Storing integer to QByteArray using only 4 bytes - c++

Question

It takes 4 bytes to represent an integer. How can I store an int in a QByteArray so that it only takes 4 bytes?
QByteArray::number(..) converts the integer to string thus taking up more than 4 bytes.
QByteArray((const char*)&myInteger,sizeof(int)) also doesn't seem to work.

Answer 1

There are several ways to place an integer into a QByteArray, but the following is usually the cleanest:
QByteArray byteArray;
QDataStream stream(&byteArray, QIODevice::WriteOnly);
stream << myInteger;
This has the advantage of allowing you to write several integers (or other data types) to the byte array fairly conveniently. It also allows you to set the endianness of the data using QDataStream::setByteOrder.
Update
While the solution above will work, the method used by QDataStream to store integers can change in future versions of Qt. The simplest way to ensure that it always works is to explicitly set the version of the data format used by QDataStream:
QDataStream stream(&byteArray, QIODevice::WriteOnly);
stream.setVersion(QDataStream::Qt_5_10); // Or use earlier version
Alternately, you can avoid using QDataStream altogether and use a QBuffer:
#include <QBuffer>
#include <QByteArray>
#include <QtEndian>
...
QByteArray byteArray;
QBuffer buffer(&byteArray);
buffer.open(QIODevice::WriteOnly);
myInteger = qToBigEndian(myInteger); // Or qToLittleEndian, if necessary.
buffer.write((char*)&myInteger, sizeof(qint32));

Answer 2

#Primož Kralj did not get around to posting a solution with his second method, so here it is:
int myInt = 0xdeadbeef;
QByteArray qba(reinterpret_cast<const char *>(&myInt), sizeof(int));
qDebug("QByteArray has bytes %s", qPrintable(qba.toHex(' ')));
prints:
QByteArray has bytes ef be ad de
on an x64 machine.

Answer 3

Recently I faced the same problem with a little variation. I had to store a vector of unsigned short into QByteArray. The trick with QDataStream did not work for unknown reason. So, my solution is:
QVector<uint16_t> d={1,2,3,4,5};
QByteArray dd((char*)d.data(),d.size()*sizeof(uint16_t));
The way to get the vector back is:
QVector<uint16_t> D;
for(int i=0; i<dd.size()/sizeof(uint16_t); ++i){
D.push_back(*(uint16_t*)(dd.data()+i*sizeof(uint16_t)) );
}