Question about [in, out] parameters
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I have been trying to write a Method on my COM interface that takes an [in, out] parameter. I expected that this would act like a I was passing in a reference. In the idl file the function is: [id(1), helpstring("method Select")] HRESULT Select([in, out] System** System); In the appropiate hear file the function is defined as: STDMETHOD(Select)(System** System); I use the function as below: //In some cases the pSystem object is created other times it will be null ISystemPtr pSystem(CLSID_System); //or ISystemPtr pSystem = NULL; //The Select Method will display a dialog with a set of options, if the object being passed in is //NOT NULL the dialog should have the correct / relavent options selected. If it is NULL the //pSystem object will be created with the selected options and passed back to the client. TheObjectWithTheMethod->Select(&pSystem); The problem is that when an valid (not NULL) pointer is passed into the function the & operator actually releases the pointer to the pSystem and returns a NULL interface pointer. So inside the implimentation of the Select function we always get a NULL pointer. Does anubody have any ideas? Cheers Andy
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I have been trying to write a Method on my COM interface that takes an [in, out] parameter. I expected that this would act like a I was passing in a reference. In the idl file the function is: [id(1), helpstring("method Select")] HRESULT Select([in, out] System** System); In the appropiate hear file the function is defined as: STDMETHOD(Select)(System** System); I use the function as below: //In some cases the pSystem object is created other times it will be null ISystemPtr pSystem(CLSID_System); //or ISystemPtr pSystem = NULL; //The Select Method will display a dialog with a set of options, if the object being passed in is //NOT NULL the dialog should have the correct / relavent options selected. If it is NULL the //pSystem object will be created with the selected options and passed back to the client. TheObjectWithTheMethod->Select(&pSystem); The problem is that when an valid (not NULL) pointer is passed into the function the & operator actually releases the pointer to the pSystem and returns a NULL interface pointer. So inside the implimentation of the Select function we always get a NULL pointer. Does anubody have any ideas? Cheers Andy
Use a ** when you want to pass a pointer to an object and you might want it to return a pointer to a different object. Consider:
HRESULT Select( System** pSystem )
{
if ( pSystem != NULL )
(*pSystem)->Release();*pSystem = new System();
}pSystem = new System();
HRESULT hr = Select( &pSystem );// pSystem now points to a different System object
If you don't want to change the value of the pointer you pass in, but instead change the value of the object, use a single *. I think that you actually do want to pass the pointer by reference, but your
ISystemPtrsmart pointer class has an override ofoperator &which I assume is releasing the held pointer. Without seeing this class, or what the typedef is, it's difficult to help. The documentation for_com_ptr_t, the environment's own smart pointer, suggests that operator&[^] releases the pointer then NULLs it. IIRC, using#importgenerates smart pointer typedefs of_com_ptr_t. If this is what you're using, I think all you can do isDetachthe pointer, call Select, thenAttachthe resulting pointer to the smart pointer object. ATL'sCComPtrallows direct access to the raw pointer through the publicpmember. Itsoperator&ASSERTs if the contained pointer is not NULL. Stability. What an interesting concept. -- Chris Maunder -
Use a ** when you want to pass a pointer to an object and you might want it to return a pointer to a different object. Consider:
HRESULT Select( System** pSystem )
{
if ( pSystem != NULL )
(*pSystem)->Release();*pSystem = new System();
}pSystem = new System();
HRESULT hr = Select( &pSystem );// pSystem now points to a different System object
If you don't want to change the value of the pointer you pass in, but instead change the value of the object, use a single *. I think that you actually do want to pass the pointer by reference, but your
ISystemPtrsmart pointer class has an override ofoperator &which I assume is releasing the held pointer. Without seeing this class, or what the typedef is, it's difficult to help. The documentation for_com_ptr_t, the environment's own smart pointer, suggests that operator&[^] releases the pointer then NULLs it. IIRC, using#importgenerates smart pointer typedefs of_com_ptr_t. If this is what you're using, I think all you can do isDetachthe pointer, call Select, thenAttachthe resulting pointer to the smart pointer object. ATL'sCComPtrallows direct access to the raw pointer through the publicpmember. Itsoperator&ASSERTs if the contained pointer is not NULL. Stability. What an interesting concept. -- Chris MaunderMike Dimmick wrote: *pSystem = new System(); *ahem* You forgot an
AddRef():) -- My name in Katakana is ヨルゲン. My name in German is Jörgen. I blog too now[^]