HW2.2 - A cleanup patch - 题目

This problem uses classes and destructors. You may leave it until after the object-oriented programming lecture.

LevelDB is a C++ library for storing data. The function below writes some text to a file. A developer proposes removing the explicit Close() call because the file object's destructor already closes it. Review this proposed cleanup.

Status represents success or failure; s.ok() is true on success. Append, Sync, and Close each return a Status. NewWritableFile creates a file object, or returns an error. RemoveFile removes the named file. Slice provides the data to write. These API notes are supplied for this exercise; you need not know how LevelDB stores data.

static Status DoWriteStringToFile(Env* env, const Slice& data,
                                  const std::string& fname, bool should_sync) {
  WritableFile* file;
  Status s = env->NewWritableFile(fname, &file);
  if (!s.ok()) {
    return s;
  }
  s = file->Append(data);
  if (s.ok() && should_sync) {
    s = file->Sync();
  }
  if (s.ok()) {
    s = file->Close();
  }
  delete file;  // Will auto-close if we did not close above
  if (!s.ok()) {
    env->RemoveFile(fname);
  }
  return s;
}

-  if (s.ok()) {
-    s = file->Close();
-  }
   delete file;  // Will auto-close if we did not close above

This is the destructor used by the POSIX implementation. fd_ >= 0 means it still holds an open file. delete file invokes this destructor.

~PosixWritableFile() override {
  if (fd_ >= 0) {
    // Ignoring any potential errors
    Close();
  }
}

Source attribution and license

Source excerpts: LevelDB, util/env.cc and util/env_posix.cc, revision bb74ef739973a70ca9f0d90788a1da89b788c0ad. The proposed deletion is authored for this exercise; it is not a claimed upstream change.

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2026 Fall HW2

E. HW2.2 - A cleanup patch

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