抽象类的运算符重载在继承时遇到问题

[Scorpiour]

using namespace std;

void displayInfo();

class MEDIA
{
	protected:
		int ID;
		string Title;
		int Year;
	public:
		MEDIA()
		{
			ID=0;
			Title="";
			Year=0;
		}
		virtual ~MEDIA(){}
		virtual void changeID(int)=0;
		virtual void Print(ostream&)const=0;
		friend ostream& operator <<(ostream&,const MEDIA&);
};

ostream& operator <<(ostream out,const MEDIA& M)
{
	M.Print(out);
	return out;
}

class DVD:public MEDIA
{
	private:
		string Director;
	public:
		DVD(int newID,const char* newTitle,int newYear,const char* name="Unknown")
		{
			ID=newID;
			Title=newTitle;
			Year=newYear;
			Director=name;
		}
		
		virtual void changeID(int newID)
		{
			ID=newID;
		}
		
		virtual void Print(ostream& out)const
		{
			out<<"DVD: ID"<<ID<<"\tTitle "<<Title<<"\t("<<Year<<")\tDirected by "<<Director<<endl;
		}

};


class Book:public MEDIA
{
	private:
		string Author;
		int NumPage;
	public:
		Book(int newID,const char* newTitle,int newYear,const char* name,int page)
		{
			ID=newID;
			Title=newTitle;
			Year=newYear;
			Author=name;
			NumPage=page;
		}
		
		virtual void changeID(int newID)
		{
			ID=newID;
		}
		
		virtual void Print(ostream& out)const
		{
			out<<"Book: ID"<<ID<<"\tTitle "<<Title<<"\t("<<Year<<")\t by "<<Author<<",\t"<<NumPage<<" pages"<<endl;
		}
};



class Journal:public MEDIA
{
	private:
		int Volumn;
		int Number;
	public:
		Journal(int newID,const char* newTitle,int newYear,int newVol,int num)
		{
			ID=newID;
			Title=newTitle;
			Year=newYear;
			Volumn=newVol;
			Number=num;
		}
				
		virtual void changeID(int newID)
		{
			ID=newID;
		}
		
		virtual void Print(ostream& out)const
		{
			out<<"Journal: ID"<<ID<<"\tTitle "<<Title<<"\t("<<Year<<")\t Volumn: "<<Volumn<<",\tNumber"<<Number<<endl;
		}
};




int main()
{	
	MEDIA *ptr[10];
	
	ptr[0] = new DVD(352, "Remember The Alamo", 1945, "George Smith");
	ptr[1] = new DVD(831, "High School Blues", 1984);
	ptr[2] = new DVD(194, "Going For The Touchdown", 1984, "Frank Madden");
	ptr[3] = new DVD(576, "Martian Hairdresser", 1992, "Debbie Gold");
	ptr[4] = new Book(608,"How to Make Money", 1987, "Phil Barton", 324);
	ptr[5] = new Book(442,"Garden Projects At Home", 1998, "Mary Freeman", 164);
	ptr[6] = new Book(185,"The Haunted House Mystery", 1996, "Bert Morgan", 53);
	ptr[7] = new Journal(294, "ACM", 2009, 6, 8);
	ptr[8] = new Journal(521, "J of Logic", 2008, 23, 14);
	ptr[9] = new Journal(630, "J of AI", 2009, 35, 11);
	
	cout << "Printing 10 items..." << endl << endl;
	
	for (int i = 0; i < 10; ++i)
	{
		cout << *ptr[i] << endl;
	}
	
	ptr[3]->changeID(707);
	ptr[5]->changeID(808);
	ptr[7]->changeID(909);
	
	cout << endl << "Printing again..." << endl << endl;
	cout << *ptr[3] << endl;
	cout << *ptr[5] << endl;
	cout << *ptr[7] << endl;
	
	return 0;
}

编译时显示 error: ambiguous overload for 'operator<<

但是我确实只有一个friend函数阿……想不明白问题所在中……求助~

[Scorpiour]

如果把函数

ostream& operator <<(ostream&,MEDIA&)

放在代码后面的话，虽然可以编译通过，但是linker依然报错
(.text+0xa30): undefined reference to `operator<<(std::ostream&, MEDIA const&)'

无法理解中

[大尾巴猫]

ostream& operator <<(ostream out,const MEDIA& M)
{
M.Print(out);
return out;
}

参数里面 ostream & out吧，漏了个&

[Scorpiour]

非常感谢。。。最近各种低级错误

[firendlys]

一个 friend 函数吗？

{
    protected:
        int ID;
        string Title;
        int Year;
    public:
        MEDIA()
        {
            ID=0;
            Title="";
            Year=0;
        }
        virtual ~MEDIA(){}
        virtual void changeID(int)=0;
        virtual void Print(ostream&)const=0;
        friend ostream& operator <<(ostream&,const MEDIA&);// here , ostream & 
};

ostream& operator <<(ostream out,const MEDIA& M)// 没有 ＆
{
    M.Print(out);
    return out;
}

其他代码没看，也没去调试，所以有没有其他错误，就不知道了。