Functional (C++)

In the context of the programming language C++, functional 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 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.

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