msdn里的C++unary_function的例子

// unary_function.cpp
// compile with: /EHsc
//
// Structure used:
//       unary_function<int, float> - allows us
//       to write operator functions accepting an
//       integer and returning floats.
////////////////////////////////////////////

#include <functional>
#include <iostream>

using namespace std ;

/* derive class from unary_function in order to use it */

class unary_test : public unary_function<int,float>
{
public:
 float value;
 unary_test(){value=10.0;}
 unary_test(float x){value=x;}
 result_type operator*(argument_type x);
 result_type operator-(argument_type x);
};

/* You can now easily create operators that accept */
/* an int and return a float.                     */

unary_test::result_type unary_test::operator*(unary_test::argument_type x)
{
 float tmp = value * (float)x;
 cout << "Value after * is " << tmp << endl ;
 return value;
}

unary_test::result_type unary_test::operator-(unary_test::argument_type x)
{
 float tmp = value - (float)x;
 cout << "Value after minus is " << tmp << endl ;
 return tmp;
}

int main(void)
{
 unary_test item;
 unary_test item2(18.0);

 cout << "Begin" << endl ;
 cout.setf(ios::fixed) ;
 item = item * 2;
 item2 = item2 - 5;
 return 0;
}
/*
Output
 Begin
 Value after * is 20.000000
 Value after minus is 13.000000
*/