std::not2

Header: <functional>

std::not2 is a helper function to create a function object that returns the complement of the binary predicate function passed. The function object created is of type std::binary_negate.

# Declarations

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

(until C++14)

template< class Predicate >
constexpr std::binary_negate<Predicate> not2( const Predicate& pred );

(since C++14) (deprecated in C++17) (removed in C++20)

# Parameters

# Return value

std::not2 returns an object of type std::binary_negate, constructed with pred.

# Example

#include <algorithm>
#include <cstddef>
#include <functional>
#include <iostream>
#include <vector>
 
struct old_same : std::binary_function<int, int, bool>
{
    bool operator()(int a, int b) const { return a == b; }
};
 
struct new_same
{
    bool operator()(int a, int b) const { return a == b; }
};
 
bool same_fn(int a, int b)
{
    return a == b;
}
 
int main()
{
    std::vector<int> v1{0, 1, 2};
    std::vector<int> v2{2, 1, 0};
    std::vector<bool> v3(v1.size());
 
    std::cout << "negating a binary_function:\n";
    std::transform(v1.begin(), v1.end(), v2.begin(), v3.begin(),
                   std::not2(old_same()));
 
    std::cout << std::boolalpha;
    for (std::size_t i = 0; i < v1.size(); ++i)
        std::cout << v1[i] << ' ' << v2[i] << ' ' << v3[i] << '\n';
 
    std::cout << "negating a standard functor:\n";
    std::transform(v1.begin(), v1.end(), v2.begin(), v3.begin(),
                   std::not2(std::equal_to<int>()));
 
    for (std::size_t i = 0; i < v1.size(); ++i)
        std::cout << v1[i] << ' ' << v2[i] << ' ' << v3[i] << '\n';
 
    std::cout << "negating a std::function:\n";
    std::transform(v1.begin(), v1.end(), v2.begin(), v3.begin(),
                   std::not2(std::function<bool(int, int)>(new_same())));
 
    for (std::size_t i = 0; i < v1.size(); ++i)
        std::cout << v1[i] << ' ' << v2[i] << ' ' << v3[i] << '\n';
 
    std::cout << "negating a std::reference_wrapper:\n";
    std::transform(v1.begin(), v1.end(), v2.begin(), v3.begin(),
                   std::not2(std::ref(same_fn)));
 
    for (std::size_t i = 0; i < v1.size(); ++i)
        std::cout << v1[i] << ' ' << v2[i] << ' ' << v3[i] << '\n';
}

# See also