QT read/write from QDataStream

Question

I think i am having a fundamental misunderstanding of how all of this works, I am trying to combine 2 hexfiles in my QDataStream and then output them to a new QFile.

QFile filea( file1 );
QFile fileb( file2 );
QByteArray ba;
QDataStream ds(ba);
ds << filea.readAll();
ds << fileb.readAll();
QFile result("C:/Users/Aaron/Desktop/mergedfile.txt");
result.open(QIODevice::ReadWrite);
result.write(ba);

The result is just an empty file, any suggestions?

score 1 · Accepted Answer · answered Aug 02 '18 at 21:41

You have the following errors:

If you are only going to read a file, you must open it with the ReadOnly mode, if you are going to write it you must use WriteOnly, in your case you do not have it with filea, fileb and result.
The QByteArray you are using it to read the data of the filea and fileb files, and then write it to result, so it must be read and write. You are using the following QDataStream constructor:

QDataStream::QDataStream(const QByteArray &a)

Constructs a read-only data stream that operates on byte array a. Use QDataStream(QByteArray*, int) if you want to write to a byte array.

Since QByteArray is not a QIODevice subclass, internally a QBuffer is created to wrap the byte array.

And since the QByteArray is read only because it is not the appropriate constructor, you must use the other constructor:

QDataStream::QDataStream(QByteArray *a, QIODevice::OpenMode mode)

Constructs a data stream that operates on a byte array, a. The mode describes how the device is to be used.

Alternatively, you can use QDataStream(const QByteArray &) if you just want to read from a byte array.

Since QByteArray is not a QIODevice subclass, internally a QBuffer is created to wrap the byte array.

Using the above we obtain the following:

QFile filea( file1 );
QFile fileb( file2 );
QFile result("C:/Users/Aaron/Desktop/mergedfile.txt");

if(filea.open(QIODevice::ReadOnly) && fileb.open(QIODevice::ReadOnly) && result.open(QIODevice::WriteOnly)){
    QByteArray ba;
    QDataStream ds(&ba, QIODevice::ReadWrite);
    ds << filea.readAll();
    ds << fileb.readAll();
    result.write(ba);
    result.flush();
}

QT read/write from QDataStream

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