Little confusion in both write() and read()

Question

Below program take three details of family members i.e name,age and occupation. Stores them in txt file.

#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <fstream>
using namespace std;

class Dewan_Family
     {

       char name[50];    // array for name of the family member
       int age;           // integer for age of the family member
       char oc[50];      // array for occupation of family member

       public:

       void getdata();         // This func. to collect information
       void displaydata();     // This func to display information
     };

void Dewan_Family::getdata()         // creating func. to get family members info
     {
         cout << "Enter the Name of the Family member: "; cin.getline(name,50);
         cout << "Enter the Age of the member: ";
         cin >> age;
         cin.ignore();
         cout << "Enter the occupation of the member: "; cin.getline(oc,50);
     }
void Dewan_Family::displaydata()     // creating func. to display info
     {
         cout << "Name of the Dewan family's member: "  << name << endl;
         cout << "Age of the member: " << age << endl;
         cout << "Occupation of the member: " << oc << endl;
     }


int main()
  {

   Dewan_Family obj[5];            // created object array of class Dewan_Family
   fstream file;                     // created object to handle file
   file.open("family.txt", ios_base::app | ios_base::in);     // created and attached file with the object.
   char ch;
   int choice; int j=0;
   cout << "Main Menu ! Choose any option 1-3.\n";
   cout << "1. To add Members\n";
   cout << "2. To Display all Members\n";
   cout << "3. Exit\nChoice: ";
   cin >> choice;
   cin.ignore();
   cout << "\n";
   switch(choice)
   {

    case 1:     ch='y';
                do{
                obj[j].getdata();
                file.write((char*)&obj[j],sizeof(obj[j]));
                j++;
                cout << "Data added successfully !.\nDo you want to continue to add: y or n: ";
                cin >> ch;
                }while(ch =='y' || ch == 'Y');
                break;

    case 2:     file.seekg(0);
                while(file)
                    {
                    file.read((char*)&obj[j],sizeof(obj[j]));
                    obj[j].displaydata();
                    j++;
                    }
                 break;

    case 3:     exit(1);

   };

   return 0;
  }

My question is as follow:- 1) This little program successfully write data(with write function) to family.txt file. But when I see data by opening this family.txt file, data appears in weird form.

for example if I enter below details

  Name = Ashok Dewan
  Age = 25
  occupation = job

then data in the text file is like this

Ashok #{}@1.^.36 ..  ) job#..#...

2) when I read data(with read func.) It display like below

Name of the Dewan family's member: Ashok Dewan
Age of the member: 25
Occupation of the member: Job
Name of the Dewan family's member: ##$8&
Age of the member: 42565571
Occupation of the member: {...&6^

It displays other data also which I did not enter. I don't know Why it is showing extra data. Is this because write() and Read() use binary mode also by default? Please check this code at your end. I am using latest codeblocks Please help me to understand this behaviour?

Why are you using C strings in a C++ program ? Use `std::string`. — Paul R, Jun 14 '20 at 09:59
I forgot to remove it. But it has nothing to do with the whole program code. — Ashok Dewan, Jun 14 '20 at 10:03
@AshokDewan, do you know what `(char*)&obj[j]` does? This might be your problem. — Incomputable, Jun 14 '20 at 10:06
No, It's not. It is pointer which pointing to address of obj's element. It's the way to use write() and read(). I know only two way to write on txt file :- 1) ofstream fout("familty.txt,ios_base::out") then fout << variable // this way variable's value will go to txt file. 2) write() function as shown in above program. — Ashok Dewan, Jun 14 '20 at 12:26
Re: why the program displays an extra record - see [this](https://stackoverflow.com/questions/5431941/why-is-while-feof-file-always-wrong) (you are not using `feof`, but `while(file)` is wrong for the same reason). Re: why there's garbage in the file - you aren't writing a text file (even though it has a .txt extension), you are writing a binary file, so you should not expect it to look reasonable when viewed as text. E.g. the program always writes 50 bytes for `name`, but only the first few are initialized, the rest contain random garbage; this is the garbage you see in the file. — Igor Tandetnik, Jun 14 '20 at 23:27
@IgorTandetnik Thank you for the help. I was really helpful link. — Ashok Dewan, Jun 15 '20 at 12:39

score 0 · Accepted Answer · answered Jun 15 '20 at 08:43

In your code you are not checking if read is successful before using it, so the last obj created from a failed read. a possible fix is checking before serializing to an object:

 case 2:     file.seekg(0);
                while(file)
                    if(file.read((char*)&obj[j],sizeof(obj[j]))){
                      obj[j].displaydata();
                      j++;
                    }
                 break;

Another solution might be to change the while statement as follows:

 case 2:     file.seekg(0);
                while(file.read((char*)&obj[j],sizeof(obj[j])))
                  {
                      obj[j].displaydata();
                      j++;
                  }
                 break;

Thank you for you help too. – Ashok Dewan Jun 15 '20 at 12:41 — Ashok Dewan, Jun 15 '20 at 12:41

Little confusion in both write() and read()

1 Answers1