replace text in middle of the file - c++

Question

I have .txt file and there are lines:
username1:123456789:etc:etc:etc:etc
username2:1234:etc:etc:etc:etc
username3:123456:etc:etc:etc:etc
username4:1234567:etc:etc:etc:etc
username1 - username;
123456789 - password;
etc - more text.
I have code to read file and find line where is username what I need. Also code change password, but there is problem if new password is longer that old one then it looks like:
username3:11111111111tc:etc:etc:etc
if new password is shorter then it looks like:
username1:111111789:etc:etc:etc:etc
I have length of new password, but how can I get length of old one and replace it properly?
My code
include<iostream>
#include<fstream>
#include <cstring>
using namespace std;
int main() {
int i=0;
bool found = false;
string line, username;
char newpass[255] = "555555555555555";
long length, plen;
cout<<"Insert username: ";
cin>>username;
username+=":";
fstream changeLINE("/.../testaDoc.txt");
if (!changeLINE) {
cout << "Can't find the file or directory!" << endl;
}
else
while (getline(changeLINE, line) && !found) {
i++;
if (line.find(username) != string::npos) {
length = changeLINE.tellg();
changeLINE.seekg((length - line.length()) + username.length() - 1);
changeLINE.write("", strlen(newpass));
cout << line << " line " << i << endl;
found = true;
}
}
if (!found)
cout << "User with username = " << username << " NOT FOUND!";
changeLINE.close();
}
I'm working on Linux, writing in C++.
Edit
Maybe there are way to add string in text, but not replace it and also way to erase string by not replacing it? Then I could read length of old password compare it with new password and delete/add letters in string to replace it properly.

Answer 1

Unless the line you want to replace is the same length as the new one, you need a different strategy-
This is a strategy you can use:
Create temporary file
Write the lines you dont want to change straight to the temporary file. The line you want to change, you replace with the new line
close both files.
delete original file
rename temporary file to the original files name
Or you could as mentioned in the comments read all lines into memory, e.g. a vector of lines, replace the one you want to change, and then write all lines back into the file, replacing the former content. If the new line is shorter than the previous, you can trunctate the file using e.g: How to truncate a file while it is open with fstream
Could this work:
std::vector<std::string> lines;
while (std::getline(changeLINE, line))
{
i++;
if (line.find(username) != std::string::npos) {
std::cout << line << " line " << i << std::endl;
found = true;
std::string newline = username + ":" + newpass + line.substr(line.find(":", username.length() + 2)) ;
lines.push_back(newline);
}
else
{
lines.push_back(line);
}
}
changeLINE.close();
std::ofstream ofs;
ofs.open("/.../testaDoc.txt", std::ofstream::out | std::ofstream::trunc);
for(auto& s: lines)
ofs << s << std::endl;
ofs.close();