Defined in header <functional>
template< class F>
/*unspecified*/ not_fn( F&& f );
(since C++17)

Creates a forwarding call wrapper that returns the complement of the callable object it holds.


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

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>.


explicit T(F&& f);
T(T&& f) = default;
T(const T& f) = default;
1) The constructor 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.

Member function operator()

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

where fd is the member object of type std::decay_t<F>


Throws no exceptions, unless the construction of fd throws.


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

See also

constructs custom std::unary_negate object
(function template)
constructs custom std::binary_negate object
(function template)

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