trouble with QByteArray

carter_james

i done this operation
quint32 registry =0x00000000;
quint8 value1l=3;
quint8 value2=0;
quint16 value3=8191;
quint16 value4=29;

registry=(value4<<19)|(value3<<6)|(value2<<4)|value1;
QByteArray b;
b.setNum(registry,10);

i get in debugger this one

b "15728579" QByteArray
[0] 49 '1' char
[1] 53 '5' char
[2] 55 '7' char
[3] 50 '2' char
[4] 56 '8' char
[5] 53 '5' char
[6] 55 '7' char
[7] 57 '9' char

why we get 8 Byte?
i think it schoud be 6 Byte because
value 1 ist 1Byte
value 2 ist 1Byte
value 3 ist 2 Byte
value 4 ist 2Byte
where may i think false ?

kshegunov

@carter_james
Hello,

why we get 8 Byte?

http://doc.qt.io/qt-5/qbytearray.html#setNum

Sets the byte array to the printed value of n in base base (10 by default) and returns a reference to the byte array. The base can be any value between 2 and 36. For bases other than 10, n is treated as an unsigned integer.

Your number has 8 digits, so the string (stored in the byte array) contains 8 characters.

i think it schoud be 6 Byte because

If I understand correctly what you're trying to do, the byte array should be 4 bytes long, because qint32 registry is 32 bits (or 4 bytes) in size.

where may i think false ?

As I suggested in your previous thread, you should mask your numbers, because if the value1 variable has a value larger than 15, then this line:

registry=(value4<<19)|(value3<<6)|(value2<<4)|value1;

could simply produce incorrect values.

That aside you can create byte array by casting the data appropriately:

QByteArray data = QByteArray::fromRawData(reinterpret_cast<char * >(&registry), sizeof(registry));

However you should be aware this doesn't take into account the endianness of the integer. If you need a portable way to transmit that data (for example through a network) you can serialize it with a data stream:

QByteArray data;
QDataStream stream(&data);
stream << registry;

So depending on what byte-order the binary data is expected to be in, you can call QDataStream::setByteOrder if applicable.

PS.
What you're trying to do with shifting can also be done with a union a struct and some bit fields as well:

union  {
    struct   {
        qint8 field1 : 4;
        qint8 field2 : 2;
        qint16 field3 : 13;
        qint16 field4 : 13;
    } flags;
    char * raw;
} dataVar;

// Fill the data
dataVar.flags.field1 = value1;
dataVar.flags.field2 = value2;
dataVar.flags.field3 = value3;
dataVar.flags.field4 = value4;

// Convert to byte array
QByteArray data = QByteArray::fromRawData(dataVar.raw, sizeof(dataVar.flags));

Which is used a bit more intuitively.