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.

#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#[]

delegate;
lambda expression.

In PHP[]

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

$a = array(3, 1, 4);
usort($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:^[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.

^[2]

Another example:

class Accumulator:
    def __init__(self, n):
        self.n = n

    def __call__(self, x):
        self.n += x
        return self.n

In Dart[]

To allow your Dart class to be called like a function, implement 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');
}

^[3]

References[]

^ PHP Documentation on Magic Methods
^ Bösch, Florian. "What is a "callable" in Python?". StackOverflow.com. Retrieved 24 September 2017.
^ "A Tour of the Dart Language". www.dartlang.org. Retrieved 2019-03-25.

External links[]

C++ Callable concept

This computing article is a stub. You can help Wikipedia by .

[phpinvoke-1] PHP Documentation on Magic Methods

[2] Bösch, Florian. "What is a "callable" in Python?". StackOverflow.com. Retrieved 24 September 2017.

[3] "A Tour of the Dart Language". www.dartlang.org. Retrieved 2019-03-25.

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