Question

我的C++友元函数为什么不能访问类的私有变量

//mytime2.h文件
#include <iostream>
using namespace std;

class Time
{
private:
int hours;
int minutes;
public:
Time();
Time(int h, int m = 0);
Time operator * (double n) const;

friend Time operator * (double m, const Time & t)
{
return t * m;
};

friend ostream & operator << (ostream & os, const Time & t);
};

#endif

//mytime2.h文件的实现文件
#include "mytime2.h"

Time::Time()
{
hours = minutes = 0;
}

Time::Time(int h, int m)
{
hours = h;
minutes = m;
}

Time Time::operator * (double mult) const
{
Time result;
long totalminutes = hours *mult *60 + minutes * mult;
result.hours = totalminutes /60;
result.minutes = totalminutes %60;
return result;
}

ostream & operator << (ostream & os, const Time & t)
{
os << t.hours << " hours, " << t.minutes << " minutes";
return os;
}

//使用类的主文件
#include <iostream>
#include "mytime2.h"
using namespace std;

int main()
{
Time A;
Time B(5,40);
Time C(2,55);

cout << "A,B,and C:\n";
/*cout << A << "; " << B << "; " << C << endl;
A = B + C;
cout << "B + C = " << A << endl;
A = B * 2.75;
cout << "B * 2.75 = " << A << endl;
cout << "10 * B = " << 10 * B << endl;*/

return 0;
}

我用VC6.0编译的文件，下面这个函数出现了编译错误
ostream & operator << (ostream & os, const Time & t)
{
os << t.hours << " hours, " << t.minutes << " minutes";
return os;
}

错误如下：
--------------------Configuration: mytime2 - Win32 Debug--------------------
Compiling...
mytime2.cpp
C:\Program Files\Microsoft Visual Studio\MyProjects\eight\mytime2\mytime2.cpp(79) : error C2248: 'hours' : cannot access private member declared in class 'Time'
c:\program files\microsoft visual studio\myprojects\eight\mytime2\mytime2.h(10) : see declaration of 'hours'
C:\Program Files\Microsoft Visual Studio\MyProjects\eight\mytime2\mytime2.cpp(79) : error C2248: 'minutes' : cannot access private member declared in class 'Time'
c:\program files\microsoft visual studio\myprojects\eight\mytime2\mytime2.h(11) : see declaration of 'minutes'
Error executing cl.exe.

mytime2.exe - 2 error(s), 0 warning(s)

我这是照着C++ primer plus的书敲进去的，一字不差的，作为初学者练习来的，但却不能运行，很郁闷！希望各位高手能赐教！

Answer 1

1. 将 #include <iostream> 改成 #include <iostream.h>
2. using namespace std; 删掉即可

刚编译通过了！

Answer 2

应该是可以的，不如你试试其他的编译器，我在dev-cpp做是可以的！

Answer 3

给你的vc6.0打上sp5补丁。
这是微软的bug。

Answer 4

我也有过同样的疑问，期待....