file created but no contents

[1vf9z8AR]

The code inputs values but when i go and open the dat file created with notepad its shows nothing

  #include<iostream>
#include<fstream>
using namespace std;
int main()
{
    int rno,marks;
    char ch;
    fstream f1("mark.dat",ios::out|ios::binary);
    f1.open("mark.dat");
    do
    {
        cout << "Enter roll number";cin >> rno;f1 << rno;
        cout << "\nEnter marks";cin >> marks;f1 << marks;
        cout << "\nDo you want to continue?(y/n)";cin >> ch;
        cout << endl;
    }while((ch=='y')||(ch=='Y'));
    f1.close();
    return 0;
}

[JLBorges]

#include<iostream>
#include<fstream>

int main()
{
    const char file_name[] = "mark.dat" ;

    {
        int rno, marks;
        char ch;

        std::ofstream file(file_name) ; // open file for output

        do
        {
            std::cout << "\nEnter roll number: ";
            std::cin >> rno;
            std::cout << "\nEnter marks: ";
            std::cin >> marks;

            if( std::cin ) // if there is no input failure
            {
                file << rno << ' ' << marks << '\n' ;

                std::cout << "\nDo you want to continue?(y/n)";
                std::cin >> ch;
            }
        } while( std::cin && ( (ch=='y') || (ch=='Y') ) ) ;

    } // file object goes out of scope; the file is automagically flushed and closed

    // verify by dumping the contents of the file on stdout
    std::cout << std::ifstream(file_name).rdbuf() ;
}

[Chervil]

In the original code, the file is opened twice. The second attempt will fail, meaning the stream is no longer useable.

    cout << boolalpha;  // print true / false as words instead of 1 / 0
    
    fstream f1("mark.dat", ios::out | ios::binary);
    cout << "open: " << f1.is_open() << "    fail: " << f1.fail() << '\n';

    f1.open("mark.dat");
    cout << "open: " << f1.is_open() << "    fail: " << f1.fail() << '\n';

Output:

open: true    fail: false
open: true    fail: true

It is usually simpler to use ifstream for input and ofstream for output, rather than specifying the mode separately. Also, binary mode is not appropriate here.

Instead of

    fstream f1("mark.dat",ios::out|ios::binary);
    f1.open("mark.dat");

just put

    ofstream f1("mark.dat");