Re: member access
On May 16, 10:23 am, Hari <pasalic.zahar...@gmail.com> wrote:
Valeriu Catina je napisao:
Hi,
consider the Shape class from the FAQ:
class Shape{
public:
Shape();
virtual ~Shape();
virtual void draw() = 0;
};
Now we have several derived shapes:
class Rectangle : public Shape{
public:
Rectangle();
void draw();
private:
double a; // width
double b; // height
};
class Circle{
public:
Circle();
void draw();
private:
double r; // radius
};
// more derived shapes follow
For each shape I would need to access its member variables. For this one
can add member functions in each derived class, for instance,
something like this in the Rectangle class:
double get_width() const
{
return a;
}
For a Shape derived class which would have many member variables, I
find it rather annoying and not very elegant to implement one get/set
member function for each member variable.
Is there any other way to gain access to the derived classes data,
(except making its member variables public)
Greetings,
Vali
You can break traditional OOP and make some member variables public.
Note that this can break OOP very badly, my style is to allow access
of member variables ONLY if changing this member variable will not
change internal class state - eg. in your class Circle, changing 'r'
will not change internal state, but if you have member function
get_area() that saves area of circle, than every change to 'r' from
outside will break class state :
class Circle{
public:
Circle();
void draw();
double get_area() { return area; }
void set_radius(double nr) { r = nr; area = r * r * pi; }
private:
double r;
double area;
};
In upper class, changing r with set_radius will change internal state
(area) and if user can access r directly, than this state will be bad.
You can change get_area like:
double get_area() { return r * r * pi; }
to allow direct access to 'r'. Note that this will cost some
additional computation, most time user will set radius once, and call
get_area when needed.
For small classes this can be easily done, but for some large, it is
not easy to see if direct change to member variable will change state
of class.
Note: this is my way of programming, and probably it is wrong way :)
Best,
Zaharije Pasalic
You don't need to store the area as a member, since that might change
anyways should the cricle's metrics/size be modified. And you don't
require a setter since you already have one - the ctor(s).
class Shape {
public:
Shape() { }
virtual ~Shape() = 0;
virtual double area() const = 0;
};
Shape::~Shape() { }
class Circle : public Shape {
double m_r;
public:
Circle(double r = 0.0)
: m_r(r) { }
double area() const { return m_r * m_r * 3.1416; }
};
class Rectangle : public Shape {
double m_w;
double m_h;
public:
Rectangle(double w = 0.0, double h = 0.0)
: m_w(w), m_h(h) { }
double area() const { return m_w * m_h; }
};
int main()
{
Circle circle(10);
std::cout << "circle's area = ";
std::cout << circle.area() << std::endl;
Rectangle rect(10, 10);
std::cout << "rect's area = ";
std::cout << rect.area() << std::endl;
}