Callable object
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A callable object, in computer programming, is any object that can be called like a function.
Contents
• In C++
• In C#
• In PHP
• In Dart
• In Swift
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In different languages
In C++
• pointer to member function;
• functor;
• std::function is a template class that can hold any callable object that matches its signature.
In C++, any class that overloads the function call operator operator() may be called using function-call syntax.
#include <iostream>
struct Foo
{
void operator()() const
{
std::cout << "Called.";
}
};
int main()
{
Foo foo_instance;
foo_instance(); // This will output "Called." to the screen.
}
In C#
In PHP
$a = array(3, 1, 4);
usort($a, function ($x, $y) { return $x - $y; });
It is also possible in PHP 5.3+ to make objects invokable by adding a magic __invoke() method to their class:cite-ref-phpinvoke-1-0[1]
class Minus
{
public function __invoke($x, $y) { return $x - $y; }
}
$a = array(3, 1, 4);
usort($a, new Minus());
In Python
In Python any object with a __call__() method can be called using function-call syntax.
class Foo:
def __call__(self):
print("Called.")
foo_instance = Foo()
foo_instance() # This will output "Called." to the screen.
cite-ref-2[2]
Another example:
class Accumulator:
def __init__(self, n):
self.n = n
def __call__(self, x):
self.n += x
return self.n
In Dart
Callable objects are defined in Dart using the call() method.
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');
}
cite-ref-3[3]
In 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.
References
cite-note-phpinvoke-11. ↑ PHP Documentation on Magic Methods
cite-note-22. ↑ citerefb-schBösch, Florian. "What is a "callable" in Python?". StackOverflow.com. Retrieved 24 September 2017.
cite-note-33. ↑ "A Tour of the Dart Language". www.dartlang.org. Retrieved 2019-03-25.
cite-note-44. ↑ "Declarations — The Swift Programming Language (Swift 5.6)". docs.swift.org. Retrieved 2022-02-28.
External links