友元类问题

[Dreamz]

下午做一个友元类的作业,要求2个无关的类Engine和Fuel,要使Fuel成员可以访问Engine中的私有和保护成员,我写的代码如下,无法通过连接,请问是哪里出了问题?

#include<iostream>
using namespace std;

class Engine
{
public:
	Engine(int m_=0,int n_=0)
	{
		m=m_;
		n=n_;
	}
	~Engine() {}
	friend class Fuel;
private:
	int m;
protected:
	int n;
};

class Fuel
{
public:
	Fuel();
	~Fuel() {}
	void GetEngine()
	{cout<<e.m<<e.n<<endl;}
private:
	Engine e;
};

int main()
{
	Engine a(5,4);
	Fuel b;
	b.GetEngine();
	return 0;
}

出错提示如下:
5_7.obj : error LNK2019: 无法解析的外部符号 "public: __thiscall Fuel::Fuel(void)" (??0Fuel@@QAE@XZ)，该符号在函数 _main 中被引用
E:\Visual Studio 2005\Projects\Exercise\5_7\Debug\5_7.exe : fatal error LNK1120: 1 个无法解析的外部命令

[ryfdizuo]

#include<iostream>
using namespace std;

class Engine
{
public:
	Engine(int m_=0,int n_=0)
	{
		m=m_;
		n=n_;
	}
	~Engine() {}
	friend class Fuel;
private:
	int m;
protected:
	int n;
};

class Fuel
{
public:
	Fuel(){};		//增加{}
	~Fuel() {}
	void GetEngine()
	{cout<<e.m<<e.n<<endl;}
private:
	Engine e;
};

int main()
{
	Engine a(5,4);
	Fuel b;
	b.GetEngine();
	return 0;
}

与友员类无关
^_^

[effective_person]

#include<iostream>
using namespace std;

class Engine
{
public:
    Engine(int m_=0,int n_=0)
    {
        m=m_;
        n=n_;
    }
    ~Engine() {}
    friend class Fuel;
private:
    int m;
protected:
    int n;
};

class Fuel
{
public:
    Fuel(){};//没实现构造函数
    ~Fuel() {}
    void GetEngine()
    {
		cout<<e.m<<e.n<<endl;
	}
private:
    Engine e;
};

int main()
{
    Engine a(5,4);
    Fuel b;
    b.GetEngine();
    return 0;
}

[Dreamz]

晕了,原来是这种低级错误-_-|||多谢2位啦:)

[starwalker]

#include<iostream>
using namespace std;

class Engine
{
public:
    Engine(int m_=0,int n_=0)
    {
        m=m_;
        n=n_;
    }
    ~Engine() {}
    friend class Fuel;
private:
    int m;
protected:
    int n;
};

class Fuel
{
public:
    Fuel() {}  // 你这个函数没定义
    ~Fuel() {}
    void GetEngine()
    {cout<<e.m<<e.n<<endl;}
private:
    Engine e;
};

int main()
{
    Engine a(5,4);
    Fuel b;
    b.GetEngine();
    return 0;
}