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Virtual Functions
Class derivation is even more valuable when combined
with virtual functions--functions that are declared in a
base class, but implemented differently in each derived
class. Consider the Shapeclass:It will need a grow
function so that every Shapecan increase its size.
class Shape {
public:
void grow();
....
};
But the grow function must be different for each class
derived from Shape. Thus we make grow a virtual function
class Shape {
public:
virtualvoid grow ();
........
};
The Circle, Square and Triangle classes will now provide
their own custom versions of grow. The circle version
might look like:
class Circle: public Shape {
public:
void grow () { radius++; }
......
private:
int radius;// radius of circle
....
}
If cis a Circleobject, the call c.grow()increases the
radius of c. When grow()is invoked through a pointer to a
base class, the virtual function uses the correct grow
function depending on the kind of object pointed to.
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