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Showing content from https://cplusplus.com/reference/functional/unary_negate/ below:

class template

std::unary_negate

template <class Predicate> class unary_negate;

Negate unary function object class

Unary function object class whose call returns the opposite of another unary function, passed to its constructor.

Object of type unary_negate are generally constructed using function not1.

The class is defined with the same behavior as:

template <class Predicate> class unary_negate
  : public unary_function <typename Predicate::argument_type,bool>
{
protected:
  Predicate fn_;
public:
  explicit unary_negate (const Predicate& pred) : fn_ (pred) {}
  bool operator() (const typename Predicate::argument_type& x) const {return !fn_(x);}
};

template <class Predicate> class unary_negate
{
protected:
  Predicate fn_;
public:
  explicit unary_negate (const Predicate& pred) : fn_ (pred) {}
  bool operator() (const typename Predicate::argument_type& x) const {return !fn_(x);}
  typedef typename Predicate::argument_type argument_type;
  typedef bool result_type;
};

Template parameters

Predicate: A unary function object class, with member argument_type defined.

Member types member type definition notes argument_type T Type of the argument in member operator() result_type T Type returned by member operator()
Member functions

constructor: Constructs an object whose functional call returns the opposite as the object passed as its argument.
operator(): Member function returning the opposite of the function object with which the object was constructed.

Example

// unary_negate example
#include <iostream>     // std::cout
#include <functional>   // std::unary_negate
#include <algorithm>    // std::count_if

struct IsOdd_class {
  bool operator() (const int& x) const {return x%2==1;}
  typedef int argument_type;
} IsOdd_object;

int main () {
  std::unary_negate<IsOdd_class> IsEven_object (IsOdd_object);
  int values[] = {1,2,3,4,5};
  int cx;
  cx = std::count_if ( values, values+5, IsEven_object );
  std::cout << "There are " << cx << " elements with even values.\n";
  return 0;
}

Output:

There are 2 elements with even values.