I'm not able to read and write to the same file - c++

Question

I'm trying to have the code read and repeat the last line of text in an input file, and put an underscore line right below it. As the code is, it will cout the last line to the terminal but won't output anything to the file. If I remove the while(getline()), the underscore line will appear but then it can't find the line string.
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
string underscoreDiv (int lengthText){
string underscores;
for(int i{}; i<lengthText; i++)
{
underscores += "_";
}
return underscores + "\n";
}
int main(){
fstream fileApp("C:\\Users\\trist\\OneDrive\\Documents\\Notes_application\\Notes_app.txt", ios::in | ios::app);
if(fileApp.is_open()){
string underscore;
string line;
underscore=underscoreDiv(80);
while(getline(fileApp, line)){} ///removing this line
cout<<line;
fileApp<<"\n"<<line<<endl;
fileApp<<underscore<<endl;
fileApp.close();
}
else{
cout<<"Text file not file";
}
system("C:\\Users\\trist\\OneDrive\\Documents\\Notes_application\\Notes_app.txt");
return 0;
}
I wrote this modified code that reads the last line and prints it and then prints the underscore line but I'm having to open, close, then reopen the input file which makes it too slow.
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
string underscoreDiv (int lengthText){
string underscores;
for(int i{}; i<lengthText; i++)
{
underscores += "_";
}
return underscores + "\n";
}
int main(){
fstream fileApp("C:\\Users\\trist\\OneDrive\\Documents\\Notes_application\\Notes_app.txt", ios::app | ios::in);
string underscore;
string line;
if(fileApp.is_open()){
underscore=underscoreDiv(80);
while(getline(fileApp, line)){}
cout<<line;
fileApp<<"\n"<<line<<endl; ///these 2 lines don't do anything
fileApp<<underscore<<endl;
fileApp.close();
}
else{
cout<<"Text file not file";
}
fileApp.open("C:\\Users\\trist\\OneDrive\\Documents\\Notes_application\\Notes_app.txt", ios::app | ios::in);
fileApp<<"\n"<<line<<endl;
fileApp<<underscore<<endl;
///system("C:\\Users\\trist\\OneDrive\\Documents\\Notes_application\\Notes_app.txt"); ///too lazy to write a close statement here.
return 0;
}

Answer 1

The problem that you encounter has to do with the state of the stream. After each IO operation the result of that operation is stored in internhal state bits of the stream.
There is a very good overview in the CPP reference here. Please especially look at the table at the bottom of this page.
After the execution of the loop while(getline(fileApp, line)){} the eof bit of the stream is set. And only because of that, the while loop stops. Look in the table. If the eof bit is set, the bool function will return false. And since the while-loop excpects a boolean value, the bool function of the stream (that will be returned by std::getline) will be called.
So, now that we reached the end of the file, the stream is in fail state.
And then, no further IO operation on this stream will be executed.
Solution: Call the streams clear function after your while loop: fileApp.clear();
Recommendation: In general you should always check the state of a stream after IO operations.