std:: not_fn

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
Function objects
Function invocation
(C++17) (C++23)
Identity function object
(C++20)
Old binders and adaptors
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( until C++17* )
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( until C++17* )
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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.

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 .

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 ) ... ) .

Exceptions

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

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 {};
}

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 )

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
}

See also

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