queryInterface

[waliop]

wt actually working of queryinterface function? sdfsdfsdfhsdfh

[Steve S]

QueryInterface is a discovery mechanism. Let's say you have an interface (any interface, it doesn't matter) to a COM component. You can, using a method in the interface, ask the component if it supports another interface that you know about. For instance, you might have an IFrooble interface on an object, and you want to know if it supports automation using the 'classic IDispatch' method. You say something like: HRESULT hr; IDispatch* pDisp = NULL; hr = pFrooble->QueryInterface( IID_IDispatch, reinterpret_cast<void**>(&pDisp)); if (SUCCEEDED(hr)) { // Yep, it does dispatch, and we now have that interface as well // as the IFrooble one. } else { // No, it doesn't or can't provide that interface } Does that help? Another scenario is versioned interfaces, where a client will ask for successive interfaces (eg IFooBar4, IFooBar3, IFooBar2, IFooBar) until it gets one it's happy with. Steve S

[Big_M]

If you look from the C++ point of view, All you interfaces to COM object are abstract base classes with pure virtual functions. And your COM object gives implementation of all these functions. So when you call QueryInterface, it simple casts the COM object(this) into the interface pointer, increase a reference count and return that pointer. This way you can only call the functions declared in that interface.