Linker Error

[Denver]

#include <iostream.h>
#include <conio.h>
#include<fstream.h>
#include <process.h>
int main()
{
int ch;
char x;

do
{
     cout <<"Hello And Welcome To Dsz Cinemas\n";
     cout <<"Please Type In The Number Of Your Choice\n";
     cout <<"1. Booking Tickets\n";
     cout <<"2. Latest Updates\n";
     cout <<"3. Gold Membership\n";          //prob is in this selection
     cout <<"4. Credits\n";
     cin >>ch;
     system("cls");
     
     if (ch==3) //must put the login info under here
     {
         cout <<"Welcome to our gold membership\n";
                using namespace std;
                void Auth();
                void Members();
                void Userchange();
                void Passchange();
                
                string inuser;
                string inpass;
                string user;
                int num = 0;
                int ch;
                char pass[BUFSIZ];
                int i = 0;
                string com;
                
                main();
                {
                       system("cls");
                       cout<<"Welcome\n---------------------------\n\n";
                       while(num==0)
                       {
                                    system("cls");
                                    cout<<"Command: ";
                                    cin>>com;
                                    if(com=="login")
                                    {
                                                    Auth();
                                    }
                                    else if(com=="change-pass")
                       {
                                    Passchange();
                       }
                       else if(com=="change-user")
                {
                       Userchange();
                }
                else if(com==com)
                {
                     cout<<"Unknown command\n";
                     }
                }
     }
     void Auth();
     {
          ifstream Passfile("password.txt", ios::in);
          Passfile>>inpass;
          ifstream Userfile("username.txt", ios::in);
          Userfile>>inuser;
          system("cls");
          cout<<"USERNAME: ";
          cin>>user;
          cout<<"PASSWORD: ";
          fflush(stdout);
          
          while ((ch = getch()) != EOF 
          && ch != '\n' 
          && ch != '\r' 
          && i < sizeof(pass) - 1)
          {
               if (ch == '\b' && i > 0) 
               {
                      printf("\b \b");
                      fflush(stdout);
                      i--;
                      pass[i] = '\0';
                      }
                      else if (isalnum(ch))
                      {
                           putchar('*');
                           pass[i++] = (char)ch;
                      }
               }
               pass[i] = '\0';
               Userfile.close();
               Passfile.close();
               if(user==inuser&&pass==inpass)
               {
                            Members();
               }
                            else
               {
                            cout<<"nope";
                            getch();
                            main();
                            }
               }
               void Userchange();
               {
                    string tempass;
                    system("cls");
                    cout<<"enter current password: ";
                    cin>>tempass;
                    if(tempass==inpass)
                    {
                                       cout<<"utility to change username stored in file\n";
                                       cout<<"CHANGE USER TO: ";
                                       cin>>user;
                                       ofstream Userfile("username.txt", ios::out);
                                       Userfile<<user;
                                       Userfile.close();
                                       cout<<"Username successfully changed";
                                       getch();
                                       main();
                    }
                                       else
                                       {
                                           cout<<"Invalid Password";
                                           getch();
                                           main();
                                       }
                    }
                    void Passchange();
                    {
                         string pass;
                         system("cls");
                         cout<<"enter current password: ";
                         fflush(stdout);
                         while ((ch = getch()) != EOF 
                         && ch != '\n' 
                         && ch != '\r' 
                         && i < sizeof(pass) - 1)
                         {
                              if (ch == '\b' && i > 0) 
                              {
                                     printf("\b \b");
                                     fflush(stdout);
                                     i--;
                                     pass[i] = '\0';
                                     }
                                     else if (isalnum(ch))
                                     {
                                          putchar('*');
                                          pass[i++] = (char)ch;
                                     }
                              }
                              pass[i] = '\0';
                              if(pass==inpass)
                              {
                                              cout<<"utility to change password stored in file\n";
                                              cout<<"CHANGE PASS TO: ";
                                              cin>>user;
                                              ofstream Passfile("password.txt", ios::out);
                                              Passfile<<user;
                                              Passfile.close();
                                              cout<<"password successfully changed";
                                              getch();
                                              main();
                              }
                                              else
                                              {
                                                  cout<<"Invalid Password: ";
                                                  getch();
                                                  main();
                                              }
                         }
                         void Members();
                         {
                              system ("cls");
                              cout <<"wassup";
                              cout<<"nothing";
                              getch ();// put members code here
                              // like a while(){} loop or somethin like that
                              // comment please
                         }
                         
}


else if (ch>4)
     cout <<"The number typed in is wrong Dum Dum =P\n\n\n\n";
     
     
     cout<<"\n\nDo you wish to redo your selection and select another option or exit? (Y/N)\n";
     cin>>x;
     system("cls");
}
while (x=='Y'||x=='y');
cout <<"Thankyou for choosing Dsz Cinemas!!!\nSorry for any inconvenience\nHope to see you again ^.^ :D\nBye"; 
getch();
exit(0);

getch();
}

[Ogoyant]

For one, your headers are not properly included. Try this:

#include <iostream>
#include <conio.h>
#include <fstream>
#include <process.h>

using namespace std;

Edit: The comments of the other members in the "General C++ Programming" section duplicate thread (http://www.cplusplus.com/forum/general/88977/#msg477667) , in conjunction to properly including the headers, should have this work for you as intended. You may want to look into the structure of C++ functions as already suggested though:

http://www.cplusplus.com/doc/tutorial/functions/