std::unary_negate

C++

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Defined in header <functional>
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)

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.

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

Member types

Type	Definition
argument_type	Predicate::argument_type
result_type	bool

Member functions

(constructor)	constructs a new unary_negate object with the supplied predicate (public member function)
operator()	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)
explicit constexpr unary_negate( Predicate const& pred );		(since C++14)

Constructs a 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).

Example

Run this code

#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;
    for (int i = 0; i < 10; ++i) v.push_back(i);

    std::unary_negate<less_than_7> not_less_than_7((less_than_7()));

    std::cout << std::count_if(v.begin(), v.end(), not_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:

3

See also

binary_negate (deprecated in C++17)(removed in C++20)	wrapper function object returning the complement of the binary predicate it holds (class template)
function (C++11)	wraps callable object of any type with specified function call signature (class template)
not1 (deprecated in C++17)(removed in C++20)	constructs custom std::unary_negate object (function template)
ptr_fun (deprecated in C++11)(removed in C++17)	creates an adaptor-compatible function object wrapper from a pointer to function (function template)
unary_function (deprecated in C++11)(removed in C++17)	adaptor-compatible unary function base class (class template)