运算符中的二元重载，为什么要调用友元函数而不是全局函数的问题

#include <iostream>

using namespace std;
//实现运算符的重载
class A
{
public:
	A(int real=0,int imaginary=0 )
	{
		this->real=real;
		this->imaginary=imaginary;
	}

	void printf()
	{
		cout<<this->real<<" + "<<this->imaginary<<"j
";
	}
	friend A operator+(A &a1,A &a2);//这里的友元函数是取决于私有属性
public:
	A  operator-(A &t1)//这个一定要表明类型的,因为这不是构造函数，所以必须有类型。
	{
		this->real=this->real-t1.real;
		this->imaginary=this->imaginary-t1.imaginary;
		return * this;
	}
private:
	int real;
	int imaginary;
};

A operator+(A &a1,A &a2)
{
	A tem(a1.real+a2.real , a1.imaginary+a2.imaginary);
	return tem;
}

int main()
{
	A c1(1,2), c2(3,4);
	A  c3;
	c3=c1+c2;
	c3.printf();
	 
//	A t1;
	//c1.operator-(c2);
	A c4 =c1-c2;
	c4.printf();
	system("pause");
	return 0;
}

　　因为，类的成员变量一般是私有的，所以，类的外部除了友元函数其他函数都不能调用类的私有变量。