unable to edit the second line of the file in cpp - file

Question

I want to edit the username and password according to userinput. I have a file exampless.txt whixh has default username and password values. I want to replace the values of username and password according to user input. I'm able to replace the username value. But password values doesnt get changed.
Please help.
My code is as below:
#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
int main()
{
std::wstring y;
std::wstring username;
std::wstring password;
std::cout<<"enter the username:";
std::wcin>>username;
std::cout<<"enter the password:";
std::wcin>>password;
std::wstring x=L"username";
std::wstring a=L"password";
std::wfstream fp("/home/aricent/Documents/exampless.txt",std::ios::in | std::ios::out );
while(!fp.eof()) {
std::getline(fp, y);
if (y.find(x, 0) != std::string::npos) {
std::wstring z=y.substr(y.find(L"-") + 1) ;
fp.seekg( 0, std::ios::beg );
fp<<y.replace(x.length()+1, z.length(), username)<<"\n";
fp.clear();
fp.seekg( 0, std::ios::beg );
}
fp.seekg(0,std::ios::beg);
std::getline(fp, y);
if (y.find(a, 0) != std::string::npos) {
std::wstring b=y.substr(y.find(L"-") + 1) ;
fp<<y.replace(a.length()+1, b.length(), password <<std::endl;
fp.clear();
}
}
fp.close();
}
my exampless.txt file contains:
username-aaa
password-bbb

Answer 1

There are many reasons why your code doesn't work. Some have to do with the way you intend to do this. Some have to do with the way you actually implemented your idea.
So I suggest you simplify the code in the following way.
#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
int main() {
std::wstring username;
std::wstring password;
std::cout<<"enter the username:";
std::wcin>>username;
std::cout<<"enter the password:";
std::wcin>>password;
std::wfstream fp("/home/aricent/Documents/exampless.txt",std::ios::in | std::ios::out );
if(!fp) { /* Failed to open file. do something */ }
<here you can check whether the file is already properly formatted>
if(!(fp << "username-" << username << "\npassword-" << password << "\n") {
//writing failed. Do something to handle this error (like throwing an exception)
}
fp.close();
}
So instead of reading the file contents, modifying it and writing it back to the file you can simply overwrite it with whatever you want it to contain.
This avoids (and fixes in this case) a lot of problems.
Also, your code should check for any exceptional states and handle them properly (like failing to open the file).
In your code you also check for whether the file is properly formatted (e.g. are the username and password lines already there) before writing to it. The above changes make this step redundant but if you still want this check to be done you can put it in between the two if's.
Finally I want to clear up a misconception. std::fstream maintains two separate positions into the filestream. One for reading and one for writing. To change these positions it has two methods seekp(pos) and seekg(pos). seekp(pos) changes at what position in the filestream any write operation (like operator << and put()) starts writing (the p stands for put I guess). seekg(pos) changes at what position in the filestream any read operation (like operator >> and get()) starts writing (the g stands for get).
Reading from a stream only changes the reading position but not the writing position and vice versa.