Can someone explain this code from C++ GUI Programming with Qt 4?

[wysota]

Suprisingly QSpinBox does include QRegExpValidator through inclusion of qabstractspinbox.h which in turn includes qvalidator.h which defines QRegExpValidator.

#include <QSpinBox>
 
int main(int argc, char **argv){
 QRegExpValidator v;
 return 0;
}

#include <QSpinBox>

int main(int argc, char **argv){
 QRegExpValidator v;
 return 0;
}

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This compiles which means the forward declaration from the original snippet is not needed.

[deejross]

This compiles which means the forward declaration from the original snippet is not needed.

You are correct. I commented out the class QRegExprValidator; line in my .h file and the project still builds and works. Thanks.

[portilhe]

Hi all,

I am also following this book and I have a question regarding this exact same example.

I was trying to play with it making it display the prefix "0x" by adding the line

setPrefix("0x")

setPrefix("0x")

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to the HexSpinbox default constructor as so:

HexSpinBox::HexSpinBox (QWidget *parent)
	: QSpinBox(parent)
{
	setPrefix("0x");
	setRange(0,255);
	validator = new QRegExpValidator(QRegExp("[0-9A-Fa-f]{1,8}"), this);
}

HexSpinBox::HexSpinBox (QWidget *parent)
	: QSpinBox(parent)
{
	setPrefix("0x");
	setRange(0,255);
	validator = new QRegExpValidator(QRegExp("[0-9A-Fa-f]{1,8}"), this);
}

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Unfortunately this compiles but immediately gives me a seg fault:

The inferior stopped because it received a signal from the Operating System.

Signal name : SIGSEGV
Signal meaning : Segmentation fault

Does this have anything to do with the validator getting the prefix value? Is it fixable?

Thanks in advance,
Manu

[norobro]

Try setting the prefix after you initialize *validator.

[portilhe]

Thanks, it works. Well, almost! I had to change the regular expression to

validator = new QRegExpValidator(QRegExp("0x[0-9A-Fa-f]{1,8}"), this);

validator = new QRegExpValidator(QRegExp("0x[0-9A-Fa-f]{1,8}"), this);

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otherwise, although it compiled and worked fined with the arrows, it didn't accept correctly any keyed-in input.

So now I have 2 questions:
1) Why does it work setting the prefix after the validator and not before?
2) From the fact that I had to include the "0x" inthe regular expression I am guessing that the reimplemented function (from QSpinBox) HexSpinBox::validate gets as a first argument the full text of the spin box, whereas the function valueFromText gets a stripped version. Am I guessing wrong?

[norobro]

Why does it work setting the prefix after the validator and not before?

setPrefix() calls HexSpinBox::validate(). In the return statement validate is an uninitialized pointer which causes a segfault.

2) ... Am I guessing wrong?

Run your code in a debugger or put in some qDebug() statements to see what's happening.