how controll system volume in qslider

[breakthecode]

i want to control system volume by using qslider i write some code but some problem with build

void dcsinter::volume()
 
    {
 
    ui->horizontalSlider->setMinimum(0);
 
    ui->horizontalSlider->setMaximum(100);
 
    ui->horizontalSlider->setSingleStep(5);
 
    int pos = ui->horizontalSlider->sliderPosition();
 
    qreal apos = pos/100;
 
 
 
    Phonon::AudioOutput *audio;
 
 
 
    audio.setVolume(apos);
 
    }

void dcsinter::volume()
     
    {
     
    ui->horizontalSlider->setMinimum(0);
     
    ui->horizontalSlider->setMaximum(100);
     
    ui->horizontalSlider->setSingleStep(5);
     
    int pos = ui->horizontalSlider->sliderPosition();
     
    qreal apos = pos/100;
     
     
     
    Phonon::AudioOutput *audio;
     
     
     
    audio.setVolume(apos);
     
    }

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build issues are

'undefined reference to `_imp___ZN6Phonon11AudioOutput9setVolumeEd'

'undefined reference to `_imp___ZNK6Phonon11AudioOutput6volumeEv'

'collect2: ld returned 1 exit status'

i can't understand what the issue here

thanks

[Zlatomir]

Do you have

QT += phonon

QT += phonon

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in your .pro file?
And another issue is that you create audio as a pointer *audio; (which is not initialized - at least in the code you showed) and then you use audio as an object, with operator . instead of -> which you should use for a pointer and don't forget to create an object before using the audio pointer to call setVolume() or other stuff.