How to properly use CComPtr in function calls? - c++

Question

I'm new to COM and smartpointers, I'm trying to convert a project from raw pointers to CComPtr to avoid the hassle with memory management. I'm looking for some advice on how to properly use CComPointers when it comes to functions and scope in general. A sample of my code.
int DisplayDeviceInformation(IEnumMoniker * pEnum, IMoniker * pMoniker)
{
CComPtr<IPropertyBag> pPropBag = NULL;
while (pEnum->Next(1, &pMoniker, NULL) == S_OK)
{
HRESULT hr = pMoniker->BindToStorage(0, 0, IID_PPV_ARGS(&pPropBag));
if (FAILED(hr))
{
}
VARIANT var;
VariantInit(&var);
// Get description or friendly name.
hr = pPropBag->Read(L"Description", &var, 0);
if (FAILED(hr))
{
hr = pPropBag->Read(L"FriendlyName", &var, 0);
}
if (SUCCEEDED(hr))
{
printf("%S\n", var.bstrVal);
VariantClear(&var);
}
hr = pPropBag->Write(L"FriendlyName", &var);
// WaveInID applies only to audio capture devices.
hr = pPropBag->Read(L"WaveInID", &var, 0);
if (SUCCEEDED(hr))
{
printf("WaveIn ID: %d\n", var.lVal);
VariantClear(&var);
}
hr = pPropBag->Read(L"DevicePath", &var, 0);
if (SUCCEEDED(hr))
{
// The device path is not intended for display.
printf("Device path: %S\n", var.bstrVal);
VariantClear(&var);
}
}
return 0;
}
CComPtr<IMoniker> pMoniker = NULL;
CComPtr<IMoniker> pMoniker2 = NULL;
CComPtr<IEnumMoniker> pEnum = NULL;
hr = EnumerateDevices(CLSID_VideoInputDeviceCategory, &pEnum);
//pEnum->Next(1, &pMoniker,&cFetched);
if (SUCCEEDED(hr))
{
DisplayDeviceInformation(pEnum, pMoniker);
}
pEnum = NULL;
hr = EnumerateDevices(CLSID_AudioInputDeviceCategory, &pEnum);
//pEnum->Next(1, &pMoniker2,&cFetched);
if (SUCCEEDED(hr))
{
DisplayDeviceInformation(pEnum, pMoniker);
}
Basiclly, the first DisplayDeviceInformation(pEnum, pMoniker); gives a p==0 error. If I however uncomment the pEnum->Next(1, &pMoniker,&cFetched); it works. With raw pointers I don't have to do that since the code just skips to the next device. Any advice or help would make me outmost grateful, thanks in advance!

Answer 1

Where CComPtr gives you an assert failure, you are likely to have an issue with raw pointers as well. You just are not pre-warned and the problem comes up later, e.g. as a reference leak.
You don't seem to need IMoniker in global scope
You need to clear IMoniker pointer before supplying it for the enumerator
See below:
int DisplayDeviceInformation(IEnumMoniker* pEnum, IMoniker** ppSelectedMoniker)
{
CComPtr<IMoniker> pMoniker;
while (pEnum->Next(1, &pMoniker, NULL) == S_OK)
{
CComPtr<IPropertyBag> pPropBag; // You need it clear to start from for every moniker
HRESULT hr = pMoniker->BindToStorage(0, 0, IID_PPV_ARGS(&pPropBag));
// ...
if(we should stop enumeration and we are good with current moniker)
{
//ATLASSERT(ppSelectedMoniker != NULL);
*ppSelectedMoniker = pMoniker.Detach();
return ...
}
// ...
pMoniker.Release(); // You have to do this, so that next Next would accept empty CComPtr as an argument
}
return 0;
}
CComPtr<IEnumMoniker> pEnumVideoMoniker;
CComPtr<IMoniker> pSelectedVideoMoniker;
hr = EnumerateDevices(CLSID_VideoInputDeviceCategory, &pEnumVideoMoniker);
if (SUCCEEDED(hr))
DisplayDeviceInformation(pEnumVideoMoniker, &pSelectedVideoMoniker);
CComPtr<IEnumMoniker> pEnumAudioMoniker;
CComPtr<IMoniker> pSelectedAudioMoniker;
hr = EnumerateDevices(CLSID_AudioInputDeviceCategory, &pEnumAudioMoniker);
if (SUCCEEDED(hr))
DisplayDeviceInformation(pEnumAudioMoniker, &pSelectedAudioMoniker);