Convert a QByteArray in Hex to a Struct

[DSpider]

@aha_1980 Thank you. I just edited the question.

[SGaist]

Hi,

How are you writing the structure to file in the first place ?

[DSpider]

@SGaist I actually don't know. I am supposed to read only, another program (I don't have access to) does the writting.

[mrjj]

@DSpider
So its very unlikely that other program used QString ?
or its an Qt app too ?

[SGaist]

Would it be possible for you to request to know how it's done ?

If the writing is done using QDataStream it will simplify things.

[DSpider]

@mrjj well its a Qt app too, but I don't have more information. I have the structure of the file, I am able to decode it using QDataStream if I do it element by element. But I was wondering if there is a more simpler way to it ?

[DSpider]

@SGaist I think it is done with QDataStream, because in the struct there is even the version of it. Supposing it is the case, how can I do then ?

[SGaist]

QDataStream in(fileData);

struct header;
in >> header;

You should check whether the QDataStream used has its version set. If not, then you should suggest to add that so that you can ensure that if a future version of Qt changes some aspect of the QDataStream protocol you can still read the old files you have generated.

[DSpider]

@SGaist Thank you, I 'll try that right now. However, the header is actually on the first 1024 bytes of the file. Is there a way to read only this part ?

[SGaist]

if the writing was done like this:

QDataStream out(file);
out << myStruct:

then there's nothing special to do.

[DSpider]

@SGaist I tried to do it this way but it says "invalid operand to binary expression ('QDataStream' and 'struct header').

[SGaist]

I knew I was forgetting something, you have to implement the QDataStream operator for the structure.

Which is something that has likely be done for the other application. The operators implementation boils down to sending each field to the output stream and read each field from the input stream.

[fcarney]

@SGaist said in Convert a QByteArray in Hex to a Struct:

The operators implementation boils down to sending each field to the output stream and read each field from the input stream.

I was wondering about that. I read a bunch of articles last week about transferring binary data. Two of the major concerns are binary alignment/packing and endianess. Does the QDataStream address both of those issues? I am more concerned with the alignment/packing as all our systems are little endian. The alignment/packing can change if compiler settings are changed.

Edit:
Sorry, I should have just looked it up:
http://doc.qt.io/qt-5/qdatastream.html

[DSpider]

@SGaist Thank you. So it should look something like this ( which I am writting now):

friend QDataStream &operator>> (QDataStream &in, header &head)
    {
        in >> head.field1 >> head.field2.....;
        return in;
    }

[SGaist]

Yes it is, but you have to ensure the the writing was done in the same order.

[DSpider]

@SGaist Thanksssssssss to the infinity. That saved me tonight ! It's working.
I'm a noob on Qt, and do not have any clue on how it works, even C++ I'm not very familiar too.
Thank you all for your help.

[SGaist]

You're welcome !

Qt is C++ so by learning the second you'll be able to master the first :)