初始化基class的引用成员

Question

经过长时间的禁欲，我目前正在尝试刷新我的 C++ 技能。因此，我正在实施一个微型绘图圆圈图像程序。

场景：我有一个名为 Shape 的基础 class，它基本上由一个成员组成，它是对 Point 的引用，表示中央。 Circle 是一个 Shape，它除了圆心之外还有一个半径。

class Shape
{

public:
    Shape(Point& p);
    ~Shape();
    Point& get_center() const;

private:
    Point& center;
};

-

class Point
{
public:
    Point();
    Point(float x, float y);
    Point(short d);
    ~Point();
    float get_x() const;
    float get_y() const;
    friend std::ostream& operator<< (std::ostream& stream, const Point& p);

private:
    short dimension;
    float x, y;
};

//.cpp
std::ostream& operator<<(std::ostream &strm, const Point &p) {
    return strm << "Point(" << p.get_x() << "," << p.get_y() << ")";
}

-

class Circle :
    public Shape
{
public:
    Circle(Point p);
    Circle(Point p, float radius);
    ~Circle();
    float get_radius() const;

private:
    float radius;
    typedef Shape super;
};

成员的初始化执行如下：

Circle::Circle(Point p, float rad) : Shape(p), radius(rad)
{
    std::cout << "Called circle constructor with center point " << p << std::endl;
}

Shape::Shape(Point& p) : center(p) {
    std::cout << "Called Shape constructor with point " << p << "a" <<std::endl;
}

如果我是运行下面的代码，似乎中心变量设置不正确，导致下面给出的输出 A。奇怪的是，如果我将更多输出附加到 Shape 构造函数（在本例中为字母 "a"），则对中心 Point[=42= 的引用] 似乎已设置（输出 B）。此外，两个 Circles 似乎引用相同的 Point?!有人可以解释为什么会这样吗？处理这种情况的适当方法是什么？

Circle& cir1 = *(new Circle(Point(400, 550), 50)); Circle& cir2 = *(new Circle(Point(300, 550), 80)); vector<Circle> circles { cir1, cir2 }; cout << "Iterating circles" << endl; for (auto& cir : circles) { cout << "Radius: "<< cir.get_radius() << " center: " << cir.get_center() << endl; }

输出A：

Called Shape constructor with point Point(400,550) Called circle constructor with center point Point(400,550) Called Shape constructor with point Point(300,550) Called circle constructor with center point Point(300,550) Iterating circles Radius: 50 center: Point(1.87436e-38,1.87419e-38) Radius: 80 center: Point(1.87436e-38,1.87419e-38)

输出 B:

Called Shape constructor with point Point(400,550)a Called circle constructor with center point Point(400,550) Called Shape constructor with point Point(300,550)a Called circle constructor with center point Point(300,550) Iterating circles Radius: 50 center: Point(300,550) Radius: 80 center: Point(300,550)

Answer 1

这里有问题：

Circle::Circle(Point p, float rad) : Shape(p), radius(rad)

{
    std::cout << "Called circle constructor with center point " << p << std::endl;
}

Shape::Shape(Point& p) : center(p) {
    std::cout << "Called Shape constructor with point " << p << "a" <<std::endl;
}

因为 Shape 存储了对 Circle 的 p 参数的引用。构建结束，参数消失

我可能不会存储一个引用，而是一个值。这样你就有了参数的副本。复制一个Point不是很贵。

Answer 2

Shape class 的正确界面可能如下所示：

class Shape
{
public:
    explicit Shape(const Point& p); //const Point& ensures you can all that constructor with an R-value
    virtual ~Shape();

    const Point& get_center() const;
private:
    Point center;

};

同样，你的Circleclass也应该在构造函数中使用const Point&。

关于你的主程序，也许做这样的事情

std::shared_ptr<Circle> cir1(new Circle(Point(400, 550), 50));
std::shared_ptr<Circle> cir2( new Circle(Point(300, 550), 80));

vector< std::shared_ptr<Circle> > circles { cir1, cir2 };

cout << "Iterating circles" << endl;
for (auto cir : circles)
{
    cout << "Radius: "<< cir->get_radius() << " center: " << cir->get_center() << endl;
}

关于您的代码的问题是您引用了 R 值（例如，参见 http://eli.thegreenplace.net/2011/12/15/understanding-lvalues-and-rvalues-in-c-and-c ）。希望对您有所帮助。

初始化基class的引用成员

Initialize reference member of base class

c++

oop

abstract-class