关于重载提取运算符'

今天编写了下列代码（仅仅显示关键代码片段），在VS2015上运行正常，但是在VC6.0下出现错误提示。

#include
using namespace std;
class vct
{
private:
	double x, y;
public:
	//vct(double X = 0, double Y = 0) { x = X, y = Y; }
	//~vct() {}
	//void set(double X = 0, double Y = 0) { x = X, y = Y; }
	//vct operator+(vct& rival);
	//vct operator-(vct& rival);
	friend ostream& operator<<(ostream& out, const vct& rival);
};
ostream& operator<<(ostream& out, const vct& rival)
{
	out << "The Vector is (" << rival.x << ',' << rival.y << ")" << endl;return out;
}

此处重载了用于输出流对象cout的提取运算符'<<'，但是在VC6.0下提示：

error C2593: 'operator <<' is ambiguous

并且出现了无法访问私有成员变量的问题，尽管已经定义了友元函数：

error C2248: 'x' : cannot access private member declared in class 'vct'

error C2248: 'y' : cannot access private member declared in class 'vct'

解决办法是，在类定义之前首先声明类，并且将友元函数的函数头放置在前面，因为友元函数不是成员函数，这么做是合理的:

#include
using namespace std;
class vct;		//此处声明类
ostream& operator<<(ostream& out,const vct& rival);		//此处声明函数头
class vct
{
private:
	double x,y;
public:
	//vct(double X = 0, double Y = 0) { x = X, y = Y; }
	//~vct() {}
	//void set(double X = 0, double Y = 0) { x = X, y = Y; }
	//vct operator+(vct& rival);
	//vct operator-(vct& rival);
	friend ostream& operator<<(ostream& out,const vct& rival);//再次声明
};
ostream& operator<<(ostream& out,const vct& rival)	//友元重载函数定义
{
	out<<"The Vector is ("<