CODEWITHSUNDEEP
c++qtqtextstream
Tomáš Zato
Tomáš Zato

Reputation: 53119

QTextStream::readAll returns empty QString

I was trying to loop through all children of specific XML node and join their name attributes. The structure:

<params>
    <param name="BLAH" />
</params>

The desired result:

PARAM1='$PARAM1',PARAM2='$PARAM2',PARAM3='$PARAM3'[...]

The code:

    // Create empty text stream
    QTextStream paramNames("");
    // Start looping child by child
    QDomElement child = params.firstChildElement();
    bool firstIteration = true;
    while( !child.isNull() ) {  
        QString param_name = child.attribute("n");
        // Skips empty names
        if(param_name.length()>0) {
          // This prevents both leading and trailing comma
          if(!firstIteration)
              paramNames<<",";
          else
              firstIteration = false;
          // This should fill in one entry
          paramNames<<param_name<<"='$"<<param_name<<'\'';
        }
        child = child.nextSiblingElement();
    }

Now even the debugger says that if I do 

QString paramNamesSTR = paramNames.readAll();

the paramNamesSTR is an empty string. However if I use std library instead, everything works:

    std::stringstream paramNames("");
    QDomElement child = params.firstChildElement();
    bool firstIteration = true;
    while( !child.isNull() ) {  
        std::string param_name = child.attribute("n").toUtf8().constData();
        if(param_name.length()>0) {
          if(!firstIteration)
              paramNames<<",";
          else
              firstIteration = false;
          paramNames<<param_name<<"='$"<<param_name<<'\'';
        }
        child = child.nextSiblingElement();
    }
    QString paramNamesSTR = QString::fromStdString( paramNames.str() );

So what's the difference? Why does the Qt QTextStream return empty string? I would really prefer to be consistent with used libraries and therefore use the QTextStream rather than std::stringstream, although presonally, I prefer the former.

Upvotes: 0

Views: 1917

Answers (1)

SingerOfTheFall
SingerOfTheFall

Reputation: 29966

To be able to use QTextStream you need to pass it something to operate on (the stream itself doesn't store any data, it just operates on a string or iodevice). Passing it a string literal is not the right thing to do. The difference is when you create the std::stringstream and pass it a string literal, an underlying stream buffer is created automatically, and that literal is used as the buffer's initial value. In case of QTextStream, that created a read-only stream that contains the passed literal. The right way to create a QTextStream would be to create the buffer first, and then create the stream to operate on that buffer, for example:

QString string; //you can also use a QByteArray, or any QIODevice
QTextStream stream(&string);

Upvotes: 3

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