How to prepend the data size to a file

summit

I am saving the QAbstract tree model to a dat file.

How can i prepend the size of total bytes to be written to the start of dat file.

void saveTreeStructureToFile(const QModelIndexList &indexes , const std::string stdstrFilePath)
{
    QMimeData *mimeData = new QMimeData;
    QByteArray data; //a kind of RAW format for datas
    QDataStream stream(&data, QIODevice::WriteOnly);
    QList<TreeItem *> nodes;
    foreach(const QModelIndex &index, indexes) {
        TreeItem *node = getItem(index);
        if (!nodes.contains(node))
            nodes << node;
    }
    stream << nodes.count();
    foreach(TreeItem *node, nodes) {
        buildTree(node, stream);
    }
    mimeData->setData(s_treeNodeMimeType, data);
    std::string st = stdstrFilePath.substr(0, stdstrFilePath.size() - 3);
    st.append("dat");
    const QString path = st.c_str();
    QFile file(path);
    file.open(QIODevice::WriteOnly);
    QDataStream out(&file);
    out << *mimeData;
}

J.Hilk

hi @summit
you have 2 possibilities,

either calculate the size beforehand and write it first or write 0 in the first few bytes (4?) and overwrite them at the end.

I you open your file in ReadWrite you can overwrite partially without loosing the rest. Just make sure to only change the reserved bytes

summit

@J-Hilk Thank you very much , i have a question if i write 0 in the beginning how do i overwrite it with the size at the end.

Can you please show me the code .

suppose if i write 0 to the file

stream << 0 // write 0 to the file and than write all other data
stream << nodes.count();

now how do i overwrite this data in the end

J.Hilk

@summit actually looking at your code, the following should work

QFile file(path);
    file.open(QIODevice::WriteOnly);
    QDataStream out(&file);
    out << static_cast<qint64 >(0) << *mimeData;

  file.close();
  if(file.open(QIODevice::ReadWrite)
      file.write(static_cast<qint64 >(file.size()));
  file.close();

summit

@J-Hilk said in How to prepend the data size to a file:

static_cast<qint64 >(0)

@J-Hilk Thank you very much , i am not able to prepend the size of the bytes.

J.Hilk

@summit you couldn't be more vague, if you tried to.

What error do you have?
The code os untested from the top of my head, just fixed a typo there may be more

JonB

@summit
@J-Hilk 's code looks good to me. You can also save closing and re-opening the file by changing the code that does that when finished writing your data to:

file.flush();
file.seek(0);
file.write(static_cast<qint64 >(file.size()));
file.close();

summit

@J-Hilk sorry i wanted to write that i am able to prepend the file now and my problem is solved.

unfortunately it was a type error from my side in my previous comment.