Question

Here’s a very simple program using the function:

#include <windows.h>
#include <tchar.h>
#include <atlstr.h>
#include <mmdeviceapi.h>
#include <devicetopology.h>
#include <functiondiscoverykeys.h>

#include <iostream>

using namespace std;

int main()
{
    HRESULT hr;
    CComPtr<IMMDeviceEnumerator> pMMDeviceEnumerator;
    pMMDeviceEnumerator->GetDefaultAudioEndpoint(eCapture, eMultimedia, 0);
    //cout << hr;
    return 0;
}

When I try to run this, I get the following error:

Debug Assertion Failed!

Program: ...
File: c:\program files\microsoft visual studio 8\vc\atlmfc\include\atlcomcli.h
Line: 154

Expression: p!=0

What’s wrong with this? I’m just now trying to learn how to use this function. Thanks!

---

EDIT:

I’ve changed the program to this:

//#include <windows.h>
//#include <tchar.h>

#include <atlstr.h>
#include <mmdeviceapi.h>
//#include <devicetopology.h>
//#include <functiondiscoverykeys.h>

#include <iostream>

using namespace std;

// helper class to CoInitialize/CoUninitialize
class CCoInitialize {
private:
    HRESULT m_hr;
public:
    CCoInitialize(PVOID pReserved, HRESULT &hr)
        : m_hr(E_UNEXPECTED) { hr = m_hr = CoInitialize(pReserved); }
    ~CCoInitialize() { if (SUCCEEDED(m_hr)) { CoUninitialize(); } }
};

int main()
{


    CComPtr<IMMDeviceEnumerator> pMMDeviceEnumerator;
HRESULT hr = pMMDeviceEnumerator.CoCreateInstance(__uuidof(MMDeviceEnumerator));
if (FAILED(hr)) {
    cout << "failed" << endl;
    return __LINE__;
    }

CCoInitialize ci(NULL, hr);
pMMDeviceEnumerator->GetDefaultAudioEndpoint(eCapture, eMultimedia, 0);
//cout << hr;
return 0;
}

When I run it, I get the output of “failed”. What’s happening?

---

EDIT:

Alright, now I’ve changed the code enough to get it running all the way through without any failures. i.e.,

HRESULT hr = S_OK;  
cout << hr;
// initialize COM
CCoInitialize ci(NULL, hr);
if (FAILED(hr)) {
    cout << "failed1" << endl;
    return __LINE__;
}
cout << hr;
// get enumerator
CComPtr<IMMDeviceEnumerator> pMMDeviceEnumerator;
hr = pMMDeviceEnumerator.CoCreateInstance(__uuidof(MMDeviceEnumerator));
if (FAILED(hr)) {
    cout << "failed2" << endl;
    return __LINE__;
}
cout << hr;
// get default render/capture endpoints
CComPtr<IMMDevice> pRenderEndpoint;
hr = pMMDeviceEnumerator->GetDefaultAudioEndpoint(eRender, eConsole, &pRenderEndpoint);
if (FAILED(hr)) {
    cout << "failed3" << endl;
    return __LINE__;
}

cout << hr;
return 0;

Some of the trouble I was having earlier with this example (see comments on the answers) was fixed just by removing some of the code. But as I run this new body of the main() function, I get the output “0000”, meaning that cout << hr always evaluates to “0”. Is this a good thing? What info can I get about the default device now? hr. and hr-> don’t really bring up any menus, so I’m kind of in the dark. Thanks!

Answer 1

pMMDeviceEnumerator variable holds a pointer, which is NULL. When you try to call an interface method on this pointer, -> operator checks this nullness and issues an assertion failure.

Windows SDK samples show how to use this function and API, check them under: \Samples\multimedia\audio, e.g. osd sample.

This sample is a Win32-based application that demonstrates the use of the Vista APIs for monitoring the default audio output device and
its current volume setting. The sample is written in C++.

OSD does not run on earlier versions of Windows, including Windows XP, Windows 2000, Windows Me, and Windows 98.

UPD: Things in main one needs to reach the GetDefaultAudioEndpoint API call – Sample: find out if your default audio playback and audio capture devices are on the same hardware.