Многофайловый код

#include <iostream>
#include <fstream>
#include <conio.h>
#include "MyListClass5A.h"
using namespace std;
void Action(List& abc){
    abc.Write();
    ...
}
int main(){
    List abc;
    Action(abc);
    ...
}

#ifndef MYLISTCLASS5A_H_INCLUDED
#define MYLISTCLASS5A_H_INCLUDED
struct t_Item;
class List;
#endif // MYLISTCLASS5A_H_INCLUDED

#include "MyListClass5A.h"
struct t_Item{
   ...
};
class List{
    ...
};

class List{
    t_Item* end;
    t_Item* items;
    unsigned count;
    public:
    List(){
        items=0;
        end=0;
        count=0;
    }
    ~List(){
        Clear();
    }
    void Clear(){
        for(unsigned i=0;i<count;++i){
            DelItem(0);
        }
    }
    unsigned GetCount(){
        return count;
    }
    ...
};

//MyListClass5A.h
#ifndef MYLISTCLASS5A_H_INCLUDED
#define MYLISTCLASS5A_H_INCLUDED
struct t_Item;
class List{
    public:
    List();
    ~List();
    unsigned GetCount();
    void Add(char val);
    void Clear();
    void DelItem(unsigned n);
    void InsItem(unsigned n,char c);
    void Write(bool reverse=0);
    void WriteFile(ofstream& fileName);
    void ChangeNum();
    void DelNum();
    void ChangeTriple();
    void ChangeDouble();
    void Enter(unsigned N);
};
#endif // MYLISTCLASS5A_H_INCLUDED

void WriteFile(ofstream& fileName);

void WriteFile(std::ofstream& fileName);