c++ - 从比较函数访问成员变量/getter

Question

我基本上是在尝试编写比较器函数，以按升序和降序对对象 P1 和 Pt2 的 x 然后 y 值进行排序。

现在我遇到了 x 和 y protected 错误，但是请问我如何正确使用 getter？我仍然很困惑。我尝试使用它但不起作用。

bool Line2D::sortLine2DPtAsc (const Point2D& left, const Point2D& right) 
{
    return (left.pt1.getX() < right.pt1.getX()) || ((left.pt1.getX() == right.pt1.getX()) && (left.pt1.getY() < right.pt1.getY()));
}

---

使用 getter 时出错:

Line2D.cpp:25:15: error: ‘const class Point2D’ has no member named ‘pt1’
Line2D.cpp:25:34: error: ‘const class Point2D’ has no member named ‘pt1’
Line2D.cpp:25:56: error: ‘const class Point2D’ has no member named ‘pt1’
Line2D.cpp:25:76: error: ‘const class Point2D’ has no member named ‘pt1’
Line2D.cpp:25:97: error: ‘const class Point2D’ has no member named ‘pt1’
Line2D.cpp:25:116: error: ‘const class Point2D’ has no member named ‘pt1’

---

这是我的代码:

class Line2D
{

    private:    
        Point2D pt1;
        Point2D pt2;

    public:
        //Constructor
        Line2D ();
        Line2D (Point2D pt1, Point2D pt2);

        //Accessors
        Point2D getPt1();
        Point2D getPt2();
        double getScalarValue();    //returns the value of attribute length

        //Mutators
        void setPt1 (Point2D pt1);
        void setPt2 (Point2D pt2);

        static bool sortLine2DPtAsc (const Point2D& left, const Point2D& right) ;
}


bool Line2D::sortLine2DPtAsc (const Point2D& left, const Point2D& right) 
{
    return (left.x < right.x) || ((left.x == right.x) && (left.y < right.y));
}

Answer 1

使用带有 getter 的版本，但将 left.pt1.getX() 替换为 left.getX() 等

我猜想在您的 Point2D 类中，您需要更改

double getX();

到

double getX() const;

getY 也是如此。

被传递 const Point2D& left, const Point2D& right，您根据契约(Contract)不得修改这些对象中的任何值。您只能调用声明为 const 的成员函数(它们不会更改对象)。