QByteArray to quint16

Creatorczyk

Hi,

I wrote code like below to change QByteArray to quint16 variable:

QByteArray data;
   data.resize(1);
   data[0] = 0x08;

   QDataStream stream(&data, QIODevice::ReadOnly);
   stream.setByteOrder(QDataStream::BigEndian);

   quint16 var;

   stream >> var;

   qDebug() << "Expected var = 8, but var = " << var;

But output is:

Expected var = 8, but var =  0

I know that quint16 is written on two bytes and QByteArray has one byte, but this is part of a larger code and I may happen that QByteArray will have two bytes as below:

QByteArray data;
    data.resize(2);
    data[0] = 0x08;
    data[1] = 0x04;

Then the output is correctly and var = 2052. How could I write a QByteArray with one byte to quint16?

jsulm

@Creatorczyk Does this work:

quint8 var;
stream >> var;
quint16 var16 = var;

Creatorczyk

@jsulm Yes, it's works, but can I do this without creating quint8?

jsulm

@Creatorczyk said in QByteArray to quint16:

but can I do this without creating quint8?

I guess you need two bytes in the QByteArray to make it work (fill it with a zero byte).
I don't know what QDataStream does if you ask it to give you a short but the underlying QByteArray contains only one byte (you can check the QDataStream source code if you want to know what happens).

JonB

@Creatorczyk said in QByteArray to quint16:

How could I write a QByteArray with one byte to quint16?

quint16 var16 = qByteArray[0]?

As @jsulm has said, I don't think you're supposed to stream one byte into a two-byte variable. Sounds like it does something like only go into the first byte at the address, or maybe even does nothing at all, at least something dodgy.

JKSH

@Creatorczyk said in QByteArray to quint16:

How could I write a QByteArray with one byte to quint16?

You cannot do that directly with a QDataStream. There are not enough bytes, so you will get a QDataStream::ReadPastEnd error: https://doc.qt.io/qt-5/qdatastream.html#Status-enum

@jsulm suggested that you study the source code; the code for QDataStream::operator>>() is at https://code.woboq.org/qt5/qtbase/src/corelib/serialization/qdatastream.cpp.html#_ZN11QDataStreamrsERs

JonB

@JKSH
Ah, I wondered whether it set an error and why the OP received 0 as the result here. Answer from the code you referenced:

    if (readBlock(reinterpret_cast<char *>(&i), 2) != 2) {
        i = 0;

That's why the result is 0, and readBlock() sets error flag from trying to read 2 bytes but only getting 1 via

    const int readResult = dev->read(data, len);
    if (readResult != len)
        setStatus(ReadPastEnd);

VRonin

@JKSH said in QByteArray to quint16:

There are not enough bytes, so you will get a QDataStream::ReadPastEnd

For Qt 5.7 or later:

quint16 var;
stream.startTransaction();
stream >> var;
if(!stream.commitTransaction()){
    quint8 tempVar;
    stream.startTransaction();
    stream >> tempVar;
    if(!stream.commitTransaction()){
        //handle this error
    }
    var=tempVar;
}