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Showing content from https://en.cppreference.com/w/cpp/language/../utility/functional/unary_negate.html below:

std::unary_negate - cppreference.com

template< class Predicate > struct unary_negate : public std::unary_function<Predicate::argument_type, bool>; (until C++11)

template< class Predicate > struct unary_negate;

(since C++11)
(deprecated in C++17)
(removed in C++20)

std::unary_negate is a wrapper function object returning the complement of the unary predicate it holds.

The unary predicate type must define a member type, argument_type, that is convertible to the predicate's parameter type. The unary function objects obtained from std::ref, std::cref, std::negate, std::logical_not, std::mem_fn, std::function, std::hash, or from another call to std::not1 have this type defined, as are function objects derived from the deprecated std::unary_function.

std::unary_negate objects are easily constructed with helper function std::not1.

[edit] Member types Type Definition argument_type Predicate::argument_type result_type bool [edit] Member functions constructs a new unary_negate object with the supplied predicate
(public member function) returns the logical complement of the result of a call to the stored predicate
(public member function) std::unary_negate::unary_negate

explicit unary_negate( Predicate const& pred );

(until C++14)

constexpr explicit unary_negate( Predicate const& pred );

(since C++14)

Constructs a std::unary_negate function object with the stored predicate pred.

Parameters pred - predicate function object std::unary_negate::operator()

bool operator()( argument_type const& x ) const;

(until C++14)

constexpr bool operator()( argument_type const& x ) const;

(since C++14)

Returns the logical complement of the result of calling pred(x).

Parameters x - argument to pass through to predicate Return value

The logical complement of the result of calling pred(x).

[edit] Example

#include <algorithm>
#include <functional>
#include <iostream>
#include <vector>
 
struct less_than_7 : std::unary_function<int, bool>
{
    bool operator()(int i) const { return i < 7; }
};
 
int main()
{
    std::vector<int> v(7, 7);
    v[0] = v[1] = v[2] = 6;
 
    std::unary_negate<less_than_7> not_less_than_7((less_than_7()));
    // C++11 solution:
    // Use std::function<bool (int)>
    // std::function<bool (int)> not_less_than_7 =
    //     [](int x)->bool { return !less_than_7()(x); };
 
    std::cout << std::count_if(v.begin(), v.end(), not_less_than_7);
}

Output:

[edit] See also

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

wrapper function object returning the complement of the binary predicate it holds
(class template) [edit] copyable wrapper of any copy constructible callable object
(class template) [edit] move-only wrapper of any callable object that supports qualifiers in a given call signature
(class template) [edit]

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

constructs custom std::unary_negate object
(function template) [edit]

(deprecated in C++11)(removed in C++17)

creates an adaptor-compatible function object wrapper from a pointer to function
(function template) [edit]

(deprecated in C++11)(removed in C++17)

adaptor-compatible unary function base class
(class template) [edit]

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