[Application crashes when I use std::reverse function to reverse a QString]

appdev

Hello again,

I hope you're doing well.

So After reading my csv file and displaying its values in a QTable which has 24 rows and 13 columns, I wanted to extract a part from the items that exist in the 9th column of every row, and then reverse them, and finally add them to a 10th column.

In fact I could only extract the parts I need and add each one to the 10 th column of every row, however when I add the reverse function , the application crashes when I run it.

Here is an explanation of what I want to do:
9th column.png

And here is what I tried to do :

for (int jx = 0; jx< colCount; ++jx) {


           setValueAt(rows, jx, values.at(jx));  //I'm adding other values

           std:: reverse(values.at(8).mid(90,8).begin(), values.at(8).mid(90,8).end());

           setValueAt(rows,9,values.at(8).mid(90,8)); // I added another column which values are some extracted QString from an initial QString at the position 90

Also in the code, initially values is a QList, when I read the csv file, I store its data in a QList then I add them to QTable.

When I remove the reverse function everything works fine, The parts are extracted and added to the 10th column however when I use it to reverse the data extracted, the app crashes.
I ran it in debug mode, the problem seems to be in this line(160) in the qarraydata.h file

And in the output of the application, I get this message:

I don't really understand what's going on and what's the real problem.
Can anyone help me please ?
Any little help is really appreciated and thank you.

JonB

@appdev said in [Application crashes when I use std::reverse function to reverse a QString]:

std:: reverse(values.at(8).mid(90,8).begin(), values.at(8).mid(90,8).end());

I don't know (believe) whether it's a good idea to std::reverse() anything in a QList (actually a QString) in place anyway. I think I'd rather take a copy, change it, and assign it back as necessary as a whole QString. Does that work without erroring?

Also, QString::mid() returns a new (sub-)string. So at best you would be reversing a temporary copy of something copied out of a QString, it won't alter the original QString. If anything you would need QStringRef QString::midRef(int position, int n = -1) const instead here. As I said, if that is even safe/allowed to be changed by std::reverse().

J.Hilk

@JonB even worse, the call is to mid() twice, meaning begin() and end() point to completely different objects.

because std::reverse should work just fine on a QString.

JonB

@J-Hilk said in [Application crashes when I use std::reverse function to reverse a QString]:

@JonB even worse, the call is to mid() twice, meaning begin() and end() point to completely different objects.

That is indeed a good observation :) Shouldn't std::reverse() complain that the begin() & end() range do not refer into the same object? :)

Then I suspect a (single!) QString::midRef() into the QString could be passed as begin/end() to std::reverse() and all should be well?

KroMignon

@appdev said in [Application crashes when I use std::reverse function to reverse a QString]:

std:: reverse(values.at(8).mid(90,8).begin(), values.at(8).mid(90,8).end());

How to you expect this to work?
You are working with temporary created instances.
From QString::mid() documentation:

Returns a string that contains n characters of this string, starting at the specified position index.

This will return a new QString instance.

This could work with QString::midRef() which returns a reference on current QString.

appdev

First of all thank you for all your replies but there is something I still didn't really get.

The mid() which was called twice and begin()/end() range that don't refer to the same object.

Well, Yesterday I saw a post where someone wanted to reverse a QString.

The answer was as follows:

QString s = "variable";
std::reverse(s.begin(), s.end());
qDebug() << s;    //output "elbairav"

link text

So I thought that begin() points to the first character of the QString and end() to the last one.

I thought by doing:

values.at(8).mid(90,8).begin()

begin () would point to the first character of the extracted QString and end() to its last.
For example: let's say the 8th element of values which is a QList of QString is 123498f1258963.
mid(7,8) would return f1258963 and begin() would point to f and end() would point to 3.

so to sum it all up,

values.at(8).mid(7,8)

would return f1258963 which is a QString

std:: reverse(values.at(8).mid(90,8).begin(), values.at(8).mid(90,8).end());

is the same as

std:: reverse(s.begin(), s.end());

where s is f1258963.

I didn't get what's wrong with mid() being called twice.

Also I included QStringRef but all I got was QStringRef file not found.

I also stored the values in the table and then tried to reverse them by accessing to their columns as follows:

reverse (mModel->item(rows,9)->text().begin(),mModel->item(rows,9)->text().end());

J.Hilk

@appdev it's not pointing to the character, its pointing to the memory where the character is, of the temporary object, that mid() returns

          setValueAt(rows, jx, values.at(jx));  //I'm adding other values
          auto subString = values.at(8).mid(90,8);
           std:: reverse(subString.begin(), subString.end());

           setValueAt(subString);

JonB

@appdev said in [Application crashes when I use std::reverse function to reverse a QString]:

Also I included QStringRef but all I got was QStringRef file not found.

There was nothing to "include". [EDIT My bad. See @jsulm's post below about Qt6 which you are using.]

As said earlier, if you wish to reverse the characters in place in the middle of your string, you will need to use QString::midRef(). You currently pass QString::mid() to std::reverse(), so that would only reverse a copy of the mid section of the original string, not the mid section of the actual source string.

KroMignon

@appdev You are mixing up several things which are (maybe similar) but not equivalent.

Yes you can use std::reverse() with QString that's not the point here.

The problem is QString::mid() returns a new QString instance, that is what is wrong with your code.
With std:: reverse(values.at(8).mid(90,8).begin(), values.at(8).mid(90,8).end()); you are using 2 different QString instances.

begin() and end() are not for the same QString instance.
It would be the same as doing:

QString str1 = "My pretty string";
QString str2 = "My pretty string";
std:: reverse(str1.begin(), str2.end());

appdev

Okay, thank you all for helping.

And yeah now I see why I'm using two different QString instances.

Also, I get no member named midRef in QString :///

jsulm

@appdev https://doc.qt.io/qt-5/qstring.html#midRef
What Qt version do you use?

appdev

@jsulm Qt6

jsulm

@appdev For Qt6 see https://doc-snapshots.qt.io/qt6-dev/qtcore-changes-qt6.html ("The QStringRef class" section).

appdev

Thank you all for your help.