std::not_fn

 

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Language support Type support (basic types, RTTI) Library feature-test macros (C++20) Dynamic memory management Program utilities Coroutine support (C++20) Variadic functions is_constant_evaluated (C++20) is_within_lifetime (C++26) initializer_list (C++11) source_location (C++20) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Date and time Function objects Formatting library (C++20)   bitset hash (C++11) Relational operators (deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23) Elementary string conversions to_chars (C++17) from_chars (C++17) chars_format (C++17) to_chars_result (C++17) from_chars_result (C++17)

 

Function objects

Function wrappers function (C++11) move_only_function (C++23) copyable_function (C++26) function_ref (C++26) mem_fn (C++11) bad_function_call (C++11)	Partial function application bind_frontbind_back (C++20)(C++23) bind (C++11) is_bind_expression (C++11) is_placeholder (C++11) _1, _2, _3, ... (C++11)
Function invocation invokeinvoke_r (C++17)(C++23) Identity function object identity (C++20)	Reference wrappers reference_wrapper (C++11) refcref (C++11)(C++11) unwrap_referenceunwrap_ref_decay (C++20)(C++20)
Operator wrappers plus minus negate multiplies divides modulus bit_and bit_or bit_not (C++14)   bit_xor equal_to not_equal_to greater less greater_equal less_equal logical_and logical_or logical_not	Transparent operator wrappers plus<> (C++14) minus<> (C++14) negate<> (C++14) multiplies<> (C++14)   divides<> (C++14) modulus<> (C++14) bit_and<> (C++14) bit_or<> (C++14) bit_not<> (C++14) bit_xor<> (C++14) equal_to<> (C++14) not_equal_to<> (C++14) greater<> (C++14) less<> (C++14) greater_equal<> (C++14) less_equal<> (C++14) logical_and<> (C++14) logical_or<> (C++14) logical_not<> (C++14)
Negators not_fn (C++17) Searchers default_searcher (C++17) boyer_moore_searcher (C++17) boyer_moore_horspool_searcher (C++17)	Constrained comparators ranges::equal_to (C++20) ranges::not_equal_to (C++20) ranges::greater (C++20) ranges::less (C++20) ranges::greater_equal (C++20) ranges::less_equal (C++20) compare_three_way (C++20)
Old binders and adaptors
unary_function (until C++17*) binary_function (until C++17*) ptr_fun (until C++17*) pointer_to_unary_function (until C++17*) pointer_to_binary_function (until C++17*)   mem_fun (until C++17*) mem_fun_tmem_fun1_tconst_mem_fun_tconst_mem_fun1_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) not1 (until C++20*) not2 (until C++20*)	binder1stbinder2nd (until C++17*)(until C++17*) bind1stbind2nd (until C++17*)(until C++17*) mem_fun_ref (until C++17*) mem_fun_ref_tmem_fun1_ref_tconst_mem_fun_ref_tconst_mem_fun1_ref_t (until C++17*)(until C++17*)(until C++17*)(until C++17*) unary_negate (until C++20*) binary_negate (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.

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

not1 (deprecated in C++17)(removed in C++20)	constructs custom std::unary_negate object (function template)
not2 (deprecated in C++17)(removed in C++20)	constructs custom std::binary_negate object (function template)