'undefine reference to'请教

[AppleStarain]

//Client.cpp
#include <iostream>
#include <string>
#include "Message.h"
#include "Folder.h"
using std::string;
using std::cin;
using std::cout;
using std::endl;

int main() {
	cout << "Input a line of character to test:" << std::flush;
	string line;
	getline(cin,line);
	Message message(line);
	Folder Fol;
	message.save(Fol);
	message.remove(Fol);
	return 0;
}

//Message.h

#ifndef _MESSAGE_H
#define _MESSAGE_H

#include <string>
#include <set>
#include "Folder.h"
#define _FOLDER_H
class Folder;
class Message {
	public:
		Message(std::string line):
			Message_str(line) { };
		Message(Message&);
		~Message();
		Message& operator=(Message&);
		void save(Folder&);
		void remove(Folder&);
	private:
		std::set<Folder*> Folders;
		std::string Message_str;
		void Put_Msg_In_Folder(const std::set<Folder*>);
		void Rem_Mes_From_Folder();
};

#endif

//Message.cpp
#include <string>
#include <set>
#include "Message.h"

Message::Message(Message& Mes):
	Message_str(Mes.Message_str) Folders(Mes.Folders) {
		Put_Mes_In_Folder(Folders);
}

Message::~Message() {
	Rem_Mes_From_Folder(Folders)
}

Message& Message::operator=(Message& Mes)  {
	Message_str = Mes.Message_str;
	Folders(Mes.Folders);
	Put_Mes_In_Folder(Folders);
}

void Message::save(Folder& folder)  {
	Folder* pfol = &folder;
	Folders.insert(pfol);
}

void Message::remove(Folder& folder) {
	Folder* pfol = &folder;
	for(std::set<Folder*>::iterator it = Folders.begin();
		it != Folders.end();++it) {
		if(*pfol == **it)
			Folders.erase(*it);
	}
}
void Message::Put_Mes_In_Folder(std::set<Folder*> Folders) {
	for(std::set<Folder*>::iterator it = Folders.begin();	
		it != Folders.end();++it) {
		(*it) -> addmes(this);
	}
}

void Message::Rem_Mes_From_Folder(std::set<Folder*> Folders) {
	for(std::set<Folder*>::iterator it = Folders.begin();
		it != Folders.end();++it) {
		(*it) -> remmes(this);
	}
}

//Folder.h

#ifndef _FOLDER_H
#define _FOLDER_H

#include <string>
#include <set>
#include "Message.h"
#define _MESSAGE_H

class Message;
class Folder {
	public:
		void addmes(Message*);
		void remmes(Message*);

	private:
		std::set<Message*> Messages;
};

#endif

//Folder.cpp

#include <set>
#include <string>
#include "Folder.h"

void Folder::addmes(Message* Message) {
	Messages.insert(Message);
}

void Folder::remmes(Message* Message) {
	Messages.erase(Message);
}

Client.cpp:(.text+0x98): undefined reference to `Message::save(Folder&)'
Client.cpp:(.text+0xae): undefined reference to `Message::remove(Folder&)'
Client.cpp:(.text+0xce): undefined reference to `Message::~Message()'
Client.cpp:(.text+0xfe): undefined reference to `Message::~Message()'
Client.cpp:(.text+0x123): undefined reference to `Message::~Message()'
collect2: error: ld returned 1 exit status
系统：Linux 编译器：g++
以前高中没什么时间学，今年上了大学才终于可以自己分配时间。所以这些问题不太懂。请多多指教了。

[proorck6]

编译器找不到相应的符号，可以看看是不是#include给忽略了。
这应该是个结构问题，仔细看看文件间的包含、调用关系。

[AppleStarain]

我找了好久。。。我在类的实现里嫁了#include "XXX.h"了，那.cpp和 .h之间应该建立联系了才对啊。。但是好像没有

[ouyh12345]

makefile文件里，有没有加入Message.cpp
或者直接用g++:
g++ -o Client Client.cpp Folder.cpp Message.cpp

[xyddz]

Message.h包含了Folder.h， 而Folder.h又包含了Message.h

你在Folder.h中声明了class Message; 用的又是指针还包含它干什么了，删了它。

[xyddz]

楼主这个代码好像是C++ Primer（第4版）的吧。

[AppleStarain]

恩。我在学习C++ Primer我现在在想后面的高级部分我现在需不需要看。。还是以后需要了再看，谢谢各位了

[AppleStarain]

多谢3L啊。。我还没从编译器用法这方面考虑过呢。。:-)