[SOLVED]Best coding practice

[Keith Hedger]

I am reasonably new to qt and c++ but am quite an experienced programmer.

My ( sort of ) problem is this I have been used to casting widgets ( gtk ) from the base widget to the specific via this sort of code:
@
combobox=(GtkCombBox*)widget
@

But I have come across this sort of thing in the various QT docs:
@
QPushButton *button = parentWidget->findChild<QPushButton *>("button1");
@

What is the better/best way to cast widgets in QT?

[Zlatomir]

To perform that type off casts in C++ you usually use "dynamic_cast":http://en.cppreference.com/w/cpp/language/dynamic_cast , but with QObject subclasses (all widgets are derived indirectly from QObject) the recommended way is "qobject_cast ":http://qt-project.org/doc/qt-4.8/qobject.html#qobject_cast

LE: findChild is a way to get a child by name (if you have the parent), is not a casting method by itself, see the documentation "here":http://qt-project.org/doc/qt-4.8/qobject.html#findChild

[Keith Hedger]

[quote author="Zlatomir" date="1420548624"]...
LE: findChild is a way to get a child by name (if you have the parent), is not a casting method by itself, see the documentation "here":http://qt-project.org/doc/qt-4.8/qobject.html#findChild[/quote]

Thanks for the links, yes I know what this function does it was just the first example I could easily find using "<QPushButton *>", sorry for any confusion.

[Keith Hedger]

Ok bit more info, I have an array of Qwidgets for the prefs some of which are boolean some are text etc so it is convenient to use a generic type and type cast the widget to get the value, all 3 ways of casting ( for instance a check box ), seem to work fine and consistently but give a few minor diffs in the size of the executable by just a few bytes, not worth worrying about.

So what is the best way:
@((QCheckBox*)prefsWidgets[widgnum])->setChecked(onoff);@
@dynamic_cast<QCheckBox *>(prefsWidgets[widgnum])->setChecked(onoff);@
@qobject_cast<QCheckBox *>(prefsWidgets[widgnum])->setChecked(onoff);@

The first is slightly less typing!

[ckakman]

Hi,

You should prefer the third option but with a pointer check:
@
QCheckBox checkBox = qobject_cast<QCheckBox>(prefsWidgets[widgnum]);
if (checkBox) checkBox->setChecked(onoff);
@

Of course you can skip the null-ptr test if you are sure of the type.

First one is good, old but too powerful C-style cast which most of the C++ programmers would advise to avoid.

Dynamic cast is costly. That's the reason there is a qobject_cast.

[Keith Hedger]

In this instance I will probably go with the qobject_cast as I don't need to do any run time checking that I gather dynamic_cast does.

Thanks for the help guys.

[SGaist]

Hi,

Indeed, don't do C style cast, there's no check at all with and you're in for a lot of problem. Also object_cast works across library boundaries.

Anyway, since it's only a matter of calling setChecked, you can even reduce your code by using the property system.

Just call:

@prefsWidgets[widgnum]->setProperty("checked", onoff);@

and you should be good to go

[Keith Hedger]

[quote author="SGaist" date="1420585048"]...Just call:

@prefsWidgets[widgnum]->setProperty("checked", onoff);@

...[/quote]

That's a much more elegant solution to the above bit of code, I didn't know I could access the property data directly, learned something new. Thanks!