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unary_function Struct

An empty base struct that defines types that may be inherited by derived classes that provides a unary function object.

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

The template struct serves as a base for classes that define a member function of the form result_type operator()(const argument_type&) const.

All such derived unary functions can refer to their sole argument type as argument_type and their return type as result_type.

// functional_unary_function.cpp
// compile with: /EHsc
#include <vector>
#include <functional>
#include <algorithm>
#include <iostream>

using namespace std;

// Creation of a user-defined function object
// that inherits from the unary_function base class
class greaterthan10: unary_function<int, bool>
{
public:
    result_type operator()(argument_type i)
    {
        return (result_type)(i > 10);
    }
};

int main()
{
    vector<int> v1;
    vector<int>::iterator Iter;

    int i;
    for (i = 0; i <= 5; i++)
    {
        v1.push_back(5 * i);
    }

    cout << "The vector v1 = ( " ;
    for (Iter = v1.begin(); Iter != v1.end(); Iter++)
        cout << *Iter << " ";
    cout << ")" << endl;

    vector<int>::iterator::difference_type result1;
    result1 = count_if(v1.begin(), v1.end(), greaterthan10());
    cout << "The number of elements in v1 greater than 10 is: "
         << result1 << "." << endl;
}
The vector v1 = ( 0 5 10 15 20 25 ) The number of elements in v1 greater than 10 is: 3.

Header: <functional>

Namespace: std

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