Compilation Error LNK2019

[Afandi]

I got this compilation error.

When I try to debug, I got these errors.

Error.jpg

main.obj:-1: error: LNK2019: unresolved external symbol "public: void __thiscall Keyboard::setWidget(class QWidget *)" (?setWidget@Keyboard@@QAEXPAVQWidget@@@Z) referenced in function _main

main.obj:-1: error: LNK2019: unresolved external symbol "public: __thiscall Keyboard::Keyboard(class QObject *)" (??0Keyboard@@QAE@PAVQObject@@@Z) referenced in function _main

And the source come from this code.

Keyboard::Keyboard(QObject *parent) : QObject(parent)
{
    theWidget = NULL;
}
 
void Keyboard::setWidget(QWidget* theWidget)
{
    this->theWidget = theWidget;
}

Keyboard::Keyboard(QObject *parent) : QObject(parent)
{
    theWidget = NULL;
}

void Keyboard::setWidget(QWidget* theWidget)
{
    this->theWidget = theWidget;
}

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I hope someone can tell me whether I got missing any things
or I need to change some things in the Build Setting.
Thank you