I have a QDialog which has a QLineEdit and a slot connected to the QLineEdit's textChanged signal.
std::string gname;
void NewMsgDialog::nameChanged(QString str) {
auto temp = str.toUtf8().constData();
gname = str.toStdString();
}
In the debugger I can see that temp has a sensible content (e. g. "my text") but at the end of the function, gname's value says "<not accessible>". And really, if I use gname from another function, I get a SEGFAULT.
What could be wrong?
Just use gname = str.toStdString();
Well, turns out, it was a bug of QT Creator's debugger. When not run in debug, gname was assigned a value just fine. Anyway, I got really useful tips from the other answers.
temp variable is wrong, it can only work by accident, possibly as a result of compiler optimizations
temp, I still see the <inaccessible> value after assignment into the std::string in a debug session in qtcreator using qt5-default v5.12.8 in Ubuntu 20.04 in Arm64. Is there a bug report I can check?Your code has undefined behavior.
str.toUtf8() returns a temporary QByteArray, which you are calling constData() on to get a pointer to its char data, which you are saving to temp. But then the QByteArray goes out of scope after that assignment, leaving temp pointing to freed memory before it can be used in the std::string{temp} construction.
You don't need the temp variable. std::string has an operator= that takes a const char* as input. By assigning the return value of constData() directly to gname, the QByteArray stays in scope until after the assignment:
std::string gname;
void NewMsgDialog::nameChanged(QString str) {
gname = str.toUtf8().constData();
}
Even simpler, you can use str.toStdString() instead, which returns a UTF-8 encoded std::string:
std::string gname;
NewMsgDialog::nameChanged(QString str) {
gname = str.toStdString();
}
temp variable and assigning directly to gname is the solution?
const in front of std::string gname;
temp variable. That is the problem.