strange problem with const char *

[alessio211734]

I have problem with conversion from QString to const char * , I know that it's not the right newsgroup but I don't know if is a c++ issue problem.

void _file(const char * file)
{
std::string fileString(file);
int a=0;
}


int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QString str("ciao");
const char * xxx=str.toLatin1().constData();
_file(str.toLatin1().constData());
_file(xxx);
MainWindow w;
w.show();

return a.exec();
}

at the first call "_file(str.toLatin1().constData())" the debugger show that the parameter passed to function _file have the correct value while in the last call _file(xxx) the parameters passed to _file function is incorrect. I didn't understand why?

 

[Marcel Müller]

Am 18.12.2013 11:28, schrieb alessio211734:

I have problem with conversion from QString to const char * , I know that it's not the right newsgroup but I don't know if is a c++ issue problem.

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QString str("ciao");
const char * xxx=str.toLatin1().constData();
_file(str.toLatin1().constData());
_file(xxx);
MainWindow w;
w.show();

return a.exec();
}

at the first call "_file(str.toLatin1().constData())" the debugger show that the parameter passed to function _file have the correct value while in the last call _file(xxx) the parameters passed to _file function is incorrect. I didn't understand why?

Whatever QString is, read the documentation of QString. Especially the
point how long the storage retuned by constData() remains valid.

Most likely you have undefined behavior because of a dangling reference,
because you passed a temporary to xxx and access it after the QString
instance, that provides the storage has been destroyed.


Marcel

 

[alessio211734]

Il giorno mercoledì 18 dicembre 2013 13:02:45 UTC+1, Marcel Müller ha scritto:

Am 18.12.2013 11:28, schrieb alessio211734:

I have problem with conversion from QString to const char * , I know that it's not the right newsgroup but I don't know if is a c++ issue problem.


int main(int argc, char *argv[])

{

QApplication a(argc, argv);

QString str("ciao");

const char * xxx=str.toLatin1().constData();


MainWindow w;


return a.exec();


at the first call "_file(str.toLatin1().constData())" the debugger showthat the parameter passed to function _file have the correct value while in the last call _file(xxx) the parameters passed to _file function is incorrect. I didn't understand why?

Whatever QString is, read the documentation of QString. Especially the

point how long the storage retuned by constData() remains valid.



Most likely you have undefined behavior because of a dangling reference,

because you passed a temporary to xxx and access it after the QString

instance, that provides the storage has been destroyed.

I think problem it's not the function constData() but the function toLatin1() that return a temporary data QByteArray