[Solved]Convert from qint32 to qbytearray

antonyprojr

How to convert from qint32 to 4bytes QByteArray and how to convert back from QByteArray to qint32?

butterface

Is that what you are looking for: "setNum":http://qt-project.org/doc/qt-4.8/qbytearray.html#setNum
If not, maybe use a QDataStream and << num
and if also not I did not understand your question.

N3Roaster

If the QByteArray is only going to contain a qint32, you could use fromRawData. Alternately you can create a char* with the address of the integer and append from there. The latter technique works in reverse as well to get the qint32 back from the byte array.

MuldeR

[quote author="butterface" date="1349040456"]Is that what you are looking for: "setNum":http://qt-project.org/doc/qt-4.8/qbytearray.html#setNum
If not, maybe use a QDataStream and << num
and if also not I did not understand your question.[/quote]

That would first convert the number to a "printable" ASCII string and then set the QByteArray these characters. If you want to create a QByteArray containing the "raw" bits of a qint32 value, you can do something like like:
@qint32 x = 42;
QByteArray foo = QByteArray::fromRawData(reinterpret_cast<char*>(&x), sizeof(qint32));@ The above won't copy the data though, it just wraps the pointer. To copy the data, you may use: @qint32 x = 42;
QByteArray foo(sizeof(qint32), '\0'); //Create QByteArray of required size
memcpy(foo.data(), &x, sizeof(qint32)); //Copy the data into the QByteArray@ Should also work: @qint32 x = 42;
QByteArray foo(reinterpret_cast<char*>(&x), sizeof(qint32));@

antonyprojr

This code works:

@QByteArray array;
qint32 x = 11;
array.append((const char*)&x, sizeof(x));@

How to convert it back to qint32?

ChrisW67

You can use MuldeR's memcpy in reverse
@
// in
qint32 x = 42;
QByteArray foo(sizeof(qint32), '\0'); //Create QByteArray of required size
memcpy(foo.data(), &x, sizeof(qint32)); //Copy the data into the QByteArray
//out
qint32 y;
Q_ASSERT(foo.size() > sizeof(y));
memcpy(&y, foo.data(), sizeof(y));
@

or even
@
//out
Q_ASSERT(foo.size() > sizeof(qint32));
qint32 y = (reinterpret_cast<qint32>(foo.data()));
@

If in and out occur on different machines then you must consider the order that bytes are written in.

andre

I'd just use a QDataStream:

@
//write
QByteArray target;
QDataStream s(&target);
qint32 value = someValue;
s << value;

//read
qint32 value;
QByteArray source;
QDataStream s(&source);
s >> value;
@

This way is much saver than using direct casts. It will work across small and big endian systems, for instance.

antonyprojr

Thanks for any answer, this is the solution for me:
@
//write
QByteArray target;
QDataStream s(&target, QIODevice::ReadWrite);
qint32 value = someValue;
s << value;

//read
qint32 value;
QByteArray source;
QDataStream s(&source, QIODevice::ReadWrite);
s >> value;
@

Mohammad Kanan

@andre ++1 for mentioning the "Endian" systems