[Qt-creator] QList and QStringList, or qDebug() odd Behaviour?

[Tamás Kiss]

Let's take a closer look on what's happening in your code (I'll
explain it with staNames, of course the case is the same with the
other two variables):

- staNames is a pointer to a QStringList

- staNames[0] happens to be the QStringList your pointer points to,
and it's equivalent with the expression (*staNames). This is because
in many kinds of expressions an array can be treated as a pointer to
its first element. Notice that you've already made a big mistake here,
but while i == 0, the expression is valid, just not what you wanted.
Also take into account, that when I write "array", I don't mean
QStringList, I mean a traditional C array, whose elements are
QStringLists. Of course it doesn't exist here.

- qDebug << staNames[0] ... orders qDebug to write your whole
QStringList out to stderr.

- Later, when i == 1, staNames[1] would be the second element of the C
array, that is, another QStringList. Of course this is an illegal
memory access, and your program dies.

- What should you write instead: (*staNames)[i] : this calls
QStringList::operator[].

- Why is the other form you wrote exactly as wrong as the first one?
Because *(staNames + i) means staNames[i] which is the same as above.
:) Again, when i == 1, (*staNames + 0) == *staNames, so you write your
whole List to stderr. When i == 1, your program dies.

Hope this helps.

--
Tamás

2010/3/23 Victor Sardina <Victor.Sardina at noaa.gov>:
> Trolls:
>
> I have just recently noticed something rather unusual going on with the
> behaviour of the "[]" operator for the QList and QStringList objects via
> qDebug(). For example:
>
> for(int i = 0; i != staNames->length(); i++)
> {  qDebug() << staNames[i] << endl; //->at(i);
>   qDebug() << staLats[i] << endl;  //->at(i);
>   qDebug() << staLons[i] << endl;  //->at(i);
> }
>
> or
>
> for(int i = 0; i != staNames->length(); i++)
> {  qDebug() << *(staNames+ i) << endl; //->at(i);
>   qDebug() << *(staLats + i) << endl;  //->at(i);
>   qDebug() << *(staLons + i) << endl;  //->at(i);
> }
>
> will produce a "serialized" output of the three objects (QStringList,
> QList<qreal>, and QList<qreal>, with all values in one object followed
> by all values in the next, and so on, as though the index "0" retrieves
> all values in memory at once, before it crashes once the "i" counter
> increments to 1. I allocated all three objects dynamically via operator
> new and populated them via "append" (push_back). The interesting thing,
> however, has to do with the fact that replacing the call to the "[]"
> operator with a call to "at" produces the intended output, namely, all
> three values one at a time for each object as the counter increments.
>
> Can someone provide some insight on the reasons behind the seemingly odd
> behaviour (for me at least)?
>
> Thank you in advance,
> Victor
>
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