c++定义一个基类Shape，在此基础上派生出Rectangle和Circle，二者都有一个GetArea()函数计算对象面积。使用Rectangle类创建一个派生类Square。

好的，针对您的问题，以下是所需的代码实现：

class Shape {
public:
    virtual double GetArea() = 0;
};

class Rectangle : public Shape {
private:
    double width, height;
public:
    Rectangle(double w, double h) {
        width = w;
        height = h;
    }
    double GetArea() {
        return width * height;
    }
};

class Circle : public Shape {
private:
    double radius;
public:
    Circle(double r) {
        radius = r;
    }
    double GetArea() {
        return 3.14159 * radius * radius;
    }
};

class Square : public Rectangle {
public:
    Square(double s) : Rectangle(s, s) {}
};

int main() {
    Shape* shape1 = new Rectangle(5, 10);
    Shape* shape2 = new Circle(5);
    Shape* shape3 = new Square(5);

    cout << "Rectangle area: " << shape1->GetArea() << endl;
    cout << "Circle area: " << shape2->GetArea() << endl;
    cout << "Square area: " << shape3->GetArea() << endl;

    delete shape1;
    delete shape2;
    delete shape3;

    return 0;
}

上述代码中，Shape是基类，Rectangle和Circle是Shape的派生类。Square则是以Rectangle为基类的派生类。每个类都有一个GetArea()函数，用于计算图形的面积。在主函数中，我们创建了一个Rectangle对象、一个Circle对象和一个Square对象，并输出它们的面积。最后，记得释放已分配的内存。