这段代码报错的原因是因为类Compare中的compareArea函数访问了类CRect的成员函数display，但是CRect并没有声明为Compare的友元类，因此无法访问其私有成员函数。
解决方法是在类Compare中添加一个友元声明：
friend void CRect::display();
完整代码如下：
#include
using namespace std;
class CRect;
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 << "}";
}
friend void CRect::display();
};
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() {
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;
}
