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.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.

import std;
struct Foo ;
int main

In C#

PHP 5.3+ has first-class functions that can be used e.g. as parameter to the usort function:

$a = array;
usort;

It is also possible in PHP 5.3+ to make objects invokable by adding a magic __invoke method to their class:

class Minus
$a = array;
usort);

In Python

In Python any object with a __call__ method can be called using function-call syntax.

class Foo:
def __call__ -> None:
print
instance: Foo = Foo
instance # This will output "Called." to the screen.

Another example:

class Accumulator:
def __init__ -> None:
self.n = n
def __call__ -> int:
self.n += x
return self.n

In Dart

Callable objects are defined in Dart using the call method.
class WannabeFunction
main

In Swift

In Swift, callable objects are defined using callAsFunction.

struct CallableStruct
let callable = CallableStruct
callable
callable.callAsFunction
// Both function calls print 208.