<![CDATA[Check if widget is pushbutton]]>Hi,

I have a widget QWidget* and I need to check is that widget is a QPushButton* (or QFrame* or any inherited from QWidget*). How to do that? The code:

if (static_cast<QPushButton*>(widget)){
    ....
}

doesn't seem to work...

]]>https://forum.qt.io/topic/117141/check-if-widget-is-pushbuttonRSS for NodeThu, 16 Apr 2026 18:56:29 GMTSat, 18 Jul 2020 13:47:54 GMT60<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 14:44:47 GMT]]>Yes it's better than string comparison, but it's 2 lines :-)

auto pushButton = qobject_cast<QPushButton*>(widget);
if (nullptr != pushButton)
]]>https://forum.qt.io/post/607584https://forum.qt.io/post/607584Sat, 18 Jul 2020 14:44:47 GMT<![CDATA[Reply to Check if widget is pushbutton on Mon, 20 Jul 2020 08:38:26 GMT]]>@SimonSchroeder said in Check if widget is pushbutton:

Has anyone any idea why the version suggested by @LeLev would work?

I've been using this feature for many years, it's definitely NOT a C++17 feature.
As I look into https://en.cppreference.com/w/cpp/language/if , it is called

declaration of a single non-array variable with a brace-or-equals initializer

]]>https://forum.qt.io/post/607781https://forum.qt.io/post/607781Mon, 20 Jul 2020 08:38:26 GMT<![CDATA[Reply to Check if widget is pushbutton on Mon, 20 Jul 2020 08:29:34 GMT]]>@LeLev said in Check if widget is pushbutton:

@hskoglund can't you do?

if(auto pushButton = qobject_cast<QPushButton*>(pushButton ))

It looks like several C++ features got mixed up in this post. The original question wasn't too far off:

if (static_cast<QPushButton*>(widget))

The only problem here is that the wrong type of cast was used. static_cast<>() means: "I know what I am doing.", i.e. assume that the cast is always allowed. The correct C++ feature is dynamic_cast<>(). So, a C++-only solution would look like this:

if(dynamic_cast<QPushButton*>(widget))

In the context of Qt it was correctly pointed out that qobject_cast<>() is preferrable over dynamic_cast<>() (as it might be faster and does not need RTTI). A normal replacement of the cast is the easiest solution here:

if(qobject_cast<QPushButton*>(widget))

I am not entirely sure what C++ feature would enable the variant suggested by @LeLev :

if(auto pushButton = qobject_cast<QPushButton*>(pushButton ))

As far as I understand variable declarations are not allowed in if statements. C++17 introduced such a feature, however it has the declaration and the condition separated:

if(auto pushButton = qobject_cast<QPushButton*>(pushButton ); pushButton)  // this works since C++17

Has anyone any idea why the version suggested by @LeLev would work?

]]>https://forum.qt.io/post/607780https://forum.qt.io/post/607780Mon, 20 Jul 2020 08:29:34 GMT<![CDATA[Reply to Check if widget is pushbutton on Sun, 19 Jul 2020 17:59:50 GMT]]>Even it works you should follow the official documentation: https://qwt.sourceforge.io/qwtinstall.html#COMPILEANDLINKAPP

]]>https://forum.qt.io/post/607689https://forum.qt.io/post/607689Sun, 19 Jul 2020 17:59:50 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 22:53:08 GMT]]>@Christian-Ehrlicher thank you! I added in .pro file:

DEFINES += QWT_DLL

Could you please explain what this line means? I've found it in internet but I don't understand how it works

]]>https://forum.qt.io/post/607626https://forum.qt.io/post/607626Sat, 18 Jul 2020 22:53:08 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 16:54:05 GMT]]>@Please_Help_me_D Then you forgot to link against the qwt library.

]]>https://forum.qt.io/post/607603https://forum.qt.io/post/607603Sat, 18 Jul 2020 16:54:05 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 16:04:42 GMT]]>@hskoglund said in Check if widget is pushbutton:

if (QString("QPushButton") == widget->metaObject()->className())
...

this works, thank you!

But when I try to execute:

auto plot = qobject_cast<QwtPlot*>(widget);

I get an error:
mainwindow.obj:-1: error: LNK2001: unresolved external symbol "public: static struct QMetaObject const QwtPlot::staticMetaObject" (?staticMetaObject@QwtPlot@@2UQMetaObject@@B)

QwtPlot is from QWT library. It is declared as:
class QWT_EXPORT QwtPlot: public QFrame, public QwtPlotDict

]]>https://forum.qt.io/post/607594https://forum.qt.io/post/607594Sat, 18 Jul 2020 16:04:42 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 15:17:41 GMT]]>Ok I guess that eliminates all arguments for the string comparison...

]]>https://forum.qt.io/post/607589https://forum.qt.io/post/607589Sat, 18 Jul 2020 15:17:41 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 15:12:38 GMT]]>@hskoglund i just tested with qt5.14/minGw, it compiles and i don't see any warnings

]]>https://forum.qt.io/post/607588https://forum.qt.io/post/607588Sat, 18 Jul 2020 15:12:38 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 14:59:59 GMT]]>@LeLev sure but doesn't the compiler complain about '=' inside an if statement?

]]>https://forum.qt.io/post/607586https://forum.qt.io/post/607586Sat, 18 Jul 2020 14:59:59 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 14:53:53 GMT]]>@hskoglund can't you do?

if(auto pushButton = qobject_cast<QPushButton*>(pushButton ))
]]>https://forum.qt.io/post/607585https://forum.qt.io/post/607585Sat, 18 Jul 2020 14:53:53 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 14:44:47 GMT]]>Yes it's better than string comparison, but it's 2 lines :-)

auto pushButton = qobject_cast<QPushButton*>(widget);
if (nullptr != pushButton)
]]>https://forum.qt.io/post/607584https://forum.qt.io/post/607584Sat, 18 Jul 2020 14:44:47 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 14:38:55 GMT]]>@Please_Help_me_D hi,
use qobject_cast

]]>https://forum.qt.io/post/607583https://forum.qt.io/post/607583Sat, 18 Jul 2020 14:38:55 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 14:35:10 GMT]]>@hskoglund no, use proper c++ instead string comparison please.

]]>https://forum.qt.io/post/607582https://forum.qt.io/post/607582Sat, 18 Jul 2020 14:35:10 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 14:33:03 GMT]]>Hi, you could try:

if (QString("QPushButton") == widget->metaObject()->className())
...
]]>https://forum.qt.io/post/607581https://forum.qt.io/post/607581Sat, 18 Jul 2020 14:33:03 GMT<![CDATA[Reply to Check if widget is pushbutton on Sat, 18 Jul 2020 14:26:25 GMT]]>@Please_Help_me_D said in Check if widget is pushbutton:

doesn't seem to work...

Because you use s a static_cast<> which (what it says) statically casts a pointer into another one ... C++ basics.

]]>https://forum.qt.io/post/607578https://forum.qt.io/post/607578Sat, 18 Jul 2020 14:26:25 GMT