A functor, also known as a function object, is an object type that can be called as if it were a function. A functor acts as an adapter, providing a common interface for invoking a callable object without regard to how that callable object is implemented. The Functor package provides a convenient, flexible, and powerful way to encapsulate callable types, including functions, lambda expressions, and other callable types.

Along with its support for functors, the Functor package also provides a powerful API to support parameter binding. Parameter binding allows for the creation of a binder object that encapsulates both a callable object as well as some or all of the parameters required to invoke that object.

The functor's interface can be independent of the encapsulated callable object. Because the information for invoking the callable type is encapsulated in an object, the invocation can be deferred or delegated. Software components can pass a functor as a parameter or return value, like any other object.

Functors make it straightforward to write a tool that can accept and carry out any arbitrary piece of work. Instead of trying to handle a variety of unknown function signatures, such a tool takes one type of functor whose arguments and return value fit the tool's own requirements. Each functor handles the details of adapting its encapsulated function to those requirements.

The Functor package provides the following features: