Problem with pointers?

[Guest]

Hi, I'm using this code below for getting the value of password from the lineEdit to be displayed on QMessageBox:

@char* pass = ui->pass->text().toAscii().data();
QMessageBox::information(0,0, pass);@

however, I get an empty message, but when I put ui->pass->text().toAscii().data() instead of pass: it works, and I get the full password value inside the message. so why am I getting this problem? thanks.

[JKSH]

Hi, and welcome to the Qt Dev Net!

toAscii() constructs a new QByteArray. data() returns the pointer to the QByteArray's internals.

In your code, you did not store the QByteArray anywhere. Therefore, it gets destroyed after line #1, and char* pass contains a dangling pointer.

[Guest]

well, so what should be the best way for storing this string into a char* pointer?

[Xander84]

Why do you even use a char*, the functions prototype is:
@
StandardButton QMessageBox::information(QWidget * parent, const QString & title, const QString & text)
@
so you would use it like
@
QMessageBox::information(this, "title", ui->pass->text());
@

[Guest]

[quote author="Xander84" date="1398529302"]Why do you even use a char*, the functions prototype is like:
@
StandardButton QMessageBox::information(QWidget * parent, const QString & title, const QString & text)
@
so you would use it like
@
QMessageBox::information(this, ,"title", ui->pass->text());
@[/quote]

you still don't understand what I mean, I just want to store the value to char* correctly without any need to display it

[Xander84]

ok sorry because you mentioned an example with QMessageBox::information ...
if you just want to use the char array you have to copy it i guess, you can use strcpy or memcpy, I don't know if Qt has some helpers for cleaner c++ usage?
Or just save a reference to the QString, depends on what you are doing with that char* and how often you need to use it.
Storing a copy of the QString (or QByteArray) is the cleanest solution in my opinion.

[Guest]

ok, I understand now why that doesn't work: I shall use @.toStdString().c_str()@ instead of @.toAscii().data()@
it's all right now, thanks all for your replies.

[JKSH]

[quote author="kaisbs" date="1398529964"]
ok, I understand now why that doesn't work: I shall use @.toStdString().c_str()@ instead of @.toAscii().data()@

it's all right now, thanks all for your replies.
[/quote]No, you still have a bug in your code. You might not see any problems now, but that's just a coincidence. Your program will likely crash in the future.

@
// 1. Here, you create a temporary std::string and get a pointer to its internal data.
char* pass = ui->pass->text().toStdString().c_str();

// 2. Here, the std::string gets destroyed because you did not store it anywhere. So, the pointer in 'pass' is no longer valid.

// 3. Here, you use the invalid pointer. The behaviour is undefined.
QMessageBox::information(0,0, pass);
@

You need to make sure that the std::string doesn't get destroyed until you have finished using the pass pointer. You can do this by storing the std::string in a local variable, and then calling c_str() to get the char-pointer from that variable.