File Streams and Content Processing

File I/O goes through streams. ifstream reads, ofstream writes, and both close in their destructor — RAII again.

#include <fstream>
#include <iostream>

std::ifstream in{"config.txt"};
if (!in) {                       // stream is false-y when open failed
    std::cerr << "cannot open config.txt\n";
    return 1;
}

std::string line;
while (std::getline(in, line)) {
    process(line);               // line excludes the trailing newline
}
// in closes here — automatically

A reference implementation for our graded pattern — filtering lines whose content starts with ERROR :

#include <sstream>
#include <string>
#include <vector>

std::string error_lines(const std::string& content) {
    std::istringstream in{content};          // same interface, in-memory
    std::string out;
    std::string line;
    while (std::getline(in, line)) {
        if (line.rfind("ERROR ", 0) == 0) {  // prefix check
            out += line;
            out += '\n';
        }
    }
    return out;
}

The graded tests in this module pass file content as a string — the sandbox has no stable working directory for real files. The parsing discipline is identical: read line-oriented, check the stream state, and never assume the last line ends with a newline.

Common file bugs: reading without checking if (!stream), mixing >> and getline (the >> leaves \n in the buffer), and forgetting that getline succeeds on the final line without a trailing \n.