Callable object

In C++

std::function is a template class that can hold any callable object that matches its signature.

In C#

In PHP

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

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

In Python

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