Functional (C++)

In the context of the programming language C++,   refers to a header file that is part of the C++ Standard Library and provides a set of predefined class templates for function objects, including operations for arithmetic, comparisons, and logic. Instances of these class templates are C++ classes that define a function call operator, and the instances of these classes can be called as if they were functions. It is possible to perform very sophisticated operations without writing a new function object, simply by combining predefined function objects and function object adaptors.

The class template  provided by C++11 is a general-purpose polymorphic function wrapper. Instances of  can store, copy, and invoke any callable target—functions, lambda expressions (expressions defining anonymous functions), bind expressions (instances of function adapters that transform functions to other functions of smaller arity by providing values for some of the arguments), or other function objects.

The algorithms provided by the C++ Standard Library do not require function objects of more than two arguments. Function objects that return Boolean values are an important special case. A unary function whose return type is is called a predicate, and a binary function whose return type is  is called a binary predicate.

Adaptable function objects
In general, a function object has restrictions on the type of its argument. The type restrictions need not be simple, though: may be overloaded or may be a member template. Similarly, there need be no way for a program to determine what those restrictions are. An adaptable function object, however, does specify what the argument and return types are, and provides nested typedefs so that those types can be named and used in programs. If a type is a model of an adaptable generator, then it must define. Similarly, if is a model of the adaptable unary function, it must define  and, and if  is a model of the adaptable binary function, it must define , , and. The C++ Standard Library provides base classes and  to simplify the definition of adaptable unary functions and adaptable binary functions.

Adaptable function objects are important, because they can be used by function object adaptors: function objects that transform or manipulate other function objects. The C++ Standard Library provides many different function object adaptors, including (that returns the logical complement of the value returned by a particular adaptable predicate), and  and, which perform composition of function object.

Predefined function objects
The C++ Standard Library includes in the header file  many different predefined function objects, including arithmetic operations (,, , , , and ), comparisons (, , , , , and ), and logical operations (, , and ).

Examples
Function wrappers can be used to make calls to ordinary functions or to functions objects created by lambda expressions.

Function wrappers also can be used to access member variables and member functions of classes.