##Purchase.h
#ifndef PURCHASE_H
#define PURCHASE_H
#include "Transaction.h"
#include "Customer.h"
#include "InvItem.h"

//foreward declaration
class Customer;

//Subclass of Transaction that specifically formats a purchase transaction
class Purchase : public Transaction { //ERROR: Purchase.h:12: error: expected class-name before ‘{’ token
//...
};
#endif

##Trade.h
#ifndef TRADE_H
#define TRADE_H
#include "Transaction.h"
#include "Customer.h"
#include "InvItem.h"

//foreward declaration
class Customer;

//Subclass of Transaction that specifically formats a trade transaction
class Trade : public Transaction {
//...
};
#endif

##Transaction.h
#ifndef TRANSACTION_H
#define TRANSACTION_H
#include <iostream>
#include <string>
#include <sstream>
#include "Customer.h"
#include "InvItem.h"

using namespace std;

//forward declaration for transaction
class Customer;

// Represents a single transaction. This is an abstract type. In practice, its subclasses
// will actually be used to differentiate between the different types of transactions (trade
// and purchase).
class Transaction {
//..
};
#endif

##Customer.h
#ifndef CUSTOMER_H
#define CUSTOMER_H
#include <iostream>
#include <string>
#include <vector>
#include "InvItem.h"
#include "Transaction.h"
#include "Trade.h"
#include "Purchase.h"

using namespace std;

//forward declaration for transaction
class Transaction;

//Represents a single customer. Responsible for keeping track of
class Customer {
//...
};