计算机小白遇到一道C++题目，麻烦大神帮忙指点迷津

Question

题目说明如上图，这个运算符重载搞不明白啊

Answer 1

#include <iostream>
#include <string>
using namespace std;
class Shape
{
public:
    string name;
    Shape(string n) : name(n) { }
    virtual void area() = 0;
};
class Rectangle : public Shape
{
private:
    int abs(int n) { return n > 0 ? n : -n; }
public:
    int width;
    int height;
    Rectangle(string n, int w, int h) : Shape(n), width(w), height(h) { }
    int operator -(const Rectangle r)    
    {
        return abs(width * height - r.width * r.height);
    }
    void area()
    {
        cout << name << " " << width * height << endl;
    }
};
class Circle : public Shape
{
public:
    int radius;
    Circle(string n, int r) : Shape(n), radius(r) { }
    void area()
    {
        cout << name << " " << radius * radius * 3.14 << endl;
    }
};
int main()
{
    Rectangle r1("r1", 3, 6), r2("r2", 1, 1);
    r1.area();
    r2.area();
    cout << r1 - r2 << endl;
    cout << r2 - r1 << endl;
    Circle c("c", 3);
    c.area();
    return 0;
}

r1 18
r2 1
17
17
c 28.26

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