CAPE-OPEN

The definition you see is for passing as input argument. As output argument - I suppose the easiest it so call .clear() on the passed output argument itself - UNDEFINED for a string value equals an empty string.

What is the context in which you are returning UNDEFINED? Which function is it?

This question arose whilst working on implementing the GetPresentPhases function of the ICapeThermoMaterial interface. The overall context is implementing a Modelica-CAPE-OPEN interface for Dymola.

Regarding this particular function, GetPresentPhases, its parameters are CapeArrayString and CapeArrayEnumeration<CAPEOPEN_1_2::CapePhaseStatus>, so there is no clear()-function available. Would a resize(0) on the output parameters comply with the specification of the function?

Yes - UNDEFINED for arrays is an empty array. Indeed resize(0) will do the trick.