c++ - 无法访问好友类的私有(private)成员

Question

有人愿意帮我解决 C++ 链接/编码难题吗？

我有一个 Shape 类。 Shape 需要使用类 Center 的私有(private)数据成员，x 和 y 坐标。我声明 friend 类 Shape；然后在 Shape.h 中#include "center.h"。在 Shape.cpp 中，我定义了我的 ostream& operator<< (ostream& ostr, const Center& c) 函数，它使用 c.xCord； c.yCord 访问中心的私有(private)数据成员。

当我尝试编译 Shape.cpp 时，我遇到了那些数据变量的访问错误，比如我没有将 Shape 声明为友元类。我觉得这与编译时的链接顺序有关。我该如何解决这个问题？

#ifndef CENTER_H
#define CENTER_H

class Center
{
    public:
        Center(double x, double y) { xCord = x; yCord = y; }
            // constructor
        friend class Shape;
            // allow Shape to use Center's x and y values

    private:
        double xCord;
            // X-coordinate
        double yCord;
            // Y-coordinate
};

#endif

#ifndef SHAPE_H
#define SHAPE_H

#include "center.h"
#include <iostream>
using namespace std;

class Shape
{
    public:
        Shape(double x, double y) : s_center(x, y) {}
            // constructor
        void moveCenter();
            // moves the center of the shape
        friend ostream& operator<< (ostream& ostr, const Center& c);
            // allows the printing of the Center object

        virtual void printCenter();
            // returns the center of the shape
        virtual double printArea();
            // returns the area of the shape

        virtual bool checkSurface(Shape& s) = 0;
            // checks if the shape can fit into
            // a given surface
        virtual double findArea() = 0;
            // calculates the area of the shape

    private:
        Center s_center;
            // center of the shape
};

#endif

// in shape.cpp
ostream& operator<< (ostream& ostr, const Center& c)
{
    ostr << "(" << c.xCord << ", " << c.yCord << ")";
    return ostr;
}

Answer 1

根据 C++11 标准的第 11.3/10 段:

Friendship is neither inherited nor transitive. [...]

如果类A是 friend类B和函数 f()是 friend类A , 这不会使 f()一个friend类B

您应该声明您的 operator <<作为friend类Center如果你想让它访问 Center 的私有(private)成员变量:

#ifndef CENTER_H
#define CENTER_H

#include <ostream>

class Center
{
public:
    Center(double x, double y) { xCord = x; yCord = y; }

    friend class Shape;
    friend std::ostream& operator<< (std::ostream& ostr, const Center& c);
//  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

private:
    double xCord;
    double yCord;
};

#endif