Re: [suse-programming-e] Help with c++ program

19 Dec 2003


      I'd like to suggest that you "serialize" objects by overloading the stream 
operators << and >>. Although this might be regarded as a somewhat advanced 
approach, it works as a general design principle. Let me show a simple 
example:

class A
{
	private:
		int i, j, k;
		std::string foo;

		// Declare operators as friends, in order to give them
		// access to private members of A
		friend ostream& operator<< (ostream&, const A&);
		friend istream& operator>> (istream&, A&);
	public:
		// ... //

		// In general, implement classes rather than structs,
		// reflecting the basic priciple of encapsulation in
		// object oriented system development. Provide public
		// member functions that access private data.
};

ostream&
operator<< (ostream& os, const A& a)
{
	return (os << i << ':' << j << ':' << k << ':' << foo);
}

istream&
operator>> (istream& is, A& a)
{
	// basically the inverse of operator<<,
	// often more difficult
}

Now, you can use these operators to save to and load from a file stream, like

int main()
{
	// ...
	A something;
	{
		std::ifstream file(FILENAME);
		file >> something;
	}
	// change somthing in something
	{
		std::ofstream file(FILENAME);
		file << something;
	}
	// ...
}

On Friday 19 December 2003 13:49, ged.suse@ntlworld.com wrote:
...
I'm currently trying to improve my knowledge of C++ and writing as many
programs as I can. My latest program has given me a problem and I can't
seem to overcome it.
My initial program has come from a book I was reading and I am building on
it to add more features. It consists of a an ordering system using
structures for stock.
At present everytime I start the program I have to populate the stock from
scratch. Idealy I want to save the stock onto disk and load it everytime
the program starts.
I am starting by populating by hand and then saving this to disk as I
assumed this would be the easier bit, however I am having a little troube
with this function.
After this, I am proposing to load the file back into the program within
the structures and need help with this too although I have not yet made a
start on this function.
I have search all my books and the net, but am strugling to find
information to help me with this. Perhaps it is there but I am not sure of
what I am looking for. Hopefully someone here can help.
For interest, here is my program at present:
#include <iostream>
#include <fstream>
using namespace std;
struct StockItem
{
    int code;
    char desc[20];
    float price;
};
/*struct bill_line
{
    StockItem product;
    float weight;
    float cost;
};
struct bill
{
    bill_line items[20];
    int num_lines;
};*/
void disp_menu(void);
struct StockItem enter_data();
void see_stock(StockItem stock[125], int num_items);
void save_data(StockItem save);
main()
{
    StockItem stock[125];
    int ans;
    int num_items = 0;
do
    {
        do
        {
            disp_menu();
            cin >> ans;
        } while ((ans<1) || (ans>5));
switch (ans)
        {
            case 1:
                stock[num_items] = enter_data();
                num_items++;
                break;
            case 2:
                see_stock(stock, num_items);
                break;
            case 3:
                save_data(*stock);
                break;
            default:
                break;
        }
    }while (ans!=6);
return 0;
system("PAUSE");
    return 0;
}
void disp_menu()
{
    cout << "\n*** The Corner Shop ***\n\n";
    cout << "Select an option: \n\n";
    cout << "\t1. Enter new stock " << endl;
    cout << "\t2. See current stock " << endl;
    cout << "\t3. Exit \n" << endl;
    cout << "option> ";
}
StockItem enter_data()
{
    StockItem stock_item;
cout << "\n\nWhat is the product code: ";
    cin >> stock_item.code;
    cout << "What is the product description: ";
    fflush(stdin);
    cin.getline(stock_item.desc, 20);
    cout << "What is the product price: ";
    cin >> stock_item.price;
return (stock_item);
}
void see_stock(StockItem stock[125], int num_items)
{
    int ctr;
    char ret;
cout << "\n\nHere is the stock listing:\n\n";
    for (ctr = 0; ctr < num_items; ++ctr)
    {
        cout << "Item " << ctr+1 << endl;
        cout << "Product Code: " << stock[ctr].code <<  endl;
        cout << "Product Description: " << stock[ctr].desc << endl;
        cout << "Price: " << stock[ctr].price << endl;
        cout << "------------------------------------------------\n";
    }
    cout << "Press any key to return to the menu";
    if (cin >> ret) //how do you make this work on a keystroke, and not
requiring input then enter?? return;
}
//Problem section
void save_data(StockItem save)
{
    fstream fp;
fp.open("items.txt", ios::in | ios::out);
    if (!fp)
    {
        cout << "\n*** Error opening file ***\n";
        exit(0);
    }
    fp << save;
    fp.close();
}
-----------------------------------------
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-- 
Ch

Re: [suse-programming-e] Help with c++ program

Christian Andersson