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
struct Foo
;

int main()

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) );

It is also possible in PHP 5.3+ to make objects invokable by adding a magic __invoke() method to their class:PHP Documentation on Magic Methods
/ref>

class Minus

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

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

main()

In Swift

In Swift, callable objects are defined using callAsFunction.

struct CallableStruct
let callable = CallableStruct(value: 100)
callable(4, scale: 2)
callable.callAsFunction(4, scale: 2)
// Both function calls print 208.

References

External links

C++ Callable concept

Articles with example C++ code
Articles with example C Sharp code
Articles with example PHP code
Articles with example Python (programming language) code
Articles with example Swift code
Object (computer science)
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