std::unary_function

C++98

Unary function object base class

Note: This class has been deprecated in C++11. 
This is a base class for standard unary function objects.

Generically, function objects are instances of a class with member function operator() defined. This member function allows the object to be used with the same syntax as a regular function call, and therefore its type can be used as template parameter when a generic function type is expected.

In the case of unary function objects, this operator() member function takes a single parameter.

unary_function is just a base class, from which specific unary function objects are derived. It has no operator() member defined (which derived classes are expected to define) - it simply has two public data members that are typedefs of the template parameters. It is defined as:

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template <class Arg, class Result>  struct unary_function {    typedef Arg argument_type;    typedef Result result_type;  };

Member types

member typedefinitionnotesargument_typeThe first template parameter (Arg)Type of the argument in member operator()result_typeThe second template parameter (Result)Type returned by member operator()

Example

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// unary_function example#include <iostream>     // std::cout, std::cin#include <functional>   // std::unary_functionstruct IsOdd : public std::unary_function<int,bool> {  bool operator() (int number) {return (number%2!=0);}};int main () {  IsOdd IsOdd_object;  IsOdd::argument_type input;  IsOdd::result_type result;  std::cout << "Please enter a number: ";  std::cin >> input;  result = IsOdd_object (input);  std::cout << "Number " << input << " is " << (result?"odd":"even") << ".\n";  return 0;}

Edit & Run

Possible output:

Please enter a number: 2Number 2 is even.