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C++: A Dialog

10.8. Implementing Safe Polymorphism

Now that we have an overview of the structure of the classes we're designing, Figure 10.23 shows the interfaces for the worker classes UndatedStockItem and DatedStockItem.
FIGURE 10.23. Safe polymorphism: The UndatedStockItem and DatedStockItem interfaces for the polymorphic version of StockItem (code\itempi.h)
class UndatedStockItem : public StockItem
{
public:
UndatedStockItem();

UndatedStockItem(std::string Name, short InStock,
short Price, short MinimumStock, short ReorderQuantity,
std::string Distributor, std::string UPC);

virtual bool CheckUPC(std::string UPC);
virtual void DeductSaleFromInventory(short QuantitySold);
virtual short GetInventory();
virtual std::string GetName();

virtual void Reorder(std::ostream& os);
virtual void FormattedDisplay(std::ostream& os);
virtual std::ostream& Write(std::ostream& os);

protected:
short m_InStock;
short m_Price;
short m_MinimumStock;
short m_MinimumReorder;
std::string m_Name;
std::string m_Distributor;
std::string m_UPC;
};

class DatedStockItem : public UndatedStockItem
{
public:

DatedStockItem(std::string Name, short InStock,
short Price, short MinimumStock, short MinimumReorder,
std::string Distributor, std::string UPC, std::string Expires);

virtual void Reorder(std::ostream& os);
virtual void FormattedDisplay(std::ostream& os);
virtual std::ostream& Write(std::ostream& os);

protected:
static std::string Today();

protected:
std::string m_Expires;
};

And Figure 10.24 shows the implementation of these classes.
FIGURE 10.24. Safe polymorphism: The implementation of the UndatedStockItem and DatedStockItem classes (code\itemp.cpp)
#include <iostream>
#include <iomanip>
#include <sstream>
#include <string>
#include "itemp.h"
#include "itempi.h"
#include <dos.h>
using namespace std;

//friend functions of StockItem

ostream& operator << (ostream& os, const StockItem& Item)
{
return Item.m_Worker->Write(os);
}

istream& operator >> (istream& is, StockItem& Item)
{
string Expires;
string Name;
short InStock;
short Price;
short MinimumStock;
short MinimumReorder;
string Distributor;
string UPC;

getline(is,Expires);
getline(is,Name);
is >> InStock;
is >> MinimumStock;
is >> Price;
is >> MinimumReorder;
is.ignore();
getline(is,Distributor);
getline(is,UPC);

if (Expires == "0")
{
Item = StockItem(Name, InStock, Price, MinimumStock,
MinimumReorder, Distributor, UPC);
}
else
{
Item = StockItem(Name, InStock, Price, MinimumStock,
MinimumReorder, Distributor, UPC, Expires);
}

return is;

}


// StockItem member functions

StockItem::StockItem()
: m_Count(0), m_Worker(new UndatedStockItem)
{
m_Worker->m_Count = 1;
}

StockItem::StockItem(const StockItem& Item)
: m_Count(0), m_Worker(Item.m_Worker)
{
m_Worker->m_Count ++;
}

StockItem& StockItem::operator = (const StockItem& Item)
{
StockItem* temp = m_Worker;

m_Worker = Item.m_Worker;
m_Worker->m_Count ++;

temp->m_Count --;
if (temp->m_Count <= 0)
delete temp;

return *this;
}

StockItem::~StockItem()
{
if (m_Worker == 0)
return;

m_Worker->m_Count --;
if (m_Worker->m_Count <= 0)
delete m_Worker;
}

StockItem::StockItem(string Name, short InStock, // Undated
short Price, short MinimumStock, short MinimumReorder,
string Distributor, string UPC)
: m_Count(0),
m_Worker(new UndatedStockItem(Name, InStock, Price,
MinimumStock, MinimumReorder, Distributor, UPC))
{
m_Worker->m_Count = 1;
}

StockItem::StockItem(int)
: m_Worker(0)
{
}

StockItem::StockItem(string Name, short InStock, // Dated
short Price, short MinimumStock, short MinimumReorder,
string Distributor, string UPC, string Expires)
: m_Count(0),
m_Worker(new DatedStockItem(Name, InStock, Price,
MinimumStock, MinimumReorder, Distributor, UPC, Expires))
{
m_Worker->m_Count = 1;
}

bool StockItem::CheckUPC(string UPC)
{
return m_Worker->CheckUPC(UPC);
}

short StockItem::GetInventory()
{
return m_Worker->GetInventory();
}

void StockItem::DeductSaleFromInventory(short QuantitySold)
{
m_Worker->DeductSaleFromInventory(QuantitySold);
}

string StockItem::GetName()
{
return m_Worker->GetName();
}

ostream& StockItem::Write(ostream& os)
{
exit(1); // should never get here
return os;
}

void StockItem::Reorder(ostream& os)
{
m_Worker->Reorder(os);
}


void StockItem::FormattedDisplay(ostream& os)
{
m_Worker->FormattedDisplay(os);
}


// UndatedStockItem member functions

UndatedStockItem::UndatedStockItem()
: StockItem(1),
m_InStock(0),
m_Price(0),
m_MinimumStock(0),
m_MinimumReorder(0),
m_Name(),
m_Distributor(),
m_UPC()
{
}

UndatedStockItem::UndatedStockItem(string Name,
short InStock, short Price, short MinimumStock,
short MinimumReorder, string Distributor, string UPC)
: StockItem(1),
m_InStock(InStock),
m_Price(Price),
m_MinimumStock(MinimumStock),
m_MinimumReorder(MinimumReorder),
m_Name(Name),
m_Distributor(Distributor),
m_UPC(UPC)
{
}

void UndatedStockItem::FormattedDisplay(ostream& os)
{
os << "Name: ";
os << m_Name << endl;
os << "Number in stock: ";
os << m_InStock << endl;
os << "Price: ";
os << m_Price << endl;
os << "Minimum stock: ";
os << m_MinimumStock << endl;
os << "Reorder quantity: ";
os << m_MinimumReorder << endl;
os << "Distributor: ";
os << m_Distributor << endl;
os << "UPC: ";
os << m_UPC << endl;
os << endl;
}

ostream& UndatedStockItem::Write(ostream& os)
{
os << 0 << endl;
os << m_Name << endl;
os << m_InStock << endl;
os << m_Price << endl;
os << m_MinimumStock << endl;
os << m_MinimumReorder << endl;
os << m_Distributor << endl;
os << m_UPC << endl;

return os;
}

void UndatedStockItem::Reorder(ostream& os)
{
short ReorderAmount;

if (m_InStock < m_MinimumStock)
{
ReorderAmount = m_MinimumStock-m_InStock;
if (ReorderAmount < m_MinimumReorder)
ReorderAmount = m_MinimumReorder;
os << "Reorder " << ReorderAmount;
os << " units of " << m_Name << " with UPC ";
os << m_UPC << " from " << m_Distributor << endl;
}
}

bool UndatedStockItem::CheckUPC(string UPC)
{
return (UPC == m_UPC);
}

short UndatedStockItem::GetInventory()
{
return m_InStock;
}

void UndatedStockItem::DeductSaleFromInventory(
short QuantitySold)
{
m_InStock -= QuantitySold;
}

string UndatedStockItem::GetName()
{
return m_Name;
}


// DatedStockItem member functions

DatedStockItem::DatedStockItem(string Name, short InStock,
short Price, short MinimumStock, short MinimumReorder,
string Distributor, string UPC, string Expires)
: UndatedStockItem(Name,InStock,Price,MinimumStock,
MinimumReorder,Distributor,UPC),
m_Expires(Expires)
{
}

ostream& DatedStockItem::Write(ostream& os)
{
os << m_Expires << endl;
os << m_Name << endl;
os << m_InStock << endl;
os << m_Price << endl;
os << m_MinimumStock << endl;
os << m_MinimumReorder << endl;
os << m_Distributor << endl;
os << m_UPC << endl;

return os;
}

void DatedStockItem::FormattedDisplay(ostream& os)
{
os << "Expiration date: ";
os << m_Expires << endl;
os << "Name: ";
os << m_Name << endl;
os << "Number in stock: ";
os << m_InStock << endl;
os << "Price: ";
os << m_Price << endl;
os << "Minimum stock: ";
os << m_MinimumStock << endl;
os << "Reorder quantity: ";
os << m_MinimumReorder << endl;
os << "Distributor: ";
os << m_Distributor << endl;
os << "UPC: ";
os << m_UPC << endl;
os << endl;
}

string DatedStockItem::Today()
{
struct date d;
unsigned short year;
unsigned short day;
unsigned short month;
string TodaysDate;
stringstream FormatStream;

getdate(&d);
year = d.da_year;
day = d.da_day;
month = d.da_mon;

FormatStream << setfill(`0') << setw(4) << year <<
setw(2) << month << setw(2) << day;
FormatStream.seekg(0);
FormatStream >> TodaysDate;

return TodaysDate;
}

void DatedStockItem::Reorder(ostream& os)
{
if (m_Expires < Today())
{
os << "Return " << m_InStock << " units of " << m_Name;
os << " with UPC " << m_UPC;
os << " to " << m_Distributor << endl;
m_InStock = 0;
}

UndatedStockItem::Reorder(os);
}

Let's start our examination of this new StockItem class by looking at the implementation of operator << in Figure 10.25.
FIGURE 10.25. Safe polymorphism: The implementation of operator << for a polymorphic