20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.16 Polymorphic function wrappers [func.wrap]

20.14.16.2 Class template function [func.wrap.func]

20.14.16.2.6 Null pointer comparison functions [func.wrap.func.nullptr]

template<class R, class... ArgTypes> bool operator==(const function<R(ArgTypes...)>& f, nullptr_t) noexcept;
Returns: !f.