unary_function<> Structure (STL Sample)

The sample code below illustrates how to use the unary_function<> structure in Visual C++.

Required Header:
< functional >

template<class _A, class _R>
     struct unary_function
     {
         typedef _A argument_type;
         typedef _R result_type;
     };

Note: The class/parameter names in the prototype do not match the version in the header file. Some have been modified to improve readability.

Description:
unary_function is used as a base class to better define operator functions in the following format: result_type classname::operatorX(argument_type).

Sample Code:

////////////////////////////////////////////
//
// Compile options needed: none
//
// 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)
{
  value*=x;
  cout << "New value after * is " << value << endl ;
  return value;
}

unary_test::result_type unary_test::operator-(unary_test::argument_type x)
{
  value-+(float)x;
  cout << "New value after minus is " << value << endl ;

  return value;
}

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

  cout << "Begin" << endl ;
  cout.setf(ios::fixed) ;
  item = item * 2;
  item2 = item2 - 5;
}

 

Program Output is:

Begin
New value after * is 20.000000
New value after minus is 18.000000