Message::simbolo uses undefined class Symbol - c++

Question

Error C2079 'Message::simbolo' uses undefined class 'Symbol'
is generated on this line when compiling
Symbol simbolo;
This is my C++ code:
class Message
#pragma once
#include <string>
#include "Symbol.h"
#include "SharedEditor.h"
class SharedEditor;
class Symbol;
class Message
{
private:
SharedEditor* sender;
int action; //1 inserted 2 deleted
Symbol simbolo;
public:
Message();
Message(SharedEditor* sender, Symbol nuovosimbolo, int action);
~Message();
};
class Symbol
#pragma once
#include "SharedEditor.h"
#include <vector>
class SharedEditor;
class Message;
class Symbol
{
char character;
int siteID;
SharedEditor* generator;
std::vector<int> position;
public:
Symbol();
Symbol(char ch, SharedEditor* source, int ID, std::vector<int> pos);
~Symbol();
};
class SharedEditor:
#pragma once
#include "NetworkServer.h"
#include "Message.h"
#include "Symbol.h"
class Message;
class Symbol;
class SharedEditor
{
private:
NetworkServer& _server;
int _siteId;
std::vector<Symbol> _symbols;
//other functions
public:
SharedEditor(NetworkServer ns);
~SharedEditor();
void process(const Message& m);
};
class NetworkServer:
#pragma once
#include <iostream>
#include <vector>
#include <queue>
#include "SharedEditor.h"
#include "Message.h"
class SharedEditor;
class Message;
class NetworkServer
{
private:
std::vector<SharedEditor*> connected;
std::queue<Message> codaMessaggi;
public:
int connect(SharedEditor* sharedEditor);
void disconnect(SharedEditor* sharedEditor);
void send(const Message& m);
NetworkServer();
~NetworkServer();
};

Answer 1

You need to rework your various header files to break the dependency cycle.
The general rule is: if you only need pointers or references to a type T, you can get away with a forward declaration (class T;) instead of a full class declaration (class T { ... }, typically behind a #include).
For the example above I will go over each file and what you need:
Symbol.h needs a forward declaration of SharedEditor, as it only uses SharedEditor*.
Message.h needs a forward declaration of SharedEditor, but a full #include "Symbol.h" (compiler needs to know how big a Symbol is to calculate the size of Message)
SharedEditor needs a forward declaration of Message (passed as a reference), a full #include "NetworkServer.h" (passed as a parameter) and a full #include "Symbol.h" (used in a vector)
NetworkServer needs a forward declaration of SharedEditor (only used with pointers), but a full #include "Message.h" (used in a queue)
If you still have two classes that need full includes of each other, search stack overflow for "C++ dependency cycle" or somesuch.