# std::experimental::not_fn

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< cpp‎ | experimental

Defined in header `<experimental/functional>` |  |
template< class F> /*unspecified*/ not_fn( F&& f ); |  | (library fundamentals TS v2)

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

### Parameters

f | - | the object from which the Callable object held by the wrapper is constructed

### Return value

Let `FD` be std::decay_t<F> and `fd` be an lvalue of type `FD` constructed from std::forward<F>(f).

`not_fn` returns a forwarding call wrapper `fn` of unspecified type such that fn(a1, a2, ..., aN) is equivalent to !INVOKE(fd, a1, ..., aN), where `INVOKE` is the operation described in Callable.

The returned call wrapper is always MoveConstructible, and is CopyConstructible if FD is CopyConstructible.

### Remarks

If `fd` is not Callable, or std::is_constructible<FD, F>::value is not `true`, the behavior is undefined.

### Exceptions

Throws no exceptions, unless the construction of `fd` throws.

### Possible implementation

    namespace detail {
        template<class F>
        struct not_fn_t {
            F f;
            template<class... Args>
            auto operator()(Args&&... args)
                noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
                -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
                return !std::invoke(f, std::forward<Args>(args)...);
            }

            // cv-qualified overload for QoI
            template<class... Args>
            auto operator()(Args&&... args) const
                noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
                -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
                return !std::invoke(f, std::forward<Args>(args)...);
            }

            template<class... Args>
            auto operator()(Args&&... args) volatile
                noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
                -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
                return !std::invoke(f, std::forward<Args>(args)...);
            }
            template<class... Args>
            auto operator()(Args&&... args) const volatile
                noexcept(noexcept(!std::invoke(f, std::forward<Args>(args)...)))
                -> decltype(!std::invoke(f, std::forward<Args>(args)...)) {
                return !std::invoke(f, std::forward<Args>(args)...);
            }
        };
    }

    template<class F>
    detail::not_fn_t<std::decay_t<F>> not_fn(F&& f) { return { std::forward<F>(f) }; }

### Notes

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

### See also

not_fn (C++17) | Creates a function object that returns the complement of the result of the function object it holds (function template)

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