autofillbackground

http://doc.qt.io/qt-5/qwidget.html#autoFillBackground-prop

Basically, it controls if the background is filled before the Paint is called.

void AddDrop(QFrame * frame )  {
if (!frame) return;
QGraphicsDropShadowEffect *effect = new QGraphicsDropShadowEffect;
    effect->setBlurRadius(5);
    effect->setXOffset(5);
    effect->setYOffset(5);
    effect->setColor(Qt::black);
    frame->setAutoFillBackground(true);
    frame->setGraphicsEffect(effect);
}

and call it from mainwin (after setupUI ) like
AddDrop(ui->frame1);
AddDrop(ui->frame2);
AddDrop(ui->frame3);
etc

#include <QFrame>

class MyFrame : public QFrame {
  Q_OBJECT
 public:
 void drawshadow(.....) /// your func
  MyFrame(QWidget* parent = 0, Qt::WindowFlags f = 0): QFrame(parent, f) {
  }
 protected:
  virtual void paintEvent(QPaintEvent* e) override {
    QFrame::paintEvent(e); // pass event to base class
  }
};

But there a note:
You cannot draw outside of the QFrame. So keep that in mind.
Also you cannot use a Painter object outside of paintEvent.
But if you use the Effect classes, it might work.
http://doc.qt.io/qt-5/qgraphicsdropshadoweffect.html

So if its ok the shadow is inside the QFrame area, then u can use the paintEvent.
Elese the qgraphicsdropshadoweffect might help you out.

If you just need a shadow, it can be done with no function inside class.

QGraphicsDropShadowEffect *effect = new QGraphicsDropShadowEffect;
    effect->setBlurRadius(5);
    effect->setXOffset(5);
    effect->setYOffset(5);
    effect->setColor(Qt::black);
    ui->frame->setAutoFillBackground(true);
    ui->frame->setGraphicsEffect(effect);

Updated code to use setAutoFillBackground so drop is not drawn inside.

This is what i wanna do:

QFrame test ;
test.drawshadow(...);

but before i use drawshadow(); function i must have to first define it in the QFrame class so that QFrame object ( in this case test ) would work.

Used it where ?
You can always subclass a QFrame.