两个类互相作为类型使用的问题
以下是C++ PRIMER中文第四版13章里消息处理机制中的课后习题，我自己扩充了下，出现了一些问题，主要是在类Message中使用了类Folder，编译器把类型当作Message::Folder来处理了，由此造成很多编译错误。我的文件代码如下：

定义类的头文件Message.h

#include <string>
#include <set>

#ifndef MESSAGEH
#define MESSAGEH

class Message
{
public:
class Folder;
Message(const std::string &str = ""):
  contents (str) {}
  Message(const Message&);
  Message& operator=(const Message&);
  ~Message();
  void save(Folder&);
  void remove(Folder&);
  std::string Show()
  { return contents; }
private:
std::string contents;
std::set<Folder*> folders;
void put_Msg_in_Folders(const std::set<Folder*>&);
void remove_Msg_from_Folders();
};

class Folder
{
public:
void remMsg(Message*);
void addMsg(Message*);
unsigned MsgNum();
std::string ShowMsg(unsigned);
private:
std::set<Message*> messages;
};

#endif

编写定义类的函数等具体文件Message.cpp

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

Message::Message(const Message &ms):
contents(ms.contents), folders(ms.folders)
{ put_Msg_in_Folders(folders); }

 Message& Message::operator=(const Message &ms)
 {
 if (&ms != this)
 {
 remove_Msg_from_Folders();
 contents = ms.contents;
 folders = ms.folders;
 put_Msg_in_Folders(ms.folders);
 }
 return *this;
 }

 void Message::put_Msg_in_Folders(std::set<Folder*> &sf)
 {
 for (std::set<Folder*>::iterator beg = sf.begin();
 beg != sf.end(); ++beg)
 (*beg)->addMsg(this);
 }

 void Message::remove_Msg_from_Folders()
 {
 for (std::set<Folder*>::iterator beg = folders.begin();
 beg != folders.end(); ++beg)
 (*beg)->remMsg(this);
 }

 void Message::save(Folder &fds)
 {
 //如果该邮件的文件夹列表中没有这个文件夹，则将该文件夹添加到邮件的文件夹集合
 if (!folders.count(&fds))
 folders.insert(&fds);
 //如果形参文件夹尚未保存该邮件指针，则予以保存
 if (!(fds->messages).count(this))
 (fds->messages).insert(this);
 return;
 }

 void Message::remove(Folder &fds)
 {
 if (folders.count(&fds))
 folders.erase(&fds);
 if ((fds->messages).count(this))
 (fds->messages).erase(this);
 return;
 }

 Message::~Message()
 { remove_Msg_from_Folders(); }

编写类Folder函数代码的文件Folder.cpp

#include "Message.h"
#include <set>
#include <string>
#include <iostream>

void Folder::remMsg(Message *mp)
{
if (messages.count(mp))
messages.erase(mp);
return;
}

void Folder::addMsg(Message *mp)
{
if (!messages.count(mp))
messages.insert(mp);
return;
}

unsigned Folder::MsgNum()
{ 
 
unsigned sum(0);
std::set<Message*>::const_iterator beg = messages.begin();
while (beg != messages.end())
{
++sum;
++beg;
}
return sum;
}

std::string Folder::ShowMsg(unsigned ix)
{
unsigned num(0);
std::set<Message*>::const_iterator beg = messages.begin();
if (beg == messages.end()){
std::cout << "该邮件夹中没有任何邮件！" << std::endl;
return std::string("Error!\n");
}
while (beg != messages.end())
{
if (num == ix)
break;
else
{
++num;
++beg;
}
}
if (num != ix){
std::cout << "超出邮件数量范围！" << std::endl;
return std::string("Out of range!\n");
}
else{
return (*beg)->Show();
}
}

测试文件main.cpp

#include "Message.h"
#include <string>
#include <iostream>
#include <vector>
using namespace std;

int main()
{
Folder MySelf, EveryOne;
vector<Message> msg;
while (true)
{
cout << "下面请写信：" << endl;
string msgtxt;
cin >> msgtxt;
msg.push_back(Message(msgtxt));
cout << "你想将信件保存在那个文件夹？（1：MySelf；2：EveryOne）" << endl;
unsigned folderName;
cin >> folderName;
if (folderName == 1)
msg.back().save(MySelf);
else msg.back().save(EveryOne);
cout << "还想继续写信吗？（y 或 n）" << endl;
char next;
cin >> next;
if (next == 'n')
break;
}
cout << "现在你的MySelf邮件夹中有：" << MySelf.MsgNum()
<< "封邮件！\n你的EveryOne邮件夹中有："
<< EveryOne.MsgNum() << "封邮件！" << endl;

return 0;
}

[解决办法]
class Message {
public:
class Folder;

将class Folder; 声明放到class Message 类的外面

class Folder;
class Message {
public:
[解决办法]
Folder明明是个嵌套类，即Message::Folder，你在Folder.cpp中的定义应加上：Message::Folder::xxx