File stream condition. To Read if file has content else write info

[Cheche01]

What condition shoul i use?
if notepad/file has contents it will display the contents.

but if the notepad/file has no contents, it will automatically let the user enter first name, last name and address then write it on the file.
here's my code. I don't know what condition to use if the file is empty.

#include<iostream>
#include<fstream>
using namespace std;

int main(){
   int size=20;
   char ans;
   char fname[size], lname[size], add[size];
   fstream fdata;
   fdata.open("Prog.txt");

   if(fdata){

             string file;
             getline(fdata,file);
             cout<<"Name: "<<file<<" ";
             getline(fdata,file);
             cout<<file<<endl;
             getline(fdata,file);
             cout<<"Address: "<<file<<endl;
             }

     else if(fdata.is_open()){
        cout<<"First Name: ";
        cin.getline(fname, size);
        cout<<"Last Name: ";
        cin.getline(lname,size);
        cout<<"Address: ";
        cin.getline(add,size);
        }
               fdata<<fname<<endl;
               fdata<<lname<<endl;
               fdata<<add<<endl;          

             fdata.close();
}

[AbstractionAnon]

I don't know what condition to use if the file is empty.

Attempt a read from the file. If after the read eof() returns true, the file is empty.

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[Cheche01]

Edited my code.
I'm sorry but how do you do it?