Abstract Class Problems

[Pratik K]

Hey guys, need some help. I'm new to the concept of abstract classes, and am getting the error 'lvalue require as left operand.." I can't seem to figure out where the compiler is running into the problem. Here's the base abstract class and one of the derived classes:

// Base Class 

class figure
{
    int area;
    float volume;
public:
    void display () { cout<<"\n Area : "<<area<<"\n Volume : "<<volume; }
    virtual void calc()=0;
protected:
   virtual void getv()=0;
    int getarea() { return area;   }
    float getvol()  { return volume; }
};


//derived class


class circle : public figure
{
    int radius;
    void getv() { cout<<"\n Enter the radius : "; cin>>radius; }
   void calc() { getv(); getarea()=4*3.14*radius*radius; getvol()=(4/3)*3/14*radius*radius*radius; display();  }
};

Any help/information/insight would be appreciated. Thanks!

[Pratik K]

Silly mistake, the getarea and getvol functions have to be passed by reference

Edited code :

class figure
{
    int area;
    float volume;
public:
    void display () { cout<<"\n Area : "<<area<<"\n Volume : "<<volume; }
    virtual void calc()=0;
protected:
   virtual void getv()=0;
    int    &getarea() { return area;   }
    float  &getvol()  { return volume; }
};

class circle : public figure
{
    int radius;
    void getv() { cout<<"\n Enter the radius : "; cin>>radius; }
    void calc() { getv(); getarea()=4*3.14*radius*radius; getvol()=(4/3)*3.14*radius*radius*radius; figure::display();  }
};

[Ericool]

This is different from c# properties , the method getarea is a getter that means it simply returns a value and does not operate on it like a setter. In your case create setarea(int value ) and setvol(float value ). Also you should add the keyword const at the end of the method like so : int getarea() const , which means your method does not modify an attribute of the class.

Did this help?

[Pratik K]

How about the second code I posted? I think that solves the problem. Cause, I think for every class that is derived from figure, a copy of area and volume are created, right? I want to minimize unnecessary variable usage, and I guess my method achieves it?

But what you said seems interesting, code you show me an example?

[Ericool]

What you did is breaking encapsulation. Yes it works, for educational purposes it might pass but in enterprise I would kill the guy who let any client access any account , if you what I mean. There is keyword call property , which is not really used in c++ but achieve the same purpose. When I examine your code I see that only your derived class access to that method so instead you should put int area and float volume as protected.

[Pratik K]

Oh okay, yeah I get what you mean, you're saying instead of declaring functions to get the area and volume I should just directly put area and volume variables into protected?

There is keyword call property , which is not really used in c++ but achieve the same purpose.

Could you elaborate further?

[Ericool]

This is an example found at this link

https://msdn.microsoft.com/en-us/library/yhfk0thd.aspx

// declspec_property.cpp
struct S {
   int i;
   void putprop(int j) { 
      i = j;
   }

   int getprop() {
      return i;
   }

   __declspec(property(get = getprop, put = putprop)) int the_prop;
};

int main() {
   S s;
   s.the_prop = 5;
   return s.the_prop;
}

where the_prop is a property , which mean acts like a setter and getter for int i , in this example . In c++ we use instead accessor and mutator -> getter and setter to get or set an attribute.

To answer to your first question , attributes should be protected , since you wont need to call method in your derived class , and you put a setter and getter for each of the attribute as public (if you want).

Was that elaborated enough ?

[MiiNiPaa]

Please note that attribute specifier "property" is not a part of C++ and only supported by Visual Studio.

[Ericool]

I should have mentionned that ..my bad

[Pratik K]

Oh okay, though, according to what MiiNiPaa said, I wouldn't be able to use this in say Code::Blocks, ie, C++ in general? :l Is there any alternative?

(ps- sorry I took a long time to reply, had exams going on..)

[coder777]

Just make the member variables protected and use them directly. So why not? Dont't produce overhead.

This property stuff is rather nonsense. Why would you want declare a pseudo variable?