std::not_fn
| Defined in header <functional>
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||
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) |
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
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) |
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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 ) & |
(since C++17) (until C++20) |
|
| template< class... Args > constexpr auto operator()( Args&&... args ) & |
(since C++20) | |
| (2) | ||
template< class... Args > auto operator()( Args&&... args ) && |
(since C++17) (until C++20) |
|
| template< class... Args > constexpr auto operator()( Args&&... 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) |
|
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) |
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 |
(since C++26) | |
Expression-equivalent to !std::invoke(ConstFn, std::forward<Args>(args)...).
Exceptions
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)
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| 202306L | (C++26) | Allow passing callable objects as non-type template arguments to std::not_fn, (2)
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Example
#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) |