C++ 类之间友元函数和函数声明的错误解决方法

#include using namespace std;

class Compare;

class CRect { int m_length; int m_width; public: friend void Compare::compareArea(const Compare& r1, const Compare& r2); CRect(int l, int w) :m_length(l), m_width(w) {} void display(); };

class Compare { int m_length; int m_width; public: void compareArea(const Compare& r1, const Compare& r2) { double s1, s2; s1 = r1.m_length * r1.m_width; s2 = r2.m_length * r2.m_width; r1.display(); if (s1 > s2) cout << "的面积大于"; else if (s1 == s2) cout << "的面积等于"; else cout << "的面积小于"; r2.display(); } Compare(int l, int w) :m_length(l), m_width(w) {} void display()const { cout << "{" << m_length << "," << m_width << "}"; } };

void CRect::display() { cout << "{" << m_length << "," << m_width << "}"; }

int main() { Compare r1(5, 2), r2(4, 3); r1.compareArea(r1, r2); CRect r3(6, 3), r4(4, 2); r3.compareArea(r3, r4); return 0; }

在类 CRect 中，需要添加一个函数声明 void display()，以便在类 Compare 中使用。同时，在类 CRect 中，需要添加一个友元函数声明 friend void Compare::compareArea(const Compare& r1, const Compare& r2);，以便在类 Compare 中使用。修改后的代码如下：

#include <iostream>
using namespace std;

class Compare;

class CRect
{
    int m_length;
    int m_width;
public:
    friend void Compare::compareArea(const Compare& r1, const Compare& r2);
    CRect(int l, int w) :m_length(l), m_width(w) {}
    void display();
};

class Compare
{
    int m_length;
    int m_width;
public:
    void compareArea(const Compare& r1, const Compare& r2)
    {
        double s1, s2;
        s1 = r1.m_length * r1.m_width;
        s2 = r2.m_length * r2.m_width;
        r1.display();
        if (s1 > s2)
            cout << "的面积大于";
        else if (s1 == s2)
            cout << "的面积等于";
        else
            cout << "的面积小于";
        r2.display();
    }
    Compare(int l, int w) :m_length(l), m_width(w) {}
    void display()const {
        cout << "{" << m_length << "," << m_width << "}";
    }
};

void CRect::display() {
    cout << "{" << m_length << "," << m_width << "}";
}

int main()
{
    Compare r1(5, 2), r2(4, 3);
    r1.compareArea(r1, r2);
    CRect r3(6, 3), r4(4, 2);
    r3.compareArea(r3, r4);
    return 0;
}