Why can't my derived class use a setter inherited from the abstract base class to change the value of a private member?

[theperson]

I have a public derived class called Rectangle and I'm trying to have a virtual function in Rectangle called calcArea() use the setter function called setArea() but I get 'double BasicShape::area' is private

I guess it's because the abstract base class is never and can never actually be instantiated but the assignment I'm doing says that it should be private and be used to hold the shape's area. I mean to me it makes much more sense to make it protected but... this is what the assignment says to do.

This is my abstract base class header

#ifndef BASICSHAPE_H
#define BASICSHAPE_H

class BasicShape
{
	private:
		double area;
	public:
		double getArea();
		
		//because area is private and is inaccessable to members of the derived
		//classes
		void setArea(double a);
		
		virtual double calcArea() const = 0; 
		

};

#endif 

And here is its .cpp file

#include "basicshape.h"

		double BasicShape::getArea()
		{
			return area;
		}
		
		//because area is private and is inaccessable to members of the derived
		//classes
		void BasicShape::setArea(double a)
		{
			area = a;
		}

This is my Rectangle header

#ifndef RECTANGLE_H
#define RECTANGLE_H

#include "basicshape.h"

class Rectangle : public BasicShape
{
	private:
		long width;
		long length;
		
	public:
		Rectangle(long w, long l);
		
		long getWidth();
		long getlength();
		
		virtual double calcArea() const;
};

#endif 

and my rectangle.cpp file

#include "rectangle.h"

Rectangle::Rectangle(long w, long l)
{
	width = w;
	length = l;
}

double Rectangle::calcArea() const
{
	//need to convert area from long to double because 
	//area variable in basicShape.h is a double
	long longArea = width * length;
	double doubleArea = static_cast<double>(longArea);
	setArea(area);
}

[MikeyBoy]

Look at the name of the variable you're using to store the results of the calculation.

Look at the name of the variable you're trying to pass into setArea().

Notice anything?

[nvrmnd]

<wrong>
the member area from the base class BasicShape is declared in the private scope, classes that will derive from it won't be able to access it.

Try specifying it's scope w/ protected
</wrong>
LOL i',m wrong, i didn't notice what setArea() is for

[theperson]

Thanks, Mikey. I changed Rectangle's calcArea() to this and now the error about area being private is gone :D I have a new error passing 'const Rectangle' as 'this' argument of 'void BasicShape::setArea(double)' discards qualifiers [-fpermissive] but at least that old one is gone!

double Rectangle::calcArea() const
{
	//need to convert area from long to double because 
	//area variable in basicShape.h is a double
	
	
	long longArea = width * length;
	double doubleArea = static_cast<double>(longArea);
	setArea(doubleArea);
	
}

[MikeyBoy]

You've defined calcArea() to be const, but you're trying to call a non-const method. That's illegal.

If the purpose of calcArea() is to change the state of the object (by changing the value of the area data member), then it shouldn't be defined as const.

[theperson]

I get the same error if I change the virtual function to not be a const though

#ifndef BASICSHAPE_H
#define BASICSHAPE_H

class BasicShape
{
	private:
		double area;
	public:
		double getArea();
		
		//because area is private and is inaccessable to members of the derived
		//classes
		void setArea(double a);
		
		virtual double calcArea() = 0; 
		

};

#endif 

[MikeyBoy]

Well, you'll need to change the declaration and definition in the derived function to also not be const.

[theperson]

Oh, yeah good point. Thanks, it compiles without errors now. =)

[MikeyBoy]

You're welcome :)