QCoreApplication: "CoInitialize has not been called" on QAudioDeviceInfo::availableDevices. QApplication is OK.

[cdwijs]

Hi All,

I'm writing a console application that will plays wav files. When I use QApplication everything works as expected, when I use QCoreApplication, I get this error: when I call QAudioDeviceInfo::availableDevices.

onecore\com\combase\objact\objact.cxx(836)\combase.dll!76F4E86C: (caller: 76F4D83B) ReturnHr(1) tid(3a70) 800401F0 CoInitialize has not been called.

Is there, besides increased memory usage, a downside of using QApplication instead of QCoreApplication for console application?
Is there another way to fix this error?
Why does QAudioDeviceInfo::availableDevices() need the qt gui framework to be loaded?

I've made a minimal program that shows the error:

//#include "mainwindow.h"

#include <QApplication>
#include <QDebug>
#include <QAudioDeviceInfo>
QAudioDeviceInfo m_deviceInfo;

int main(int argc, char *argv[])
{
    //QApplication a(argc, argv); //no warnings
    QCoreApplication a(argc, argv); /*generates these warnings:
onecore\com\combase\objact\objact.cxx(836)\combase.dll!76F4E86C: (caller: 76F4D83B) ReturnHr(1) tid(3a70) 800401F0 CoInitialize has not been called.
onecore\com\combase\objact\objact.cxx(836)\combase.dll!76F4E86C: (caller: 76F4D83B) ReturnHr(2) tid(3a70) 800401F0 CoInitialize has not been called.
onecore\com\combase\objact\objact.cxx(836)\combase.dll!76F4E86C: (caller: 76F4D83B) ReturnHr(3) tid(3a70) 800401F0 CoInitialize has not been called.
onecore\com\combase\objact\objact.cxx(836)\combase.dll!76F4E86C: (caller: 76F4D83B) ReturnHr(4) tid(3a70) 800401F0 CoInitialize has not been called.*/
    int idx = 1;
    const QAudio::Mode mode = idx == 0 ? QAudio::AudioInput : QAudio::AudioOutput;
    for (auto &deviceInfo: QAudioDeviceInfo::availableDevices(mode))
    {
        qDebug()<<deviceInfo.deviceName();
    } /*debug output in both cases:
"Speakers (High Definition Audio Device)"
"1 - CF791 (AMD High Definition Audio Device)"
"Digital Audio (S/PDIF) (High Definition Audio Device)"
"Speakers/Headphones (Realtek(R) Audio)"*/
    //MainWindow w;
    //w.show();
    return a.exec();
}

My versions:

Qt Creator 4.12.0
Based on Qt 5.14.2 (MSVC 2017, 32 bit)
Built on Apr 22 2020 01:29:13
From revision 10fece1795

Windows 10 Enterprise version 1903 OS build 18362.1016

My compile output:

Starting: "C:\Qt\Tools\mingw730_32\bin\mingw32-make.exe" -j12
C:/Qt/Tools/mingw730_32/bin/mingw32-make -f Makefile.Debug
mingw32-make[1]: Entering directory '[..]/New folder/build-untitled5-Desktop_Qt_5_14_2_MinGW_32_bit-Debug'
g++ -c -fno-keep-inline-dllexport -g -std=gnu++11 -Wall -Wextra -Wextra -fexceptions -mthreads -DUNICODE -D_UNICODE -DWIN32 -DMINGW_HAS_SECURE_API=1 -DQT_DEPRECATED_WARNINGS -DQT_QML_DEBUG -DQT_MULTIMEDIA_LIB -DQT_WIDGETS_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_CORE_LIB -DQT_NEEDS_QMAIN -I..\untitled5 -I. -IC:\Qt\5.14.2\mingw73_32\include -IC:\Qt\5.14.2\mingw73_32\include\QtMultimedia -IC:\Qt\5.14.2\mingw73_32\include\QtWidgets -IC:\Qt\5.14.2\mingw73_32\include\QtGui -IC:\Qt\5.14.2\mingw73_32\include\QtANGLE -IC:\Qt\5.14.2\mingw73_32\include\QtNetwork -IC:\Qt\5.14.2\mingw73_32\include\QtCore -Idebug -I. -IC:\Qt\5.14.2\mingw73_32\mkspecs\win32-g++  -o debug\main.o ..\untitled5\main.cpp
g++ -Wl,-subsystem,windows -mthreads -o debug\untitled5.exe debug/main.o debug/mainwindow.o debug/moc_mainwindow.o  C:\Qt\5.14.2\mingw73_32\lib\libQt5Multimedia.a C:\Qt\5.14.2\mingw73_32\lib\libQt5Widgets.a C:\Qt\5.14.2\mingw73_32\lib\libQt5Gui.a C:\Qt\5.14.2\mingw73_32\lib\libQt5Network.a C:\Qt\5.14.2\mingw73_32\lib\libQt5Core.a  -lmingw32 C:\Qt\5.14.2\mingw73_32\lib\libqtmain.a -LC:\openssl\lib -LC:\Utils\my_sql\mysql-5.7.25-win32\lib -LC:\Utils\postgresql\pgsql\lib -lshell32 
mingw32-make[1]: Leaving directory '[..]/New folder/build-untitled5-Desktop_Qt_5_14_2_MinGW_32_bit-Debug'

[Christian Ehrlicher]

I would guess that QtMultimedia at least needs a QGuiApplication

[Bonnie]

It's quite usual to have some COM warnings in debug mode.
I would ignore those if there're no other problems.