plz help me with virtual functions file handling code

here is the code

plz help me with the case 2 present in main...

#include<iostream.h>
#include<conio.h>
#include<fstream.h>
#include<stdlib.h>

class student
{
protected:

int rollno;
char name[50],degree;

public:

void virtual getdata()=0;
void virtual putdata()=0;
void virtual calresult()=0;
};


class acadmic:public student
{
protected:

int imk1,imk2,emk1,emk2;
float result;

public:

void getdata();
void putdata();
void calresult();
};

void acadmic::getdata()
{
cout<<"\n Enter Roll No:";
cin>>rollno;
cout<<"\n Enter Name:" ;
cin>>name;
cout<<"\n Enter Course:";
cout<<"\n E for Enginnering \n M for MCA:";
cin>>degree;
cout<<"\n Enter Internal Marks for test1 and test 2:" ;
cin>>imk1>>imk2;
cout<<"\n Enter Marks For Extrenal Test1 and Test 2:";
cin>>emk1>>emk2;

}
void acadmic::putdata()
{
cout<<"\n Roll No:"<<rollno;
cout<<"\n Name:"<<name;
cout<<"\n Course:"<<degree;
cout<<"\n Internal Marks";
cout<<"\n "<<imk1<<"\n "<<imk2;
cout<<"\n External Marks";
cout<<"\n "<<emk1<<"\n "<<emk2;
cout<<"\n The Result Is "<<result;
}
void acadmic::calresult()
{
result=(imk1+imk2+emk1+emk2)/4;
}

void main()
{
student *ptr;
acadmic a;
ptr=&a;
fstream file;

file.open("stud.txt",ios::in|ios::out);//|ios::binary);
if(file==NULL)
{
cout<<"\nFile opening error....";
getch();
exit(1);
}

char ans;
do
{
clrscr();
cout<<"\n******** MENU ********\n\t1.Insert\n\t2.Display\n\t3.Search\n\t4.Modify\n\t5.Logical Delete\nEnter your choice?";

switch(getche())
{
case '1':

file.seekp(0,ios::end);
ptr->getdata();
ptr->calresult();
file.write((char *)ptr,sizeof(a));

file.clear();
break;

case '2': //DISPLAYING
file.seekg(0,ios::beg);

while(file)
{

file.read((char *)ptr,sizeof(a));
ptr->putdata();//wat is the reason that it ddnt work
//a.putdata(); //even this call works
}
//ptr->putdata(); //this works alone

file.clear();
break;

}
cout<<"\n Do you want to continue?";
ans=getche();
}while(ans=='y'||ans=='Y');
file.close();
cout<<"\nFile closed Successfully...";
}



thanks

Nilesh

Please use code tags for posting code.

Code:

---

file.read((char *)ptr,sizeof(a));

---

You are casting a class pointer to a char pointer. What do you think will happen ? Also, sizeof(a). You are trying to take the size of a class. DON'T.

we always have to cast class pointer to char pointer while reading or writing to a file

and also need to specify size of the object which i did

plz tell me whats wrong with that function call

Quote:

---

we always have to cast class pointer to char pointer while reading or writing to a file

---

But what do you expect to happen ? You have a pointer to a class and you are telling the compiler that it is a character buffer. Do you expect to write the class to disk ? You can't write a class to disk. You can only write the data in the class members to disk (for example the 'name' and 'degree') but you need them to write them individually.

So this..

Code:

---

file.write((char *)ptr,sizeof(a));

---

and this...

Code:

---

file.read((char *)ptr,sizeof(a));

---

is incorrect.

This...

Code:

---

file.write(ptr->name,50);

---

writes the name (50 chars) to disk.

but i want to write entire object
and not the individual fields

You cannot do that. If it's a class, you must write each member individually. Philip has already said this in your other (duplicate) post:

http://www.codeguru.com/forum/showthread.php?t=495486

Viggy

ok
i got what u r trying to explain

now problem is since i am writing in file from main() hence i cant write each data member individually

can u suggest any other approach

thank you

Quote:

---

now problem is since i am writing in file from main() hence i cant write each data member individually

---

Why not ? What have the main function and writing to a file have to do with each other ? You can write to a file from every function you want.

thanks a lot sir

i finally found a solution for my problem

thanks a lot