AnalogClock::staticMetaObject': definition of dllimport static data member not allowe

[giorgik]

I'm using Qt 6.4.3 and took the example of the analog clock customwidgetplugin included in the Qt installation package. I compiled the plugin and placed it in the path C:\Qt\Tools\QtCreator\bin\plugins\designer. I create a new qt project based on QWidget. In its .ui form I select and insert the created plugin. So far, so good. When I compile the plugin test program, I get this error message:

D:\dev\Qt\ProvaEsempioPlugin\debug\moc_analogclock.cpp:59: error: C2491: 'AnalogClock::staticMetaObject': definition of dllimport static data member not allowed

D:\dev\Qt\ProvaEsempioPlugin\debug\moc_analogclock.cpp:59: error: C2491: 'AnalogClock::staticMetaObject': definition of dllimport static data member not allowed

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I don't understand where is the problem... You can help me ?

[d_stranz]

Google is your friend here.

Either you are building the plugin with the wrong dllimport / dllexport declaration or you are trying to use it in a program with the wrong declaration. You use dllexport when you are building the DLL and use dllimport when your are using the DLL in a program.

And in Microsoft world, you cannot mix debug and release mode code. If the DLL was built in release mode to use with Qt Designer, you cannot link with that same release mode DLL if you are building a debug mode program. You must build a debug mode DLL to link with your debug mode program.

[giorgik]

d_stranz
No, the test project (like the plugin itself) are Release. I am attaching the .pro file of the test:

QT       += core gui
 
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
 
CONFIG += c++17
 
# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0
 
SOURCES += \
    main.cpp \
    widget.cpp
 
HEADERS += \
    widget.h \
    analogclock.h
 
FORMS += \
    widget.ui

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++17

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
    main.cpp \
    widget.cpp

HEADERS += \
    widget.h \
    analogclock.h

FORMS += \
    widget.ui

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If it can be useful, I also put the .pro of the plugin:

QT          += widgets uiplugin
 
QTDIR_build {
# This is only for the Qt build. Do not use externally. We mean it.
PLUGIN_TYPE = designer
PLUGIN_CLASS_NAME = AnalogClockPlugin
load(qt_plugin)
CONFIG += install_ok
} else {
# Public example:
 
CONFIG      += plugin
TEMPLATE    = lib
 
TARGET = $$qtLibraryTarget($$TARGET)
 
target.path = $$[QT_INSTALL_PLUGINS]/designer
INSTALLS += target
}
 
HEADERS     = analogclock.h \
              customwidgetplugin.h
 
SOURCES     = analogclock.cpp \
              customwidgetplugin.cpp

QT          += widgets uiplugin

QTDIR_build {
# This is only for the Qt build. Do not use externally. We mean it.
PLUGIN_TYPE = designer
PLUGIN_CLASS_NAME = AnalogClockPlugin
load(qt_plugin)
CONFIG += install_ok
} else {
# Public example:

CONFIG      += plugin
TEMPLATE    = lib

TARGET = $$qtLibraryTarget($$TARGET)

target.path = $$[QT_INSTALL_PLUGINS]/designer
INSTALLS += target
}

HEADERS     = analogclock.h \
              customwidgetplugin.h

SOURCES     = analogclock.cpp \
              customwidgetplugin.cpp

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Where do I find the reference of dllimport in Qt code ? Please note that here I'm doing everything only in Qt environment, including IDE, for the moment I don't use Visual Studio C++ (here the same plugin project works fine, including the test program).
I want to be able to make everything work in Qt environment, so I can finally verify the old QLed problem that you will remember well :-))

[d_stranz]

No, the test project (like the plugin itself) are Release.

Then why does your original post say this?

D:\dev\Qt\ProvaEsempioPlugin\debug\moc_analogclock.cpp:59: error: C2491

Anyway, the error is because you are including the header file "analogclock.h" as part of the HEADERS section in your .pro file. This causes Qt to create the moc_analogclock.cpp and try to compile it. But that file has already been compiled and is in your plugin DLL. So the compiler is complaining that you have defined the same static variable in two different places, your DLL and your application. Just remove the "analogclock.h" line from your .pro file. You are using the header, not building it.

Where do I find the reference of dllimport in Qt code ?

The header analogclock.h decalares the AnalogClock class this way:

class QDESIGNER_WIDGET_EXPORT AnalogClock : public QWidget

class QDESIGNER_WIDGET_EXPORT AnalogClock : public QWidget

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The macro QDESIGNER_WIDGET_EXPORT expands to:

// qdesignerexportwidget.h
 
#if defined(QDESIGNER_EXPORT_WIDGETS)
#  define QDESIGNER_WIDGET_EXPORT Q_DECL_EXPORT
#else
#  define QDESIGNER_WIDGET_EXPORT Q_DECL_IMPORT
#endif

// qdesignerexportwidget.h

#if defined(QDESIGNER_EXPORT_WIDGETS)
#  define QDESIGNER_WIDGET_EXPORT Q_DECL_EXPORT
#else
#  define QDESIGNER_WIDGET_EXPORT Q_DECL_IMPORT
#endif

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and Q_DECL_IMPORT and Q_DECL_EXPORT expand to:

// qcompilerdetection.h
 
#  define Q_DECL_EXPORT __declspec(dllexport)
#  define Q_DECL_IMPORT __declspec(dllimport)

// qcompilerdetection.h

#  define Q_DECL_EXPORT __declspec(dllexport)
#  define Q_DECL_IMPORT __declspec(dllimport)

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The .pro file for the plugin causes QDESIGNER_EXPORT_WIDGETS to be defined for the plugin build, probably by some combination of the QTDIR_build section and CONFIG += plugin and TEMPLATE = lib declarations.

If you are going to try to build a Qt Designer plugin in a Visual Studio project, then you need to add QDESIGNER_EXPORT_WIDGETS as a Preprocessor Definition in the C/C++ -> Preprocessor section of your plugin project Properties. That means the __declspec( dllexport ) declaration is active and causes the compiler to export the AnalogClock class from the DLL so you can use it in projects.

When you are using the plugin in an application project, you do not use this define. If QDESIGNER_EXPORT_WIDGETS is not defined, then the __declspec( dllimport ) declaration is active and your class is imported from the DLL.