二进制文件两次写入数据

Question

I have the code for binary file handling.

  class info
        {
        public:
            int pno, qty;
            char pname[50];
            float price;

        void getdata()
        {
            cout << "Enter product number: "; cin >> pno;
            cout << "Enter product name: "; cin >> pname;
            cout << "Enter the price: "; cin >> price;
            cout << "Enter the quantity: "; cin >> qty;
        }

        void display()
        {
            cout << "Product number: " << pno << endl;
            cout << "Product name: " << pname << endl;
            cout << "Price: " << price << endl;
            cout << "Quantity Available: " << qty << endl;
            cout << "\n";
        }

    };

void createfile()
{
    info obj; char flag='y';
    fstream fin("project.dat", ios::out|ios::binary);
    cout << "Enter the values to be stored in the file" << endl;
    while(flag=='y')
    {
        obj.getdata();
        fin.write((char*)&obj ,sizeof(obj));
        cout << "Data has been added to the file." << endl;
        cout << "Do you want to continue adding more? :  "; cin >> flag;
    }
    fin.close();
   [![OUTPUT][1]][1] cout << "Proceeding to program..." << endl;
}

The above program is writing the last entered record twice, How do I stop the program writing the data twice to the binary file

Answer 1

You need to pass ios::trunc when opening the file. This will cause the contents of the file to be deleted. This does NOT happen automatically.

fstream fin("project.dat", ios::out|ios::binary|ios::trunc);

What's currently happening is that you are opening an existing file containing two (I assume) items, overwriting the first item, then closing the file. The second item is there because it was written by a previous run of your code.