QDataStream to return QByteArray set

Kevin470

Is there a possibility to find which QByteArray is set to the QDataStream object?
This method QDataStream::device() returns the device set to it. But is there a way to return if the QDataStream object is connected to a QByteArray and finding out which QByteArray it is set to?
Thanks.

ChrisW67

I do not know why you would want to do this. Your code constructed the QDataStream with an associated QByteArray so I am not sure how you would not know which byte array that was.

Anyway, something like this might work:

QBuffer *internalBuffer = qobject_cast<QBuffer*>(stream.device());
if (internalBuffer) {
  QByteArray data = internalBuffer->buffer();
  ...
} 
else {
  // not  an underlying QByteArray.  Panic?
}

Kevin470

@ChrisW67 The idea was that I want to write a function that needs the QDataStream and the size of the QByteArray that it is connected to. An example would be like this:

void sampleFn(QDataStream &ds, QByteArray &ba)
{
    for (int i = 0; i < ba.size() ; i++)
    {
        qint8 int_8;
        ds >> int_8;
        qInfo() << int_8;
    }
}

This is just a sample function. I just want to make sure that the user of this function does not give the wrong QByteArray here, so that I can take just one parameter QDataStream &ds and work with it.

Kevin470

@ChrisW67 And also, this works! Thank you so much.

Kevin470

@ChrisW67 Is it possible to get the pointer or reference of the QByteArray corresponding to the QDataStream?

Christian Ehrlicher

@Kevin470 said in QDataStream to return QByteArray set:

Is it possible to get the pointer or reference of the QByteArray corresponding to the QDataStream?

Why would someone need it? Why do you want to modify the data in a function in a function where it should not be done? What's the (strange) use case for this?

ChrisW67

@Kevin470 That is what the code I gave you does. Not sure I understand the question (or, as @Christian-Ehrlicher and I have said, the original problem).

I just want to make sure that the user of this function does not give the wrong QByteArray here, so that I can take just one parameter QDataStream &ds and work with it.

You can derive the QByteArray, and hence its size, inside the function.
The function is misleading though; you cannot pass an arbitrary QDataStream and expect the function to work.

This should fly and does not need to know the size or type of the underlying data source:

void sampleFn(QDataStream &ds)
{
    while (!ds.atEnd()) 
    {
        qint8 int_8;
        ds >> int_8;
        qInfo() << int_8;
    }
}

Have you considered passing the function the QByteArray by const reference? If the stream in the array is complex you can still do something like this:

void sampleFn(const QByteArray &ba)
{
    QDataStream ds(ba);
    while (!ds.atEnd()) 
    {
        qint8 int_8;
        qint32 int_32;
        ds >> int_8 >> int_32;
        qInfo() << int_8 << int_32;
    }
}

If you are using QDataStream just to read bytes from the buffer then you can do without QDataStream entirely.

Kevin470

@Christian-Ehrlicher @ChrisW67 I have not found a right solution to my problem yet.
My actual use case is to read and write strings to a QByteArray but without the extra information of the string size and everything else in it.
For example if I am to write a QString str = "Hello World" to a QByteArray, I am trying to find a solution to write just the bytes of each character in the string in the QByteArray.
Which means, it must contain only 48:65:6C:6C:6F:20:57:6F:72:6C:64 and not 00:00:00:0B:48:65:6C:6C:6F:20:57:6F:72:6C:64 where 0B gives the size of the String.
Similarly, the other way around. To read each byte to a QString without the need of knowing the size. Because the size is already known during reading and writing.

I was thinking of maybe manipulating the QByteArray so that the bytes containing the size of the QString is removed. But I know it is a very bad idea to do so. That is when I was asking if it is possible to know which QByteArray is corresponding with the QDataStream.

Is there a possibility or workarounds for my use case?

jsulm

@Kevin470 said in QDataStream to return QByteArray set:

My actual use case is to read and write strings to a QByteArray but without the extra information of the string size and everything else in it.

Then you should not use QDataStream at all.
QString has some methods to convert a QByteArray to a QString (for example https://doc.qt.io/qt-6/qstring.html#fromUtf8-2).
It also has methods to convert a QString to a QByteArray: https://doc.qt.io/qt-6/qstring.html#toUtf8

Kevin470

@jsulm Therein lies my problem unfortunately. (The actual source of the bytearray that I receive is actually not written using the Qt Framework, and so it does not have any QString QByteArray concepts in it, so I have to read every byte separately in a uniform order and strings do not have their size mentioned before them)
My QByteArray consists of data from different datatypes.
For example in this instance:

QByteArray ba;
QDataStream write(&ba,QDataStream::WriteOnly);
QDataStream read(&ba,QDataStream::ReadOnly);

quint8 int8 = 14;
quint16 int16 = 22;
write << int8;
write << int16;

qInfo() << QString("Hello World").toUtf8().toHex(':');
write << QString("Hello World").toUtf8();

qInfo() << ba.toHex(':');

Here I expect "0e:00:16:48:65:6c:6c:6f:20:57:6f:72:6c:64" and not "0e:00:16:00:00:00:0b:48:65:6c:6c:6f:20:57:6f:72:6c:64"
I can't use QByteArray::append() because the flexibility to choose the Endianness is lost I think. And anyway, append makes the quint16 to single byte if the number can be written in a single byte.

Kevin470

I made a rudimentary solution for the problem.

//Read data to QString with a known length
void readDataQString(QDataStream &ds, QString &str, uint length)
{
    QByteArray buffer(length,Qt::Uninitialized);
    ds.readRawData(buffer.data(),length);
    str = QString(buffer);
    qInfo() << str;
}

//Write data from QString without length info
void writeDataQString(QDataStream &ds, QString &str)
{
    QByteArray ab = str.toUtf8();
    for(int i = 0; i < ab.size(); ++i)
    {
        quint8 int_8 = static_cast<quint8>(ab.at(i));
        ds << int_8;
    }
    QBuffer *internalBuffer = qobject_cast<QBuffer*>(ds.device());
    QByteArray ba = internalBuffer->buffer();
    qInfo() << ba.toHex(':');
}

I hope this works for everyone.