Visual C++ :: Member Function - Add Vertical Colour Text In Popup Menu

I was reading a c++ code written in embarcadero ide to add vertical colour text in popup menu.

While reading the code, I had seen following instructions.

private:

Code:
void __fastcall ExpandMenuItemWidth(TObject *Sender, TCanvas *ACanvas,int &Width, int &Height);

Code:
void __fastcall DrawNewItem(TObject *Sender, TCanvas *ACanvas, const TRect &ARect, bool Selected);

The above two methods used in the code like below:

Code:
PopupMenu1->Items->Items[i]->OnMeasureItem = ExpandMenuItemWidth;
PopupMenu1->Items->Items[i]->OnDrawItem = DrawNewItem;

What is happening when we are using member function without parameters like above?

I found above code at: [URL] ....