[Qt-interest] How to deal with pointers for a QDataStream?

[Oliver Heins]

Hi André,

Andre Somers <andre at familiesomers.nl> writes:

> On 19-4-2010 14:41, Oliver Heins wrote:
>> I have a data structure which holds some pointers:
>>
>> class Token
>> {
>> public:
>>      enum Type { Space, Paragraph, Kern, Char, Format, Note, End };
>>      enum SubType { Bold, Italic,
>>                     Header1, Header2, Header3, Header4, Header5, Header6,
>>                     Itemize, Enumerate, Description, Item,
>>                     Note1, Note2, Note3, Note4, Note5 };
>>
>>      [...]
>>
>>      Type type;
>>      SubType subType;
>>      QChar chr;
>>      Token *parent;
>>      QLinkedList<Token *>  children;
>>      Token *partner;
>> };
>>
>> As saving pointers directly to a QDataStream will obviously not work,
>> what would be a good strategy to save the structure?
>>
>>    
>
> One way is to give each item you need to store a pointer to an ID that
> is unique for the file (if such a thing is not already there), and
> store these ID's instead of the pointers to the objects. On loading,
> you can reconstruct the pointers by using the ID's to lookup the
> addresses from some (temporary?) data structure such as a QHash<int,
> Token*> that you keep around at loading time.

Thanks, that's the way I intent to go.  However, I run into another
problem: The data I write is not read properly.  

This is how I go: I write my MagicNumber and the highest unique id to
the file.  Then I traverse my QLinkedList<Token*>, writing the tokens: 

    while (i.hasNext())
        out << *(i.next());

On reading, MagicNumber and the max id are set properly.  Then I try to
read the data into a QVector<Token> tokenVec:

    in >> tokenVec;

But the Tokens get filled with garbage.  This is how the << and >>
operators are defined:

QDataStream &operator<<(QDataStream &out, const Token &token)
{
    quint32 partner = token.partner ? token.partner->unique : 0;
    quint32 parent = token.parent ? token.parent->unique : 0;

    out << qint16(token.type) << qint16(token.subType) << token.chr 
        << quint32(token.unique) << parent << partner;
    return out;
}

QDataStream &operator>>(QDataStream &in, Token &token)
{
    quint32 partner, parent;
    qint16 type, subType;

    token.parent = 0;
    token.partner = 0;
    in >> type >> subType >> token.chr >> token.unique >> parent >> partner;
    token.type = static_cast<Token::Type>(type);
    token.subType = static_cast<Token::SubType>(subType);

    return in;
}

What is going wrong here?

TIA,
 olli

-- 
Oliver Heins heins at sopos.org    http://oliverheins.net/
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