Callable object

A '''callable object''', in computer programming, is any object that can be called like a function.

==In different languages ==

=== In C++ === * pointer to function; * pointer to member function; * functor; * lambda expression. * <code>std::function</code> is a template class that can hold any callable object that matches its signature.

In C++, any class that overloads the function call operator <code>operator()</code> may be called using function-call syntax. <syntaxhighlight lang="cpp"> import std;

struct Foo { void operator()() const { std::println("Called."); } };

int main() { Foo instance; instance(); // This will output "Called." to the screen. } </syntaxhighlight>

=== In C# === * delegate; * lambda expression.

=== In PHP ===

PHP 5.3+ has first-class functions that can be used e.g. as parameter to the <code>usort()</code> function:

<syntaxhighlight lang="php"> $a = array(3, 1, 4); usort($a, function ($x, $y) { return $x - $y; }); </syntaxhighlight>

It is also possible in PHP 5.3+ to make objects invokable by adding a magic <code>__invoke()</code> 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"> class Minus { public function __invoke($x, $y) { return $x - $y; } }

$a = array(3, 1, 4); usort($a, new Minus()); </syntaxhighlight>

=== In Python === In Python any object with a <code>__call__()</code> method can be called using function-call syntax.

<syntaxhighlight lang="python"> class Foo: def __call__(self) -> None: print("Called.")

instance: Foo = Foo() instance() # This will output "Called." to the screen. </syntaxhighlight><ref>{{cite web|last1=Bösch|first1=Florian|title=What is a "callable" in Python?|url=https://stackoverflow.com/a/111255/5513567|website=StackOverflow.com|access-date=24 September 2017}}</ref>

Another example: <syntaxhighlight lang="python"> class Accumulator: def __init__(self, n: int) -> None: self.n = n

def __call__(self, x: int) -> int: self.n += x return self.n </syntaxhighlight>

=== In Dart === Callable objects are defined in Dart using the <code>call()</code> method.<syntaxhighlight lang="dart"> class WannabeFunction { call(String a, String b, String c) => '$a $b $c!'; }

main() { var wf = new WannabeFunction(); var out = wf("Hi","there,","gang"); print('$out'); } </syntaxhighlight><ref>{{Cite web|url=https://www.dartlang.org/guides/language/language-tour|title=A Tour of the Dart Language|website=www.dartlang.org|access-date=2019-03-25}}</ref>

=== In Swift === In Swift, callable objects are defined using <code>callAsFunction</code>.<ref>{{Cite web |title=Declarations — The Swift Programming Language (Swift 5.6) |url=https://docs.swift.org/swift-book/ReferenceManual/Declarations.html |access-date=2022-02-28 |website=docs.swift.org}}</ref> <syntaxhighlight lang="swift"> struct CallableStruct { var value: Int func callAsFunction(_ number: Int, scale: Int) { print(scale * (number + value)) } } let callable = CallableStruct(value: 100) callable(4, scale: 2) callable.callAsFunction(4, scale: 2) // Both function calls print 208. </syntaxhighlight>

==References== {{Reflist}}

== External links == * [https://web.archive.org/web/20151115042817/http://en.cppreference.com/w/cpp/concept/Callable C++ Callable concept]

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