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geometricalObject.h:
#pragma once
class GeometricalObject
{
public:
    double Length = 0;
    double SurfaceArea = 0;

protected:
    virtual double GetSurfaceArea();
};

Square.h:
#pragma once
#include "GeometricalObject.h"
class Square :
    public GeometricalObject
{
public:
    Square(double length);

    double GetLength();
    void SetLength(double value);

    double GetSurfaceArea() override;

};

Square.cpp:
#include "Square.h"


Square::Square(double length)
{
    this->Length = length;
    this->SurfaceArea = GetSurfaceArea();
}

double GetLength()
{
    return Length;
}

void SetLength(double value)
{
    if (value >= 0)
    {
        GeometricalObject::Length = value;
    }
}

double GetSurfaceArea()
{
    return this->Length * this->Length;
}