How to pass an object of a class to a function of the same class?

Question

int main(){

   int choice;
   fstream in, out;                             

  switch(choice)
  {

  case 1: 
      filename = s.studentRegister();
      out.open(filename.c_str(), ios::out);
      out.write((char *)&s, sizeof(Student));
      cout<<endl<<"Registered successfully."<<endl;

  case 2:
      s.studentLogin();
  }                                   
}

class Student
{
std::string studentName, roll, studentPassword;
fstream in, out;

public:

std::string studentRegister()
{
    cout<<"Enter roll number"<<endl;
    cin>>roll;
    cout<<"Enter current semester"<<endl;
    cin>>ch;
    cout<<"Enter your name"<<endl;
    cin>>studentName;
    cout<<"Enter password"<<endl;
    cin>>studentPassword;

    return (roll+".dat");
}

void studentLogin()
{
            Student s;
    cout<<"Enter roll number: "<<endl;
            cin>>roll;
    cout<<"Enter password: "<<endl;
    cin>>studentPassword;

    filename = roll + ".dat";
    in.open(filename.c_str(), ios::in);
    in.read((char *)&s, sizeof(Student));

    read.close();

    if((studentPassword.compare(s.studentPassword)==0))
    {
        system("cls");
        cout<<"Welcome "<<s.studentName<<"!"<<endl;
        displayStudentMenu();
    }

    else
    {
        cout<<"Invalid user";
        exit(0);
    }

}

I have two functions in my Student class: studentRegister() and studentLogin(). When studentRegister is invoked, it accepts all details of the student and then writes the object of the respective class to a DAT file. Now, when logging in, I'm trying to read the contents of the file into an object "s" using in.read((char *)&s, sizeof(Student));

However, this leads to a run-time error and console closes abruptly. What is going wrong?

As mentioned in another of your questions, you simply cannot write and read your class with these methods. (Also, the question title doesn’t have anything to do with the actual question.) — Konrad Rudolph
– Konrad Rudolph, Commented Mar 19, 2013 at 13:01
Please provide a short, complete example program. See SSCCE.org for more details. — Robᵩ
– Robᵩ, Commented Mar 19, 2013 at 13:09

john · Accepted Answer · 2013-03-19 13:15:10Z

1

Read and write don't work the way you are trying. They are only good for classes with no pointers, but string has pointers and your class has string so you cannot use them.

You are going to have to find a different way to read and write your data. What's wrong with this

out << studentName << ' ' << roll << ' ' << studentPassword << '\n';

and

in >> studentName >> roll >> studentPassword;`

Also not the question you asked but in and out should not be declared in your Student class. They should be declared in the functions where you use them.

answered Mar 19, 2013 at 13:15

john

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2 Comments

Slashr Over a year ago

Thanks a lot! I was storing several strings in the file and didn't know how to access them randomly (without using seekg()). in works fine. BTW, can in read correctly from DAT and binary files too or just text files?

john Over a year ago

Read will read bytes from any sort of file, and write will write bytes to any sort of file. The issue is you have is something else. Namely you cannot regard objects like string (or your Student) as flat sequences of bytes. They have internal structure (using pointers etc.) that read and write know nothing about.

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