20 General utilities library [utilities]

20.14 Function objects [function.objects]

20.14.17 Polymorphic function wrappers [func.wrap]

20.14.17.3 Class template function [func.wrap.func]

20.14.17.3.4 Capacity [func.wrap.func.cap]

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