How to make a frameless QWidget draggable?

main.cpp

// Qt
#include <QApplication>

// MyWidget
#include "MyWidget.h"

int main( int argc, char* argv[] )
{
    QApplication app( argc, argv );    
    
    MyWidget widget;
    widget.show();
    
    return app.exec();
}

MyWidget.cpp

// Qt
#include <QMouseEvent>

// MyWidget
#include "MyWidget.h"

MyWidget::MyWidget( QWidget *parent ) :
    QWidget( parent ),
    _mousePressed(false),
    _mousePosition(QPoint())
{
    setWindowFlags( Qt::FramelessWindowHint );    
}

MyWidget::~MyWidget()
{
    // delete stuff.
}

void MyWidget::mousePressEvent( QMouseEvent *e )
{
    if ( e->button() == Qt::LeftButton ) {
        _mousePressed = true;
        _mousePosition = e->pos();
    }   
}

void MyWidget::mouseMoveEvent( QMouseEvent *e )
{
    if ( _mousePressed ) {
        move( mapToParent( e->pos() - _mousePosition ) );
    }    
}

void MyWidget::mouseReleaseEvent( QMouseEvent *e )
{
    if ( e->button() == Qt::LeftButton ) {
        _mousePressed = false;
        _mousePosition = QPoint();
    }
}

MyWidget.h

#ifndef MYHEADER_H 
#define MYHEADER_H

// Qt
#include <QWidget>

class MyWidget : public QWidget
{
    Q_OBJECT
public:
    explicit MyWidget( QWidget *parent = 0 );
    ~MyWidget();
    
protected:
    virtual void mousePressEvent( QMouseEvent *e );
    virtual void mouseMoveEvent( QMouseEvent *e );
    virtual void mouseReleaseEvent( QMouseEvent *e );
    
private:
    bool _mousePressed;
    QPoint _mousePosition;
};

#endif

Nice!