Alternating between reading and writing repeatedly - c++

Question

My objective is to read a file line by line, check if that line contains some number, and if so rewrite that line. Then continue reading the file.
I've successfully been able to do this for one line, but I can't figure out how to continue reading the rest of the file.
Here's how I replace one line (every line is a known fixed size):
while(getline(fs, line)){
if(condition){
pos = fs.tellg(); //gets current read position (end of the line I want to change)
pos -= line.length()+1; //position of the beginning of the line
fs.clear(); //switch to write mode
fs.seekp(pos); //seek to beginning of line
fs << new_data; //overwrite old data with new data (also fixed size)
fs.close(); //Done.
continue;
}
}
How do I switch back to read and continue the getline loop?

Answer 1

I had the same problem, TB-scale files and I wanted to modify some header information in the beginning of the file.
Obviously one has to leave enough room when one initially creates the file for any new content, because there is no way to increase the file size (besides appending to it) and the new line has to have the exact same line length as the original one.
Here is a simplification of my code:
#include <iostream>
#include <fstream>
using namespace std;
bool CreateDummy()
{
ofstream out;
out.open("Dummy.txt");
// skip: test if open
out<<"Some Header"<<endl;
out<<"REPLACE1 12345678901234567890"<<endl;
out<<"REPLACE2 12345678901234567890"<<endl;
out<<"Now ~1 TB of data follows..."<<endl;
out.close();
return true;
}
int main()
{
CreateDummy(); // skip: test if successful
fstream inout;
inout.open("Dummy.txt", ios::in | ios::out);
// skip test if open
bool FoundFirst = false;
string FirstText = "REPLACE1";
string FirstReplacement = "Replaced first!!!";
bool FoundSecond = false;
string SecondText = "REPLACE2";
string SecondReplacement = "Replaced second!!!";
string Line;
size_t LastPos = inout.tellg();
while (getline(inout, Line)) {
if (FoundFirst == false && Line.compare(0, FirstText.size(), FirstText) == 0) {
// skip: check if Line.size() >= FirstReplacement.size()
while (FirstReplacement.size() < Line.size()) FirstReplacement += " ";
FirstReplacement += '\n';
inout.seekp(LastPos);
inout.write(FirstReplacement.c_str(), FirstReplacement.size());
FoundFirst = true;
} else if (FoundSecond == false && Line.compare(0, SecondText.size(), SecondText) == 0) {
// skip: check if Line.size() >= SecondReplacement.size()
while (SecondReplacement.size() < Line.size()) SecondReplacement += " ";
SecondReplacement += '\n';
inout.seekp(LastPos);
inout.write(SecondReplacement.c_str(), SecondReplacement.size());
FoundSecond = true;
}
if (FoundFirst == true && FoundSecond == true) break;
LastPos = inout.tellg();
}
inout.close();
return 0;
}
The input is
Some Header
REPLACE1 12345678901234567890
REPLACE2 12345678901234567890
Now ~1 TB of data follows...
The output is:
Some Header
Replaced first!!!
Replaced second!!!
Now ~1 TB of data follows...