QBuffer from void pointer and memory size

Question

I'm looking for the most simple and or elegant way to create a QBuffer in C++ Qt5.6 from void * data and long data_size.

I tried casting the void to a char pointer and using the QByteArray::fromRawData() as well as using QDataStream to fill a QByteArray. In both cases I didn't succeed.

Answer 1

QByteArray::QByteArray(const char *data, int size) will copy the data.

QByteArray::fromRawData(const char *data, int size) will use the already existing data.

Depending on your implementation, not copying the data might end up being problematic.

After you have the data in a byte array, there are several ways to go, you can directly construct a buffer:

QBuffer(QByteArray *byteArray, QObject *parent = Q_NULLPTR)

or more likely, since you are playing audio, you might want to reusing the same buffer and use one of the following to refill it:

setBuffer(QByteArray *byteArray)
setData(const QByteArray &data)

Lastly, there is also void QBuffer::setData(const char *data, int size) for which you don't even need the byte array step at all.

Lastly, remember that QBuffer is an IO device - it needs to be opened in order for it to work. A quick test shows that it works as expected:

char data[] = {1, 2, 3};
void * vdata = data;
QBuffer buffer;
buffer.setData((char *)vdata, sizeof(data));
buffer.open(QIODevice::ReadOnly);
qDebug() << buffer.readAll(); // outputs "\x01\x02\x03"

Answer 2

Since you are asking for an elegant way to create a QBuffer from data and data_size, I would simply use this:

QBuffer buffer;
buffer.setData(static_cast<const char*>(data), data_size);

Note: this copies the memory pointed to by data.

---

I have no idea why you are unable to succeed, maybe you forgot to add the data_size as size argument for QByteArray::fromRawData? In that case strlen(data) would be used to calculate the size.

And if you are using something like this:

QByteArray byteArray = QByteArray::fromRawData(static_cast<const char*>(data), data_size);
QBuffer buffer(&byteArray);

Then the byteArray must remain valid until the QBuffer is destroyed. And since QByteArray::fromRawData(...) does not copy, the memory pointed to by data must remain valid as well. Failing to meet that requirement would also explain any failure.