Peek on QTextStream

Question

I would like to peek the next characters of a QTextStream reading a QFile, in order to create an efficient tokenizer.

However, I don't find any satisfying solution to do so.

QFile f("test.txt");
f.open(QIODevice::WriteOnly);
f.write("Hello world\nHello universe\n");
f.close();

f.open(QIODevice::ReadOnly);
QTextStream s(&f);
int i = 0;
while (!s.atEnd()) {
  ++i;
  qDebug() << "Peek" << i << s.device()->peek(3);
  QString v;
  s >> v;
  qDebug() << "Word" << i << v;
}

Gives the following output:

Peek 1 "Hel" # it works only the first time
Word 1 "Hello" 
Peek 2 "" 
Word 2 "world" 
Peek 3 "" 
Word 3 "Hello" 
Peek 4 "" 
Word 4 "universe" 
Peek 5 "" 
Word 5 ""

I tried several implementations, also with QTextStream::pos() and QTextStream::seek(). It works better, but pos() is buggy (returns -1 when the file is too big).

Does anyone have a solution to this recurrent problem? Thank you in advance.

Answer 1

You peek from QIODevice, but then you read from QTextStream, that's why peek works only once. Try this:

while (!s.atEnd()) {
      ++i;
      qDebug() << "Peek" << i << s.device()->peek(3);
      QByteArray v = s.device()->readLine ();
      qDebug() << "Word" << i << v;
}

Unfortunately, QIODevice does not support reading single words, so you would have to do it yourself with a combination of peak and read.