Registry vs INI: Choosing QSettings Format for Portable Backups

Direct Answer

Default to QSettings::IniFormat on Windows. It produces a plain‑text .ini file that can be copied and restored on Windows, Linux, and macOS without conversion. The registry format is opaque and cannot be migrated directly to another OS.

Why IniFormat Wins for Portability

• Registry: Windows‑only, binary, requires re‑creating keys on restore; not portable.
• IniFormat: Text‑based, hierarchical groups and all Qt data types supported since Qt 4.6. Identical file structure on all platforms.
• Performance: Negligible difference for typical configuration sizes (a few KB). Read/write times differ by a few milliseconds.

Reliable Integrity Verification

QSettings::status() only reports the last sync result. For backup verification, compute a deterministic SHA‑256 hash of the serialized key/value pairs:

1. Open the .ini with QSettings in IniFormat.
2. Retrieve all keys via allKeys(), sort them (e.g., std::sort) to guarantee consistent order.
3. Iterate sorted keys, concatenate key=value lines using UTF‑8 encoding (no BOM).
4. Hash the resulting byte array with QCryptographicHash::hash(data, QCryptographicHash::Sha256).
5. Store the hex digest in a companion .sha256 file alongside the .ini.

// Example verification routine (Qt 5/6)
QByteArray computeHash(const QString &iniPath) {
    QSettings settings(iniPath, QSettings::IniFormat);
    QStringList keys = settings.allKeys();
    std::sort(keys.begin(), keys.end());
    QByteArray serialized;
    for (const QString &k : keys) {
        serialized.append(k.toUtf8());
        serialized.append('=');
        serialized.append(settings.value(k).toString().toUtf8());
        serialized.append('\n');
    }
    return QCryptographicHash::hash(serialized, QCryptographicHash::Sha256).toHex();
}

Assumptions & Uncertainty

• Assumes Qt ≥ 4.6 (IniFormat on Windows).
• Assumes configuration values are convertible to string via toString(). If you store custom QVariant types, you must provide a custom serialization before hashing.
• UTF‑8 without BOM is used by default on Windows; ensure the same encoding on verification.

Verification Steps

1. Write a sample config with IniFormat, call sync(), compute hash, save .sha256.
2. Copy .ini and .sha256 to Linux/macOS, run the same hash routine; digests must match.
3. Measure write/read latency for your actual config size on Windows with both formats; expect ≤ 5 ms difference.

One Missing Detail

What is the typical size of your configuration (number of keys, total bytes) and do you use any custom QVariant types? This affects whether the default string serialization is sufficient or a custom routine is needed.