19 General utilities library [utilities]

19.14 Function objects [function.objects]

19.14.16 Polymorphic function wrappers [func.wrap]

19.14.16.2 Class template function [func.wrap.func]

19.14.16.2.3 Capacity [func.wrap.func.cap]

explicit operator bool() const noexcept;
Returns: true if *this has a target, otherwise false.