|C++ Standard Library|
|Standard Template Library|
|C standard library|
||This article may require cleanup to meet Wikipedia's quality standards. (December 2010)|
In the context of the C++ programming language,
functional refers to a header file that is part of the C++ Standard Library and provides a number of predefined class templates for function objects, including arithmetic operations, comparisons, and logical operations. 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 actually writing a new function object, simply by combining predefined function objects and function object adaptors.
The class template
std::function provided by C++11 is a general-purpose polymorphic function wrapper. Instances of
std::function 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
bool is called a predicate, and a binary function whose return type is
bool 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:
operator() 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
F0 is a model of an adaptable generator, then it must define
F0::result_type. Similarly, if
F1 is a model of the adaptable unary function, it must define
F1::result_type, and if
F2 is a model of the adaptable binary function, it must define
F2::result_type. The C++ Standard Library provides base classes
binary_function 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
unary_negate (that returns the logical complement of the value returned by a particular adaptable predicate), and
binary_compose, which perform composition of function object.
Predefined function objects
The C++ Standard Library includes in the header file
functional many different predefined function objects, including arithmetic operations (
negate), comparisons (
less_equal), and logical operations (