C++ Files and Stream I/O

std::ifstream owns an input file handle and std::ofstream owns an output handle. Construction attempts to open the path; stream state reports whether it succeeded. When the stream leaves scope, its destructor closes the handle even after an early return.

Opening can fail because a path is missing, permissions deny access, or a device is unavailable. Report enough context for the user to act, but avoid exposing secrets or assuming every missing file is an error—a first-run data file may be optional.

#include <fstream>
#include <iostream>
#include <string>

int main()
{
  std::ifstream input{"notes.txt"};
  if (!input)
  {
      std::cerr << "notes.txt is unavailable\n";
      return 1;
  }

  std::string line;
  while (std::getline(input, line))
      std::cout << line << '\n';
}

Line-oriented formats should use getline; whitespace-delimited numeric data can use formatted extraction; structured formats should use a parser that implements their grammar. Drive the loop from the read result so processing happens only after successful input.

Distinguish normal end-of-file from a malformed record or I/O failure. If a malformed line can be skipped, report its line number. If partial data would be dangerous, reject the whole load and keep the previous valid state.

Core calculations should not discover files from hidden working-directory assumptions. Pass a path or stream into the storage boundary. Tests can then use a temporary location or string stream without changing domain logic.

The filesystem library provides path operations and directory queries. Keep path values as paths rather than assembling separators by hand, and never trust an external path without considering traversal and overwrite rules.

A saved format becomes an interface with earlier versions of the program. Define field meaning, escaping, optional values, and version changes. Hand-written delimiter splitting fails when user text contains that delimiter.

For important data, write a complete replacement to a temporary file, check the result, then replace the target according to platform guarantees. Backups, locking, and concurrent writers are product decisions beyond the syntax of ofstream.