22 General utilities library [utilities]

22.10 Function objects [function.objects]

22.10.17 Polymorphic function wrappers [func.wrap]

22.10.17.3 Class template function [func.wrap.func]

22.10.17.3.7 Null pointer comparison operator functions [func.wrap.func.nullptr]

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