how to display the contents of a binary file at an indexed location?

[ayushigarg]

the rror i have is it does not read contents from the employee.dat file

#ifndef main_h
#define main_h
struct date{
int day;
int month;
int year;
};
class employees
{
int ID;
char name[30];
date hiredate;
int salary;
public:
void insert();
void deleteemp();
void modify();
void show();
void display(employees,int,fstream);
};
#endif main_h
 
#include<iostream.h>
#include<fstream.h>
#include<conio.h>
#include"main.h"
 
void employees::insert()
{
employees emp;
cout<<"enter the details:";
cout<<"ID";
cin>>ID;
cout<<"name:";
cin>>name;
cout<<"salary:";
cin>>salary;
cout<<"hiredate:";
cin>>hiredate.day>>hiredate.month>>hiredate.year;
 
}
void employees::show()
{ cout<<"ID:"<<ID<<endl;
cout<<"name:"<<name<<endl;
cout<<"salary:"<<salary<<endl;
cout<<"date hired on:"<<hiredate.day<<"-"<<hiredate.month<<"-"<<hiredate.year<<endl;
}
 
 
void main()
{
employees e;
char ch;
do{
cout<<"enter your choice!\n1.insert new record\n2.display the existing record\n3.delete a record\n4.modify existing\n";
int choice;
cin>>choice;
switch(choice)
{
case 1:
e.insert();
fstream fio;
fio.open("employee.dat",ios::binary|ios::app|ios::out);
fio.write((char*)& e,sizeof(e));
fio.seekg(0,ios::beg);
fio.close();
break;
case 2:
int n;
cout<<"which employee number do you want details for?";
cin>>n;
fio.open("employee.dat",ios::binary|ios::in);
int y=(n-1)*sizeof(e);
fio.seekg(y);
fio.read((char*)& e,sizeof(e));
break;}
cout<<"continue?";
 
cin>>ch;
}while(ch=='y');
getch();
}
 

[weaknessforcats]

I think you are running afoul of slack bytes. These padding bytes are inserted by the compiler to enforce int boundary alignment for class and struct members.

A sizeof(employees) is 52 bytes but a sizeof each of the data members is 50.

That means you have to read back in 52 bytes and then fiddle with that to find your data. Very ugly.

I recommend that you write out a buffer you have defined as an array. That gives you a fixed size. In this case the buffer is 50 bytes. Write a function that takes an employees& as an argument and inside that function extract the data members from your object and pack them into the buffer. Then write the buffer.

On the readback, write a function that takes an employees& as an argument plus a second argument for the offset. Read into the buffer and then unpack the buffer into the arguent object.

[ayushigarg]

could you please write an exact code block for the fix that you have offered. i have not learnt much about file handling...so if you could pls help me...thanks a lot for your help

[weaknessforcats]

Maybe you can try it this way:

class employees
  {
 int ID;
 char name[30];
 unsigned  char pad1;      //pad byte
 unsigned  char pad2;      //pad byte
 date hiredate;
  int salary;
  public:
  void insert();
  void deleteemp();
  void modify();
  void show();
  void display(employees,int,fstream);
  };

Since alignment is on an int boundary, ID is on an int boundary since it is the ifrst member. ID is an int so the next member (name) also is on an int boundary. However, since it is only 30 bytes, the next int boundary would be 32 bytes away. So I added 2 pad bytes after the array to force hiredate to begin 2 bytes later. When I did a sizeof(employees) I still got 52 but this time I have defined 52 bytes in the class. That means no pad bytes stuffed in by the compiler. I believe you can now write as you were indending and your data will be where you expect it.

I didn't test this so let me knw what happened.