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Difference between revisions of "cpp/utility/functional/not fn"

From cppreference.com
< cpp‎ | utility‎ | functional
(copy from libfun2)
 
(Oh, use of invoke_result_t is incorrect since C++20 (but was exactly specified in C++17).)
 
(45 intermediate revisions by 11 users not shown)
Line 1: Line 1:
−
{{cpp/utility/functional/title|not_fn}}
+
{{cpp/title|not_fn}}
 
{{cpp/utility/functional/navbar}}
 
{{cpp/utility/functional/navbar}}
 
{{dcl begin}}
 
{{dcl begin}}
−
{{dcl header | functional }}
+
{{dcl header|functional}}
−
{{dcl | since=c++17 |
+
{{dcla|num=1|since=c++17|notes={{mark constexpr since c++20}}|
−
template< class F>
+
template< class F >
−
/*unspecified*/ not_fn( F&& f );
+
/* unspecified */ not_fn( F&& f );
 +
}}
 +
{{dcla|num=2|since=c++26|
 +
template< auto ConstFn >
 +
constexpr /* unspecified */ not_fn() noexcept;
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
Creates a forwarding call wrapper that returns the complement of the callable object it holds.
+
@1@ Creates a forwarding call wrapper that returns the negation of the callable object it holds.
 +
@2@ Creates a forwarding call wrapper that returns the negation of the statically determined callable target. The program is ill-formed if {{tt|ConstFn}} is a null pointer or null pointer-to-member.
  
−
=== Parameters ===
+
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | f | the object from which the {{concept|Callable}} object held by the wrapper is constructed }}
+
{{par|f|the object from which the {{named req|Callable}} object held by the wrapper is constructed}}
 +
{{par hreq}}
 +
{{par req named|{{c/core|std::decay_t<F>}}|MoveConstructible|Callable}}
 +
{{par req|{{c|std::is_constructible_v<std::decay_t<F>, F>}} is required to be {{c|true}}.}}
 
{{par end}}
 
{{par end}}
  
 
===Return value===
 
===Return value===
−
Let {{tt|FD}} be {{c|std::decay_t<F>}} and {{tt|fd}} be an lvalue of type {{tt|FD}} constructed from {{c|std::forward<F>(f)}}.  
+
@1@ A function object of unspecified type {{tt|T}}. It has the following members.
 +
{{member|{{small|std::not_fn}} ''return type''|2=
 +
====Member objects====
 +
The return type of {{tt|std::not_fn}} holds a member object of type {{c/core|std::decay_t<F>}}.
  
−
{{tt|not_fn}} returns a forwarding call wrapper {{tt|fn}} of unspecified type such that {{c|fn(a1, a2, ..., aN)}} is equivalent to {{c|!INVOKE(fd, a1, ..., aN)}}, where {{tt|INVOKE}} is the operation described in {{concept|Callable}}.
+
====Constructors====
 +
{{dcl begin}}
 +
{{dcl|num=1|since=c++17|notes={{mark constexpr since c++20}}<br>{{mark expos}}|1=
 +
explicit T( F&& f );
 +
}}
 +
{{dcl|num=2|since=c++17|1=
 +
T( T&& f ) = default;
 +
T( const T& f ) = default;
 +
}}
 +
{{dcl end}}
 +
@1@ The constructor direct-non-list-initializes the member object (of type {{c/core|std::decay_t<F>}}) from {{c|std::forward<F>(f)}}. Throws any exception thrown by the constructor selected.
 +
@2@ Because {{c/core|std::decay_t<F>}} is required to be {{named req|MoveConstructible}}, the returned call wrapper is always {{named req|MoveConstructible}}, and is {{named req|CopyConstructible}} if {{c/core|std::decay_t<F>}} is {{named req|CopyConstructible}}.
 +
{{rrev multi|rev1=
 +
The explicitly defaulted definitions make the return type not assignable.
 +
|since2=c++20|rev2=
 +
It is unspecified whether these constructors are explicitly defaulted and whether the return type is assignable.
 +
}}
  
−
The returned call wrapper is always {{concept|MoveConstructible}}, and is {{concept|CopyConstructible}} if FD is {{concept|CopyConstructible}}.
+
====Member function {{c/core|operator()}}====
 +
{{dcl begin}}
 +
{{dcl rev multi|num=1|since1=c++17|dcl1=
 +
template< class... Args >
 +
auto operator()( Args&&... args ) &
 +
-> decltype(!std::declval<
 +
    std::invoke_result_t<std::decay_t<F>&, Args...>>());
 +
template< class... Args >
 +
auto operator()( Args&&... args ) const&
 +
-> decltype(!std::declval<
 +
    std::invoke_result_t<std::decay_t<F> const&, Args...>>());
 +
|since2=c++20|dcl2=
 +
template< class... Args >
 +
constexpr auto operator()( Args&&... args ) &
 +
    noexcept(/* see below */)
 +
-> decltype(!std::invoke(
 +
    std::declval<std::decay_t<F>&>(), std::declval<Args>()...));
 +
template< class... Args >
 +
constexpr auto operator()( Args&&... args ) const&
 +
    noexcept(/* see below */)
 +
-> decltype(!std::invoke(
 +
    std::declval<std::decay_t<F> const&>(), std::declval<Args>()...));
 +
}}
 +
{{dcl rev multi|num=2|since1=c++17|dcl1=
 +
template< class... Args >
 +
auto operator()( Args&&... args ) &&
 +
-> decltype(!std::declval<
 +
    std::invoke_result_t<std::decay_t<F>, Args...>>());
 +
template< class... Args >
 +
auto operator()( Args&&... args ) const&&
 +
-> decltype(!std::declval<
 +
    std::invoke_result_t<std::decay_t<F> const, Args...>>());
 +
|since2=c++20|dcl2=
 +
template< class... Args >
 +
constexpr auto operator()( Args&&... args ) &&
 +
    noexcept(/* see below */)
 +
-> decltype(!std::invoke(
 +
    std::declval<std::decay_t<F>>(), std::declval<Args>()...));
 +
template< class... Args >
 +
constexpr auto operator()( Args&&... args ) const&&
 +
    noexcept(/* see below */)
 +
-> decltype(!std::invoke(
 +
    std::declval<std::decay_t<F> const>(), std::declval<Args>()...));
 +
}}
 +
{{dcl end}}
  
−
=== Remarks ===
+
Let {{c|fd}} be the member object of type {{c/core|std::decay_t<F>}}.
−
If {{tt|fd}} is not {{concept|Callable}}, or {{c|std::is_constructible<FD, F>::value}} is not {{tt|true}}, the behavior is undefined.
+
{{rrev multi
 +
|since1=c++17|rev1=
 +
@1@ Equivalent to {{c|return !std::invoke(fd, std::forward<Args>(args)...);}}
 +
@2@ Equivalent to {{c|return !std::invoke(std::move(fd), std::forward<Args>(args)...);}}
  
−
=== Exceptions ===
+
While invoking the result, if the substitution into the return type of the originally selected {{c/core|operator()}} overload fails, [[cpp/language/sfinae|another overload may be selected]].
−
Throws no exceptions, unless the construction of {{tt|fd}} throws.
+
|since2=c++20|rev2=
 +
@1@ [[cpp/language/expressions#Expression-equivalence|Expression-equivalent]] to {{c|!std::invoke(fd, std::forward<Args>(args)...)}}
 +
@2@ [[cpp/language/expressions#Expression-equivalence|Expression-equivalent]] to {{c|!std::invoke(std::move(fd), std::forward<Args>(args)...)}}
 +
 
 +
While invoking the result, if the substitution into the return type of the originally selected {{c/core|operator()}} overload fails, the invocation is ill-formed, which can also be a [[cpp/language/sfinae|substitution failure]].
 +
}}
 +
}}
 +
@2@ A value of the following type.
 +
{{member|{{small|std::not_fn}} ''stateless return type''|2=
 +
The return type is a {{named req|CopyConstructible}} stateless class. It is unspecified whether the return type is assignable.
 +
 
 +
====Member function {{c/core|operator()}}====
 +
{{ddcl|since=c++26|
 +
template< class... Args >
 +
constexpr auto operator()( Args&&... args ) const
 +
    noexcept(/* see below */)
 +
-> decltype(!std::invoke(ConstFn, std::declval<Args>()...));
 +
}}
 +
 
 +
[[cpp/language/expressions#Expression-equivalence|Expression-equivalent]] to {{c|!std::invoke(ConstFn, std::forward<Args>(args)...)}}.
 +
}}
 +
 
 +
===Exceptions===
 +
@1@ Throws no exceptions, unless the construction of {{c|fd}} throws.
 +
 
 +
===Possible implementation===
 +
{{eq impl
 +
|title1=(1) not_fn|ver1=1|1=
 +
namespace detail
 +
{
 +
    template<class V, class F, class... Args>
 +
    constexpr bool negate_invocable_impl = false;
 +
    template<class F, class... Args>
 +
    constexpr bool negate_invocable_impl<std::void_t<decltype(
 +
        !std::invoke(std::declval<F>(), std::declval<Args>()...))>, F, Args...> = true;
 +
 
 +
    template<class F, class... Args>
 +
    constexpr bool negate_invocable_v = negate_invocable_impl<void, F, Args...>;
  
−
=== Possible implementation ===
 
−
{{eq fun|1=
 
−
namespace detail {
 
 
     template<class F>
 
     template<class F>
−
     struct not_fn_t {
+
     struct not_fn_t
 +
    {
 
         F f;
 
         F f;
−
         template<class... Args>
+
       
−
         auto operator()(Args&&... args)
+
         template<class... Args,
 +
            std::enable_if_t<negate_invocable_v<F&, Args...>, int> = 0>
 +
         constexpr decltype(auto) operator()(Args&&... args) &
 
             noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
 
             noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
−
            -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
+
        {
 
             return !std::invoke(f, std::forward<Args>(args)...);
 
             return !std::invoke(f, std::forward<Args>(args)...);
 
         }
 
         }
−
 
+
          
−
         // cv-qualified overload for QoI
+
         template<class... Args,
−
         template<class... Args>
+
            std::enable_if_t<negate_invocable_v<const F&, Args...>, int> = 0>
−
         auto operator()(Args&&... args) const
+
         constexpr decltype(auto) operator()(Args&&... args) const&
 
             noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
 
             noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
−
            -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
+
        {
 
             return !std::invoke(f, std::forward<Args>(args)...);
 
             return !std::invoke(f, std::forward<Args>(args)...);
 
         }
 
         }
−
 
+
       
−
         template<class... Args>
+
         template<class... Args,
−
         auto operator()(Args&&... args) volatile
+
            std::enable_if_t<negate_invocable_v<F, Args...>, int> = 0>
−
             noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
+
         constexpr decltype(auto) operator()(Args&&... args) &&
−
            -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
+
             noexcept(noexcept(!std::invoke(std::move(f), std::forward<Args>(args)...)))
−
             return !std::invoke(f, std::forward<Args>(args)...);
+
        {
 +
             return !std::invoke(std::move(f), std::forward<Args>(args)...);
 
         }
 
         }
−
         template<class... Args>
+
       
−
         auto operator()(Args&&... args) const volatile
+
         template<class... Args,
−
             noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
+
            std::enable_if_t<negate_invocable_v<const F, Args...>, int> = 0>
−
            -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
+
         constexpr decltype(auto) operator()(Args&&... args) const&&
−
             return !std::invoke(f, std::forward<Args>(args)...);
+
             noexcept(noexcept(!std::invoke(std::move(f), std::forward<Args>(args)...)))
 +
        {
 +
             return !std::invoke(std::move(f), std::forward<Args>(args)...);
 
         }
 
         }
 +
 +
        // Deleted overloads are needed since C++20
 +
        // for preventing a non-equivalent but well-formed overload to be selected.
 +
 +
        template<class... Args,
 +
            std::enable_if_t<!negate_invocable_v<F&, Args...>, int> = 0>
 +
        void operator()(Args&&...) & = delete;
 +
 +
        template<class... Args,
 +
            std::enable_if_t<!negate_invocable_v<const F&, Args...>, int> = 0>
 +
        void operator()(Args&&...) const& = delete;
 +
 +
        template<class... Args,
 +
            std::enable_if_t<!negate_invocable_v<F, Args...>, int> = 0>
 +
        void operator()(Args&&...) && = delete;
 +
 +
        template<class... Args,
 +
            std::enable_if_t<!negate_invocable_v<const F, Args...>, int> = 0>
 +
        void operator()(Args&&...) const&& = delete;
 
     };
 
     };
 
}
 
}
  
 
template<class F>
 
template<class F>
−
detail::not_fn_t<std::decay_t<F>> not_fn(F&& f) { return { std::forward<F>(f) }; }
+
constexpr detail::not_fn_t<std::decay_t<F>> not_fn(F&& f)
 +
{
 +
    return {std::forward<F>(f)};
 +
}
 +
|title2=(2) not_fn|ver2=2|2=
 +
namespace detail
 +
{
 +
    template<auto ConstFn>
 +
    struct stateless_not_fn
 +
    {
 +
        template<class... Args>
 +
        constexpr auto operator()(Args&&... args) const
 +
            noexcept(noexcept(!std::invoke(ConstFn, std::forward<Args>(args)...)))
 +
            -> decltype(!std::invoke(ConstFn, std::forward<Args>(args)...))
 +
        {
 +
            return !std::invoke(ConstFn, std::forward<Args>(args)...);
 +
        }
 +
    };
 +
}
 +
 
 +
template<auto ConstFn>
 +
constexpr detail::stateless_not_fn<ConstFn> not_fn() noexcept
 +
{
 +
    if constexpr (std::is_pointer_v<decltype(ConstFn)> {{!!}}
 +
                  std::is_member_pointer_v<decltype(ConstFn)>)
 +
        static_assert(ConstFn != nullptr);
 +
 
 +
    return {};
 +
}
 
}}
 
}}
  
−
=== Notes ===
+
===Notes===
−
{{tt|not_fn}} is intended to replace the C++03-era negators {{lc|std::not1}} and {{lc|std::not2}}.
+
{{tt|std::not_fn}} is intended to replace the C++03-era negators {{lc|std::not1}} and {{lc|std::not2}}.
  
−
=== See also ===
+
{{ftm begin}}
 +
{{ftm|__cpp_lib_not_fn|std=C++17|value=201603L|rowspan="2"|{{tt|std::not_fn()}}, {{vl|1}}}}
 +
{{ftm|-|std=C++26|value=202306L|Allow passing callable objects as non-type template arguments to {{tt|std::not_fn}}, {{vl|2}}}}
 +
{{ftm end}}
 +
 
 +
===Example===
 +
{{example
 +
|code=
 +
#include <cassert>
 +
#include <functional>
 +
 
 +
bool is_same(int a, int b) noexcept
 +
{
 +
    return a == b;
 +
}
 +
 
 +
struct S
 +
{
 +
    int val;
 +
    bool is_same(int arg) const noexcept { return val == arg; }
 +
};
 +
 
 +
int main()
 +
{
 +
    // Using with a free function:
 +
    auto is_differ = std::not_fn(is_same);
 +
    assert(is_differ(8, 8) == false); // equivalent to: !is_same(8, 8) == false
 +
    assert(is_differ(6, 9) == true); // equivalent to: !is_same(8, 0) == true
 +
 
 +
    // Using with a member function:
 +
    auto member_differ = std::not_fn(&S::is_same);
 +
    assert(member_differ(S{3}, 3) == false); //: S tmp{6}; !tmp.is_same(6) == false
 +
 
 +
    // Noexcept-specification is preserved:
 +
    static_assert(noexcept(is_differ) == noexcept(is_same));
 +
    static_assert(noexcept(member_differ) == noexcept(&S::is_same));
 +
 
 +
    // Using with a function object:
 +
    auto same = [](int a, int b) { return a == b; };
 +
    auto differ = std::not_fn(same);
 +
    assert(differ(1, 2) == true); //: !same(1, 2) == true
 +
    assert(differ(2, 2) == false); //: !same(2, 2) == false
 +
 
 +
#if __cpp_lib_not_fn >= 202306L
 +
    auto is_differ_cpp26 = std::not_fn<is_same>();
 +
    assert(is_differ_cpp26(8, 8) == false);
 +
    assert(is_differ_cpp26(6, 9) == true);
 +
 
 +
    auto member_differ_cpp26 = std::not_fn<&S::is_same>();
 +
    assert(member_differ_cpp26(S{3}, 3) == false);
 +
 
 +
    auto differ_cpp26 = std::not_fn<same>();
 +
    static_assert(differ_cpp26(1, 2) == true);
 +
    static_assert(differ_cpp26(2, 2) == false);
 +
#endif
 +
}
 +
}}
 +
 
 +
===See also===
 
{{dsc begin}}
 
{{dsc begin}}
−
{{dsc inc | cpp/utility/functional/dsc not1}}
+
{{dsc inc|cpp/utility/functional/dsc not1}}
−
{{dsc inc | cpp/utility/functional/dsc not2}}
+
{{dsc inc|cpp/utility/functional/dsc not2}}
−
{{dsc inc | cpp/experimental/dsc not_fn}}
+
 
{{dsc end}}
 
{{dsc end}}
 +
 +
{{langlinks|es|ja|ru|zh}}

Latest revision as of 03:02, 11 January 2024

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Function objects
Function invocation
(C++17)(C++23)
Identity function object
(C++20)
Transparent operator wrappers
(C++14)
(C++14)
(C++14)
(C++14)  
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)
(C++14)

Old binders and adaptors
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)
(until C++17*)  
(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
(until C++17*)(until C++17*)
(until C++17*)(until C++17*)

(until C++17*)
(until C++17*)(until C++17*)(until C++17*)(until C++17*)
(until C++20*)
(until C++20*)
 
Defined in header <functional>
template< class F >
/* unspecified */ not_fn( F&& f );
 (1) (since C++17)
(constexpr since C++20)
template< auto ConstFn >
constexpr /* unspecified */ not_fn() noexcept;
 (2) (since C++26)
1) Creates a forwarding call wrapper that returns the negation of the callable object it holds.
2) Creates a forwarding call wrapper that returns the negation of the statically determined callable target. The program is ill-formed if ConstFn is a null pointer or null pointer-to-member.

Contents

[edit] Parameters

f - the object from which the Callable object held by the wrapper is constructed
Type requirements
-
std::decay_t<F> must meet the requirements of Callable and MoveConstructible.
-
std::is_constructible_v<std::decay_t<F>, F> is required to be true.

[edit] Return value

1) A function object of unspecified type T. It has the following members.

std::not_fn return type

Member objects

The return type of std::not_fn holds a member object of type std::decay_t<F>.

Constructors

explicit T( F&& f );
 (1) (since C++17)
(constexpr since C++20)
(exposition only*)
T( T&& f ) = default;
T( const T& f ) = default;
 (2) (since C++17)
1) The constructor direct-non-list-initializes the member object (of type std::decay_t<F>) from std::forward<F>(f). Throws any exception thrown by the constructor selected.
2) Because std::decay_t<F> is required to be MoveConstructible, the returned call wrapper is always MoveConstructible, and is CopyConstructible if std::decay_t<F> is CopyConstructible.

The explicitly defaulted definitions make the return type not assignable.

(until C++20)

It is unspecified whether these constructors are explicitly defaulted and whether the return type is assignable.

(since C++20)

Member function operator()

(1)
template< class... Args >

auto operator()( Args&&... args ) &
-> decltype(!std::declval<
    std::invoke_result_t<std::decay_t<F>&, Args...>>());
template< class... Args >
auto operator()( Args&&... args ) const&
-> decltype(!std::declval<

    std::invoke_result_t<std::decay_t<F> const&, Args...>>());
 (since C++17)
(until C++20)
template< class... Args >

constexpr auto operator()( Args&&... args ) &
    noexcept(/* see below */)
-> decltype(!std::invoke(
    std::declval<std::decay_t<F>&>(), std::declval<Args>()...));
template< class... Args >
constexpr auto operator()( Args&&... args ) const&
    noexcept(/* see below */)
-> decltype(!std::invoke(

    std::declval<std::decay_t<F> const&>(), std::declval<Args>()...));
 (since C++20)
(2)
template< class... Args >

auto operator()( Args&&... args ) &&
-> decltype(!std::declval<
    std::invoke_result_t<std::decay_t<F>, Args...>>());
template< class... Args >
auto operator()( Args&&... args ) const&&
-> decltype(!std::declval<

    std::invoke_result_t<std::decay_t<F> const, Args...>>());
 (since C++17)
(until C++20)
template< class... Args >

constexpr auto operator()( Args&&... args ) &&
    noexcept(/* see below */)
-> decltype(!std::invoke(
    std::declval<std::decay_t<F>>(), std::declval<Args>()...));
template< class... Args >
constexpr auto operator()( Args&&... args ) const&&
    noexcept(/* see below */)
-> decltype(!std::invoke(

    std::declval<std::decay_t<F> const>(), std::declval<Args>()...));
 (since C++20)

Let fd be the member object of type std::decay_t<F>.

1) Equivalent to return !std::invoke(fd, std::forward<Args>(args)...);
2) Equivalent to return !std::invoke(std::move(fd), std::forward<Args>(args)...);

While invoking the result, if the substitution into the return type of the originally selected operator() overload fails, another overload may be selected.

(since C++17)
(until C++20)
1) Expression-equivalent to !std::invoke(fd, std::forward<Args>(args)...)
2) Expression-equivalent to !std::invoke(std::move(fd), std::forward<Args>(args)...)

While invoking the result, if the substitution into the return type of the originally selected operator() overload fails, the invocation is ill-formed, which can also be a substitution failure.

(since C++20)
2) A value of the following type.

std::not_fn stateless return type

The return type is a CopyConstructible stateless class. It is unspecified whether the return type is assignable.

Member function operator()

template< class... Args >

constexpr auto operator()( Args&&... args ) const
    noexcept(/* see below */)

-> decltype(!std::invoke(ConstFn, std::declval<Args>()...));
 (since C++26)

Expression-equivalent to !std::invoke(ConstFn, std::forward<Args>(args)...).

[edit] Exceptions

1) Throws no exceptions, unless the construction of fd throws.

[edit] Possible implementation

(1) not_fn 
namespace detail
{
    template<class V, class F, class... Args>
    constexpr bool negate_invocable_impl = false;
    template<class F, class... Args>
    constexpr bool negate_invocable_impl<std::void_t<decltype(
        !std::invoke(std::declval<F>(), std::declval<Args>()...))>, F, Args...> = true;
 
    template<class F, class... Args>
    constexpr bool negate_invocable_v = negate_invocable_impl<void, F, Args...>;
 
    template<class F>
    struct not_fn_t
    {
        F f;
 
        template<class... Args,
            std::enable_if_t<negate_invocable_v<F&, Args...>, int> = 0>
        constexpr decltype(auto) operator()(Args&&... args) &
            noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
        {
            return !std::invoke(f, std::forward<Args>(args)...);
        }
 
        template<class... Args,
            std::enable_if_t<negate_invocable_v<const F&, Args...>, int> = 0>
        constexpr decltype(auto) operator()(Args&&... args) const&
            noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
        {
            return !std::invoke(f, std::forward<Args>(args)...);
        }
 
        template<class... Args,
            std::enable_if_t<negate_invocable_v<F, Args...>, int> = 0>
        constexpr decltype(auto) operator()(Args&&... args) &&
            noexcept(noexcept(!std::invoke(std::move(f), std::forward<Args>(args)...)))
        {
            return !std::invoke(std::move(f), std::forward<Args>(args)...);
        }
 
        template<class... Args,
            std::enable_if_t<negate_invocable_v<const F, Args...>, int> = 0>
        constexpr decltype(auto) operator()(Args&&... args) const&&
            noexcept(noexcept(!std::invoke(std::move(f), std::forward<Args>(args)...)))
        {
            return !std::invoke(std::move(f), std::forward<Args>(args)...);
        }
 
        // Deleted overloads are needed since C++20
        // for preventing a non-equivalent but well-formed overload to be selected.
 
        template<class... Args,
            std::enable_if_t<!negate_invocable_v<F&, Args...>, int> = 0>
        void operator()(Args&&...) & = delete;
 
        template<class... Args,
            std::enable_if_t<!negate_invocable_v<const F&, Args...>, int> = 0>
        void operator()(Args&&...) const& = delete;
 
        template<class... Args,
            std::enable_if_t<!negate_invocable_v<F, Args...>, int> = 0>
        void operator()(Args&&...) && = delete;
 
        template<class... Args,
            std::enable_if_t<!negate_invocable_v<const F, Args...>, int> = 0>
        void operator()(Args&&...) const&& = delete;
    };
}
 
template<class F>
constexpr detail::not_fn_t<std::decay_t<F>> not_fn(F&& f)
{
    return {std::forward<F>(f)};
}
(2) not_fn 
namespace detail
{
    template<auto ConstFn>
    struct stateless_not_fn
    {
        template<class... Args>
        constexpr auto operator()(Args&&... args) const
            noexcept(noexcept(!std::invoke(ConstFn, std::forward<Args>(args)...)))
            -> decltype(!std::invoke(ConstFn, std::forward<Args>(args)...))
        {
            return !std::invoke(ConstFn, std::forward<Args>(args)...);
        }
    };
}
 
template<auto ConstFn>
constexpr detail::stateless_not_fn<ConstFn> not_fn() noexcept
{
    if constexpr (std::is_pointer_v<decltype(ConstFn)> ||
                  std::is_member_pointer_v<decltype(ConstFn)>)
        static_assert(ConstFn != nullptr);
 
    return {};
}

[edit] Notes

std::not_fn is intended to replace the C++03-era negators std::not1 and std::not2.

Feature-test macro Value Std Feature
__cpp_lib_not_fn 201603L (C++17) std::not_fn(), (1)
202306L (C++26) Allow passing callable objects as non-type template arguments to std::not_fn, (2)

[edit] Example

Run this code
#include <cassert>
#include <functional>
 
bool is_same(int a, int b) noexcept
{
    return a == b;
}
 
struct S
{
    int val;
    bool is_same(int arg) const noexcept { return val == arg; }
};
 
int main()
{
    // Using with a free function:
    auto is_differ = std::not_fn(is_same);
    assert(is_differ(8, 8) == false); // equivalent to: !is_same(8, 8) == false
    assert(is_differ(6, 9) == true); // equivalent to: !is_same(8, 0) == true
 
    // Using with a member function:
    auto member_differ = std::not_fn(&S::is_same);
    assert(member_differ(S{3}, 3) == false); //: S tmp{6}; !tmp.is_same(6) == false
 
    // Noexcept-specification is preserved:
    static_assert(noexcept(is_differ) == noexcept(is_same));
    static_assert(noexcept(member_differ) == noexcept(&S::is_same));
 
    // Using with a function object:
    auto same = [](int a, int b) { return a == b; };
    auto differ = std::not_fn(same);
    assert(differ(1, 2) == true); //: !same(1, 2) == true
    assert(differ(2, 2) == false); //: !same(2, 2) == false
 
#if __cpp_lib_not_fn >= 202306L
    auto is_differ_cpp26 = std::not_fn<is_same>();
    assert(is_differ_cpp26(8, 8) == false);
    assert(is_differ_cpp26(6, 9) == true);
 
    auto member_differ_cpp26 = std::not_fn<&S::is_same>();
    assert(member_differ_cpp26(S{3}, 3) == false);
 
    auto differ_cpp26 = std::not_fn<same>();
    static_assert(differ_cpp26(1, 2) == true);
    static_assert(differ_cpp26(2, 2) == false);
#endif
}

[edit] See also

(deprecated in C++17)(removed in C++20)
 constructs custom std::unary_negate object
(function template) [edit]
(deprecated in C++17)(removed in C++20)
 constructs custom std::binary_negate object
(function template) [edit]
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