基类指针，调到各自的版本

（本条路线统一用 g++ -std=c++17 -O0 -Wall 编译）

#include <iostream>

struct Shape {
    virtual int area() const { return 0; }
    virtual ~Shape() = default;
};

struct Rect : Shape {
    Rect(int w, int h) : w_(w), h_(h) {}
    int area() const override { return w_ * h_; }
private:
    int w_, h_;
};

struct Square : Shape {
    explicit Square(int s) : s_(s) {}
    int area() const override { return s_ * s_; }
private:
    int s_;
};

int main() {
    Rect r(3, 4);
    Square q(5);

    /* 基类指针，调到的是各自的版本 */
    const Shape *ps[2] = {&r, &q};
    int total = 0;
    for (const Shape *p : ps) total += p->area();

    std::cout << r.area() << "/" << q.area() << "/" << total << "\n";
    return 0;
}