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Stefan Bender

VCOM Interfaces: Release them or not? (refcount warning)

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IN the constructor of one of our classes, we query for a VCOM interface:


 Boolean succ = VCOM_SUCCEEDED( ::GS_VWQueryInterface( VectorWorks::EnergyAnalysis::IID_EnergyBuildingElementWinDoorEx, (IVWUnknown**) & m_energy_2021 ) );


As requested in the SDK documentation, we release that interface in the destructor of the class:


if (m_energy_2021) {
      //  m_energy_2021 = nullptr;


Before releasing, the refcount (m_energy2021->fPtr) is 1, after releasing it is 0 which looks OK.


But after the end of the destructor, it seems that the VCOM destructor is called again, tries to release again and gives a debug warning saying refcount <0.


Is this OK? Should I release the interface by myself or wait for the automatic destructor? SDK doc says  I should release, but looking at the source code in various SDK plug-ins I get a mixed feeling, in some routines the interface is released, in others it is not.


What is the correct workflow? If I am supposed to release the interface by myself, why do I get the debug warning?


Thanks for any help,


Stefan Bender


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Here's how I would tackle this:

class SomeClass

    IEnergyBuildingElementWinDoorExPtr m_energy_2021;



SomeClass::SomeClass() : m_energy_2021(IID_EnergyBuildingElementWinDoorEx)
    // If querying the interface fails for some reason, m_enery_2021 would be nullptr

The default destructor of SomeClass() will take care of releasing the acquired interface. Even if you create your own destructor, m_energy_2021 will still be released. So, to the best of my knowledge, there's no need for you to manually release it.


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