how to read and write objects from/in binary file in c++ correctly

Question

Below is the basic structure code for my super market billing and stock editing program class:

class Admin
{
public:
     admin();
private:
     int Pid;
     char name[20];
     double quant;
     double price;
     double disc;
     double net_price;

     friend istream &read(istream&, Admin&);
     friend ostream &show(ostream&, const Admin);
};

istream &read(istream&, Admin&);
ostream &show(ostream&, const Admin);

definition of friend functions:

istream &read(istream &is, Admin &commodity)
{
    double dis;

    fflush(stdin);
    is>>commodity.Pid;

    is.getline(commodity.name,30);

    is>>commodity.quant
      >>commodity.price
      >>commodity.disc;

    dis=grs_pr*(commodity.disc/100);

    commodity.net_price=grs_pr-dis;

    return is;
}

ostream &show(ostream &os, const Admin thing)
{
    os << thing.name <<"   " << thing.quant <<"   "
       << thing.price <<"   " << thing.disc <<"   "
       << thing.net_price << endl;

    return os;
}

Main function:

int main()
{
    admin item;

    while(read(cin,item)
    {
        ofstream file;
        file.open("Stock.dat",ios::binary | ios::app);
        file.write(reinterpret_cast<const char*>(&item),sizeof(Admin));
    }
    ifstream readFile("Stock.dat",ios::in|ios::binary);

    while(!readFile.eof())
    {
        readFile.read(reinterpret_cast<char*> (&item),sizeof(Admin));
        Admin readedItem;
        read(readFile,readedItem);    /*i have used read and show function to watch the values that are read by readFile but every time show function output some five exponential values like 5.23689e-301*/
        show(cout,readedItem);
    }

    return 0;
}

Please tell me what is wrong in above code and what should i do to correct it, my motive is to read from the file created and to alter or edit the data of the item that is to be selected by mentioning the product ID by the user, please help me how to attain this functionality.

@n.m.my problem is that i am not able to read the data (objects) from the file — Udit
– Udit, Commented Mar 23, 2013 at 9:58
"I am not able to do this or that" is not specific. "I am getting this error (paste the text of the error)" is. "I expect the program to print FOO but it prints BAR" is. — n. m. could be an AI
– n. m. could be an AI, Commented Mar 23, 2013 at 10:06
One problem is that the code you've shown won't compile. For example, in main(), you have while(read(cin,item) {. This is simply wrong, missing a parenthesis. Your code should be an SSCCE (Short, Self-Contained, Correct Example). Amongst other things, that means it should compile! — Jonathan Leffler
– Jonathan Leffler, Commented Mar 23, 2013 at 10:47

Community · Accepted Answer · 2017-05-23 11:49:50Z

1

you shouldn't implement a naiive serialization yourself. Use a library.

See i.e. answers 1, 2+comments, 3, 4

Also, don't put logic of data manipulation (i.e. commodity.net_price=grs_pr-dis;) in your data serialization

edited May 23, 2017 at 11:49

CommunityBot

11 silver badge

answered Aug 13, 2013 at 14:45

Dmitry Ledentsov

3,67020 silver badges30 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

john · Accepted Answer · 2013-03-23 10:06:32Z

0

Assuming I'm understanding right then your read and show loop should be

    for (;;)
    {
        readFile.read(reinterpret_cast<char*> (&item),sizeof(Admin));
        if (readFile.eof())
            break;
        show(cout,item);
    }

For some reason you were reading twice, first into a variable called item and then into another variable called readItem. You only need to read once. Also while (!readFile.eof()) is wrong because you must test for end of file after you read not before.

answered Mar 23, 2013 at 10:06

john

88.3k5 gold badges61 silver badges95 bronze badges

Collectives™ on Stack Overflow

how to read and write objects from/in binary file in c++ correctly

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related