Generalized Function Pointers</a>" by <a href="Herb_Sutter" title="Herb Sutter">Herb Sutter</a></li>

<li><a rel="nofollow" class="external text" href="https://jga.sourceforge.net/">Generic Algorithms for Java</a></li>

<li><a rel="nofollow" class="external text" href="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</a></li>

<li><a rel="nofollow" class="external text" href="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?</a> (C++ FAQ)</li></ul></div><div><div class="zim-footer">

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