29 The Power of Inheritance and Polymorphism

WindowRep and Window classes 1027Frame+------------------+|Health 100|+------------------++------------------+|Direction |+------------------+LabelNumberoutput hereCursor positioned so thatinput characters appear hereFigure 29.3NumberItem and EditText WindowsNumberItemThe class declaration for NumberItem is:class NumberItem : public Window {public:NumberItem(int x, int y, int width, char *label,long initval = 0);void SetVal(long newVal);long GetVal() { return fVal; }private:void SetLabel(int s, char*);void ShowValue();long fVal;int fLabelWidth;};In addition to the character arrays and dimension data that a NumberItem objectalready has because it is a kind of Window, a NumberItem owns a long integerholding the value to be displayed (and, also, an integer to record the number ofcharacters used by the label so that numeric outputs don't overwrite the label).The constructor for class NumberItem completes the normal initializationprocesses of class Window. The auxiliary private member function SetLabel() isused to copy the given label string into the background array. The inheritedPrepareContent() function loads the current array from the background and addsthe frame. Finally, using the auxiliary ShowValue() function, the initial number isconverted into characters that are added to the current image array.Once constructed, a NumberItem can be used by the program. Usually, usagewill be restricted to just three functions – GetVal() (return fVal;), SetVal()(changes fVal and uses ShowValue() to update the current image array), andShowAll() (the function, inherited from class Window, that gets the windowdisplayed).What does aNumberItem own?What does aNumberItem do?