How to extract characters from QString...?

[Juan Dev]

I have this simple code to extract the characters and concatenate them to another string.

QString extract;
QString stringA = "2024ABCDEF";
extract = "01-01-";
extract += stringA.left(4);

And under QT Creator (width QT 5.15.2) I have this message "Use leftRef() instead [clazy-qstring-ref]"
But I read that "leftRef" is deprecated as of QT 6.x

So I should continue to use left() and not leftRef() and ignore the message...?
Or use QStringView...?

[Pl45m4]

@Juan-Dev said in How to extract characters from QString...?:

But I read that "leftRef" is deprecated as of QT 6.x

The whole QStringRef API/module will be removed in Qt7.

Or use QStringView...?

Yes, see what @SGaist wrote here
(very last comment of the topic. He also linked to the to-be-released Qt6.9 documentation in Qt's dev branch)

[Juan Dev]

@Pl45m4
First of all, thank you for this response.
Then I looked at QStringView but if I use my previous code how can I extract characters and concatenate them to a QString ?
I can simply display extraction like that :
qDebug() << QStringView{ stringA }.left(4);
But how to transform this into QString...?

[richferrara]

@Juan-Dev Try first() instead.

[J.Hilk]

@Juan-Dev QString has an overload for operator+ that allows for a QStringView object.

So you don't have to change much at all:

int main() {
    QString stringA = "2024ABCDEF";
    QStringView stringViewA(stringA);

    QString extract = "01-01-";
    extract += stringViewA.left(4);

    qDebug() << extract;

    return 0;
}

[Pl45m4]

@Juan-Dev said in How to extract characters from QString...?:

I can simply display extraction like that :
qDebug() << QStringView{ stringA }.left(4);
But how to transform this into QString...?

Isn't the whole point of using QStringView to not have to create additional sub-string variables or refs in the process?

QString stringA = "ABCDEFGH";
QStringView abcd = QStringView{ stringA }.left(4);
// some further string manipulation
// ...

// make string again once you are done
QString str = abcd.toString();
qDebug() << str;

[Juan Dev]

@Pl45m4
Thank you for your answer. I finally opted for the method provided by @J-Hilk but it is true that it is not necessarily useful to create additional variables with QStringView

[Juan Dev]

@J-Hilk
Thank you for your answer :)
I hadn't seen that the + operator overload existed.

[Pl45m4]

@Juan-Dev

If you are satisfied with the result, please mark this topic as resolved.

[JoeCFD]

@Juan-Dev
Is it easier to use std::string to handle it and convert it to QString when needed? Then Qt version issue will not matter any more.
https://cplusplus.com/reference/string/string/substr/

[Juan Dev]

@JoeCFD
I think this is another good solution. Thanks

[SimonSchroeder]

@JoeCFD said in How to extract characters from QString...?:

Is it easier to use std::string to handle it and convert it to QString when needed?

I had bad experiences with mixing QString and std::string. There have been times (on Windows with MSVC) where some references to std::string could not be found when using QString::toStdString(). Now, we have set the regular C++ locale to be UTF-8 (which means all std::strings are UTF-8). This also means that we need to use QString::fromUtf8(stdstring.c_str()) and qtstring.toUtf8().data() to convert between QString and std::string. (Under some circumstances the conversion from QString to std::string has some weird behaviour because of calling data() on a temporary object. It would be better to store the result of toUtf8() in a QByteArray before.)

So, bad experiences with QString::toStdString/fromStdString() together with UTF-8 std::strings tell me it is not easier to go back and forth between std::string and QString. (Also, QString::mid() might have worse performance than QString::midRef(), but certainly better performance than converting back and forth.)