[Qt-interest] How to save the content in tree structure to file

[Andre Somers]

Hi,

On 4-5-2010 11:39, Ramesh wrote:
>
> Ok.. I'm sorry,
>
> I know how to use Qdatastream , which we use to write the data to 
> file.. but its not serialized..
>
QDataStream does data serialization. What do you mean if you say "it is 
not serialized"?
>
> The way you write is the same wy you need to access the data from the 
> system..
>
I have no idea what you mean by this.
>
> But in tree hierarchy, I need the key value pairs.. I need to know 
> which one is parent and child..
>
> How can I do it.. by using Qdatastream?
>
You can use QDataStream to stream any file structure you like. 
QDataStream does not do the structuring for you, unless the class you 
are streaming out already contains such structure and it supports the 
QDataStream stream operators << and >>, such as QList or QHash do. What 
do you mean by "the key value pairs"? Which pairs?

You don't talk about how your data is structured in your program. 
Unfortunately, Qt does not supply a QTree<> container. So, I guess you 
are using some other structure. Could you tell us what you have now?

I would probably do something according to these lines:
The first step is a format for the tree itself. I would choose to stream 
out the number of items at top level in the tree as an unsigned int 
(quint32), followed by each of the top level tree items. That is really 
all you need.

How each item looks exactly, depends on what data you want to store in 
each tree item. In the example below, I will assume only a name that is 
stored in a QString and an unsigned integer ID. Each tree item is then 
an instance of a class, like this (all code is pseudo-code):

class TreeItem {
     unsigned int id;
     QString name;
     QList<TreeItem> children();
     unsigned int childCount();
}

If you want to write that to a file using QDataStream, you could do 
something like this:

QDataStream operator()<<(QDataStream& out, const TreeItem& item) {
   out << quint32(item.id);
   out << item.name;
   out << quint32(item.childCount());
   foreach(TreeItem, item.children()) {
       out << item; // note: this is a recursive call!
   }
   return out;
}

That is, you have each TreeItem write out its own data, the number of 
its children, and then recursively write out each of its children.

Reading back is the opposite process, where you need to be VERY CAREFULL 
that the procedures work on exactly the same structure, with the same 
data types.

If you would considder not using QDataStream (you are not telling why 
you want to use it), you could perhaps also considder the suggestion by 
Srdjan to use XML. It still requires you to do your own reading and 
writing somewhere, so I am not sure if it is a benefit for you. 
Personally, I think XML is nice, but mainly if you:
* need to have the data sort-of human readable, or
* need to have interoperability and extensibility.

André

> *From:* qt-interest-bounces at trolltech.com 
> [mailto:qt-interest-bounces at trolltech.com] *On Behalf Of *Andre Somers
> *Sent:* Tuesday, May 04, 2010 3:01 PM
> *To:* qt-interest at trolltech.com
> *Subject:* ***SPAM*** Re: [Qt-interest] How to save the content in 
> tree structure to file
>
> On 4-5-2010 11:25, Ramesh wrote:
>
> Hi,
>
> I want to save the data to a file in tree hierarchy, Same way I want 
> to display it on UI.
>
> How can I achieve that in Qt..
>
> This question can not be answered the way you formulate it.
> Please read and understand 
> http://catb.org/~esr/faqs/smart-questions.html 
> <http://catb.org/%7Eesr/faqs/smart-questions.html>, and reformulate 
> your question.
>
> André
>
>
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