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# std::not2

From cppreference.com
< cpp‎ | utility‎ | functional

Defined in header `<functional>` |  |
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)

`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<Predicate>.

The binary predicate type must define two member types, `first_argument_type` and `second_argument_type`, that are convertible to the predicate's parameter types. The function objects obtained from std::owner_less, std::ref, std::cref, std::plus, std::minus, std::multiplies, std::divides, std::modulus, std::equal_to, std::not_equal_to, std::greater, std::less, std::greater_equal, std::less_equal, std::logical_not, std::logical_or, std::bit_and, std::bit_or, std::bit_xor, std::mem_fn, std::map::value_comp, std::multimap::value_comp, std::function, or from another call to std::not2 have these types defined, as are function objects derived from the deprecated std::binary_function.

### [edit] Parameters

pred | - | binary predicate

### [edit] Return value

`std::not2` returns an object of type std::binary_negate<Predicate>, constructed with `pred`.

### [edit] Exceptions

None.

### [edit] Example

Run this code

    #include <algorithm>
    #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';

    }

Output:

    negating a binary_function:
    0 2 true
    1 1 false
    2 0 true
    negating a standard functor:
    0 2 true
    1 1 false
    2 0 true
    negating a std::function:
    0 2 true
    1 1 false
    2 0 true
    negating a std::reference_wrapper:
    0 2 true
    1 1 false
    2 0 true

### [edit] See also

not_fn (C++17) | Creates a function object that returns the complement of the result of the function object it holds (function template) [edit]
binary_negate (deprecated) | wrapper function object returning the complement of the binary predicate it holds (class template) [edit]
function (C++11) | wraps callable object of any type with specified function call signature (class template) [edit]
not1 (deprecated) | constructs custom std::unary_negate object (function template) [edit]
ptr_fun (until C++17) | creates an adaptor-compatible function object wrapper from a pointer to function (function template) [edit]
binary_function (until C++17) | adaptor-compatible binary function base class (class template) [edit]

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