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std::not_fn(3) C++ Standard Libary std::not_fn(3) NAME std::not_fn - std::not_fn Synopsis Defined in header <functional> template< class F > (1) (since C++17) /* unspecified */ not_fn( F&& f ); (constexpr since C++20) template< auto ConstFn > (2) (since C++26) constexpr /* unspecified */ not_fn() noexcept; 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 MoveCon- structible. - 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 (since C++17) explicit T( F&& f ); (1) (constexpr since C++20) (exposition only*) T( T&& f ) = default; (2) (since C++17) T( const T& f ) = default; 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 (until C++20) assignable. It is unspecified whether these constructors are explicitly de- faulted (since C++20) and whether the return type is assignable. Member function operator() template< class... Args > auto operator()( Args&&... args ) & -> decltype(!std::declval< (since std::invoke_result_t<std::decay_t<F>&, Args...>>()); C++17) template< class... Args > (until auto operator()( Args&&... args ) const& C++20) -> decltype(!std::declval< std::invoke_result_t<std::decay_t<F> const&, Args...>>()); template< class... Args > constexpr auto operator()( Args&&... args ) & noexcept(/* see below */) -> decltype(!std::invoke( std::declval<std::decay_t<F>&>(), std::declval<Args>()...)); (since template< class... Args > C++20) constexpr auto operator()( Args&&... args ) const& noexcept(/* see below */) -> decltype(!std::invoke( std::declval<std::decay_t<F> const&>(), std::declval<Args>()...)); (1) template< class... Args > auto operator()( Args&&... args ) && -> decltype(!std::declval< (since std::invoke_result_t<std::decay_t<F>, Args...>>()); C++17) template< class... Args > (until auto operator()( Args&&... args ) const&& C++20) -> decltype(!std::declval< std::invoke_result_t<std::decay_t<F> const, Args...>>()); template< class... Args > (2) constexpr auto operator()( Args&&... args ) && noexcept(/* see below */) -> decltype(!std::invoke( std::declval<std::decay_t<F>>(), std::declval<Args>()...)); (since template< class... Args > C++20) constexpr auto operator()( Args&&... args ) const&& noexcept(/* see below */) -> decltype(!std::invoke( std::declval<std::decay_t<F> const>(), std::declval<Args>()...)); Let fd be the member object of type std::decay_t<F>. 1) Equivalent to return !std::invoke(fd, std::for- ward<Args>(args)...); 2) Equivalent to return !std::invoke(std::move(fd), std::forward<Args>(args)...); (since C++17) (un- til C++20) While invoking the result, if the substitution into the return type of the originally selected operator() overload fails, another overload may be selected. 1) Expression-equivalent to !std::invoke(fd, std::forward<Args>(args)...) 2) Expression-equivalent to !std::invoke(std::move(fd), std::forward<Args>(args)...) (since C++20) 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. 2) A value of the following type. std::not_fn stateless return type The return type is a CopyConstructible stateless class. It is un- specified whether the return type is assignable. Member function operator() template< class... Args > constexpr auto operator()( Args&&... args ) const (since C++26) noexcept(/* see below */) -> decltype(!std::invoke(ConstFn, std::declval<Args>()...)); Expression-equivalent to !std::invoke(ConstFn, std::for- ward<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::for- ward<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::for- ward<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::for- ward<Args>(args)...))) { return !std::invoke(std::move(f), std::for- ward<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::for- ward<Args>(args)...))) { return !std::invoke(std::move(f), std::for- ward<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::for- ward<Args>(args)...))) -> decltype(!std::invoke(ConstFn, std::for- ward<Args>(args)...)) { return !std::invoke(ConstFn, std::for- ward<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 201603L (C++17) std::not_fn(), (1) __cpp_lib_not_fn 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 constructs custom std::unary_negate object (deprecated in C++17) (function template) (removed in C++20) not2 constructs custom std::binary_negate object (deprecated in C++17) (function template) (removed in C++20) http://cppreference.com 2024.06.10 std::not_fn(3)
NAME | Synopsis | Parameters | Type requirements | Return value | Member objects | Member function operator() | Member function operator() | Exceptions | Possible implementation | Notes | Example | See also
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