Is it necessary to flush a QTextStream before closing a QFile?

Question

I need to log some text messages to a file with following requirements :

1. Each text messages is written in a new line at the end of the file.

2. Be reasonably sure that each message was correctly written to the file.

So far, the function is using QTextStream and QFile :

bool FileManager::appendLine(const QString &line)
{
    if(!m_file.open(QIODevice::Append | QIODevice::Text)) // m_file is a QFile
        return false;

    QTextStream ts(&m_file);

    ts << line << endl;

    bool status = (ts.status() == QTextStream::Ok);

    m_file.close();

    return status;
}

Point 1 is satisfied but i have doubts about Point 2.

Even Qt Doc says that it is sufficient to close() the QFile to flush all its internal buffers :

void QFileDevice::close()

Reimplemented from QIODevice::close().

Calls QFileDevice::flush() and closes the file. Errors from flush are ignored.

What about the internal buffer of the QTextStream ?

Is it necessary to call QTextStream::flush() before closing the file ?

About Point 2, i guess that reading back the line just after it has been written would be the only way to be 100% sure of that. (for example a power failure may occur while the kernel has still datas in its buffers )

Thanks.

Answer 1

In your case, its not, because you are appending &endl in each write!

• Writting &endl to the QTextStream writes '\\n' to the stream and flushes the stream. It is Equivalent to: stream << '\\n' << flush;

• Further, when QTextStream is flushed due to &endl , it will empty all data from its write buffer into the device and call flush() on the device.

Answer 2

While this particular code will work because operations with QTextStream end with an endl , it's still better to ensure that QTextStream is completely and utterly finished working with the file when you close it. Just use scopes.

bool FileManager::appendLine(const QString &line)
{
    if(!m_file.open(QIODevice::Append | QIODevice::Text)) // m_file is a QFile
        return false;
    
    bool status {false};
    {
        QTextStream ts(&m_file);
        
        ts << line << endl;
        
        status = (ts.status() == QTextStream::Ok);
    }
    
    m_file.close();
    
    return status;
}