Negators

Function Object

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Function adaptors and function objects used to reverse the sense of predicate function objects.

None

#include <functional>
template <class Predicate>
class unary_negate;

template <class Predicate>
unary_negate<Predicate> not1(const Predicate&);

template <class Predicate>
class binary_negate;

template <class Predicate>
binary_negate<Predicate> not2(const Predicate&);

Negators not1 and not2 are functions that take predicate function objects as arguments and return predicate function objects with the opposite sense. Negators work only with function objects defined as subclasses of the classes unary_function and binary_function. not1 accepts and returns unary predicate function objects. not2 accepts and returns binary predicate function objects.

unary_negate and binary_negate are function object classes that include return types for the negators, not1 and not2.

template <class Predicate>
class unary_negate
  : public unary_function
           <typename Predicate::argument_type, bool> {

public:
  explicit unary_negate (const Predicate&);
  bool operator() (const argument_type&) const;
};

template<class Predicate>
unary_negate <Predicate> not1 (const Predicate&);

template<class Predicate>
class binary_negate
  : public binary_function
           <typename Predicate::first_argument_type,
            typename Predicate::second_argument_type, bool>
{
public:
  explicit binary_negate (const Predicate&);
  bool operator() (const first_argument_type&,
                   const second_argument_type&) const;
};

template <class Predicate>
binary_negate<Predicate> not2 (const Predicate&);

//
// negator.cpp
//
 #include<functional>
 #include<algorithm>
 #include <iostream>
 using namespace std;

 //Create a new predicate from unary_function
 template<class Arg>
 class is_odd : public unary_function<Arg, bool>
 {
   public:
   bool operator()(const Arg& arg1) const
   {
     return (arg1 % 2 ? true : false);
   }
 };

 int main()
 {
   less<int> less_func;

   // Use not2 on less
   cout << (less_func(1,4) ? "TRUE" : "FALSE") << endl;
   cout << (less_func(4,1) ? "TRUE" : "FALSE") << endl;
   cout << (not2(less<int>())(1,4) ? "TRUE" : "FALSE") 
        << endl;
   cout << (not2(less<int>())(4,1) ? "TRUE" : "FALSE") 
        << endl;   

   //Create an instance of our predicate
   is_odd<int> odd;

   // Use not1 on our user defined predicate
   cout << (odd(1) ? "TRUE" : "FALSE") << endl;
   cout << (odd(4) ? "TRUE" : "FALSE") << endl;
   cout << (not1(odd)(1) ? "TRUE" : "FALSE") << endl;
   cout << (not1(odd)(4) ? "TRUE" : "FALSE") << endl;

   return 0;
 }

Program Output

TRUE
FALSE
FALSE
TRUE
TRUE
FALSE
FALSE
TRUE

Algorithms, binary_function, Function Objects, unary_function