Negators

Library:  General utilities

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

#include <functional>

namespace std {
  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().

namespace std {

  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>   // for unary_function
#include <iostream>     // for boolalpha, cout, endl


// Create a new predicate from unary_function.
template <class Arg>
struct is_odd : public std::unary_function<Arg, bool>
{
    bool operator() (const Arg &arg1) const {
        return arg1 % 2 != 0;
    }
};


int main ()
{
    std::less<int> less_func;

    // Use not2 on less.
    std::cout << std::boolalpha
              << less_func (1, 4) << '\n'
              << less_func (4, 1) << '\n'
              << std::not2 (std::less<int>())(1, 4) << '\n'
              << std::not2 (std::less<int>())(4, 1) << '\n';

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

    // Use not1 on our user defined predicate.
    std::cout << odd (1) << '\n'
              << odd (4) << '\n'
              << std::not1 (odd)(1) << '\n'
              << std::not1 (odd)(4) << std::endl;

    return 0;
}


Program Output:

true
false
false
true
true
false
false
true

Algorithms, binary_function, Function Objects, unary_function

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 20.3.5