*Description*

*Description*

The basic function object concepts are Generator, Unary Function, and Binary Function: these describe, respectively, objects that can be called as *f()*, *f(x)*, and *f(x,y)*. (This list could obviously be extended to *ternary function *and beyond, but, in practice, no STL algorithms require function objects of more than two arguments.) All other function object concepts defined by the STL are refinements of these three.

Function objects that return *bool* 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.

There is an important distinction, but a somewhat subtle one, between function objects and *adaptable function objects*. [1] 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, or both. 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 *typedef*s so that those types can be named and used in programs. If a type *F0* is a model of Adaptable Generator, then it must define *F0::result_type*. Similarly, if *F1* is a model of Adaptable Unary Function then it must define *F1::argument_type* and *F1::result_type*, and if *F2* is a model of Adaptable Binary Function then it must define *F2::first_argument_type*, *F2::second_argument_type*, and *F2::result_type*. The STL provides base classes *unary_function* and *binary_function* to simplify the definition of Adaptable Unary Functions and Adaptable Binary Functions. [2]

Adaptable function objects are important because they can be used by *function object adaptors*: function objects that transform or manipulate other function objects. The STL provides many different function object adaptors, including *unary_negate* (which returns the logical complement of the value returned by a particular AdaptablePredicate), and *unary_compose* and *binary_compose*, which perform composition of function object.

Finally, the STL includes many different predefined function objects, including arithmetic operations (*plus*, *minus*, *multiplies*, *divides*, *modulus*, and *negate*), comparisons (*equal_to*, *not_equal_to, greater*, *less*, *greater_equal*, and *less_equal*), and logical operations (*logical_and*, *logical_or*, and *logical_not*). 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.