Resolved: C++ Problem with overriding base class variable

Question:

I have 3 classes that derive from each other:

class Shape
{
public:
    float r;
};

class ThreeDimentional : public Shape
{
public:
    virtual float area() = 0;
    virtual float volume() = 0;
};

class Sphere : public ThreeDimentional
{
public:
    float r;

    float area() {
        return 4*pi*pow(r, 2);
    }

    float volume() {
        return float(4)/3*pi*pow(r, 3);
    }
};

In main, I create an instance pointer of ThreeDimentional and set its value with a Sphere. And then change its r to 2. I think it somehow changes r of the base class? because it returns the volume as 0. Isn’t sphere supposed to override the r of base class? how can I change r of sphere?

int main()<br />
{<br />
    ThreeDimentional* s1 = new Sphere;<br />
    s1->r = 2;<br />
    cout << s1->volume() << endl;
}
[/code]

</p>



Output:<br> 


<p>[code]0

Answer:

You can’t override data members. You can override only virtual member functions.
If every Shape is supposed to have a radius, then Sphere shouldn’t declare another r (which will just hide the one in Shape depending on the context from where it is named).
If only a Sphere is supposed to have a radius, then it shouldn’t be possible to set r through a ThreeDimentional pointer, which ought to be agnostic about what kind of ThreeDimentional object the pointer is pointing to. (In circumstances where a decision must still be taken based on the derived type nonetheless, dynamic_cast can be used.)
Which of the two applies depends on your intended interpretation for “radius”/r, but typically only spheres have a radius in the strict sense.

If you have better answer, please add a comment about this, thank you!

Resolved: C++ Problem with overriding base class variable

Resolved: How to set consistent decimal separators in R data frame?

Resolved: How to resolve LNK2001 in c++ projects

Resolved: How to correctly initialize vector of number in Typescript

Leave A Reply