C++ 将指针从一个派生类转换为另一个派生类

Question

假设我有一个类 shape 和 2 个派生类 circle 和 square。代码是:

Shape* s1 = new circle;

现在我想将 s1 赋值给 square，同时保留两者共有的变量。

Shape* s1 = new Square;

我该怎么做？

Answer 1

通过使用引用基类的构造函数，您可以轻松复制常见的 Shape 数据:

#include <assert.h>

enum class Color { red, green, blue };

class Shape {
  public:
    Shape() : color(red) { }
    void setColor(Color new_color) { color = new_color; }
    Color getColor() const { return color; }
  private:
    Color color;
};

class Square : public Shape {
  public:
    Square() { }
    // Using explicit constructor to help avoid accidentally
    // using the wrong type of shape.
    explicit Square(const Shape &that) : Shape(that) { }
};

class Circle : public Shape {
  public:
    Circle() { }
    explicit Circle(const Shape &that) : Shape(that) { }
};

int main(int,char**)
{
  Circle circle;
  circle.setColor(Color::blue);
  Square square(circle);
  assert(circle.getColor()==square.getColor());
}