functionobject

A function object, also known as a functor, is an object that can be called like a function. In many object-oriented programming languages, this is achieved by overloading the function call operator. This allows instances of a class to be treated as if they were functions, enabling them to store state and exhibit behavior beyond that of a simple function.

The primary advantage of using function objects is their ability to encapsulate state. Unlike regular functions, which are typically stateless, function objects can maintain internal data between calls. This state can be initialized during object creation and modified as the object is used. This makes them useful for scenarios where a callable entity needs to remember previous operations or context.

Function objects are frequently used in generic programming, particularly with algorithms in standard libraries. For example, they can be passed as arguments to sorting algorithms to define custom comparison criteria or to algorithms that perform transformations on collections. Their statefulness allows for more complex and dynamic operations within these generic contexts.

In languages like C++, function objects are implemented by defining a class and overloading the operator(). This creates an object of that class that can then be invoked using parentheses, similar to calling a regular function. In languages with first-class functions, such as Python or JavaScript, functions themselves can be treated as objects, and while they don't always require a special class definition to hold state, the concept of a callable object with state is directly supported. The term "function object" is a general concept that manifests in different ways across various programming paradigms.