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Showing content from https://cplusplus.com/reference/functional/not1/ below:

function template

std::not1

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

Return negation of unary function object

Constructs a unary function object (of a unary_negate type) that returns the opposite of pred (as returned by operator !).

It is defined with the same behavior as:

template <class Predicate> unary_negate<Predicate> not1 (const Predicate& pred)
{
  return unary_negate<Predicate>(pred);
}

Parameters

pred: Unary function object of a class type with member argument_type defined.

Return value An unary function object with the opposite behavior of pred.
See unary_negate.

Example

// not1 example
#include <iostream>     // std::cout
#include <functional>   // std::not1
#include <algorithm>    // std::count_if

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

int main () {
  int values[] = {1,2,3,4,5};
  int cx = std::count_if (values, values+5, std::not1(IsOdd()));
  std::cout << "There are " << cx << " elements with even values.\n";
  return 0;
}

Output:

There are 2 elements with even values.