How to resolve error LNK2019?

A.Michael

Hello everybody,

my system is follows:

OS:WIN10
Qt:5.8.0
Compiler:MSVC2015
qtcreator:4.3.0

I created a qtcreator custom control,CEditer(cediterplugin.dll and cediterplugin.lib). It works well.

.h file as follows:

CEditer.h:

#include <QtUiPlugin/QDesignerExportWidget>

namespace Ui {
class CEditerForm;
} // namespace Ui

class QDESIGNER_WIDGET_EXPORT CEditer: public QWidget
{
   Q_OBJECT
public:
   CEditer();
   XXX
    .
    .
    .
private:
   Ui::CEditerForm *ui;
};

Now,I'm going to create another custom control,CEditPanel(ceditpanelplugin.dll and ceditpanelplugin.lib). It contains the CEditer control.

CEditPanel.h:

#include <QtUiPlugin/QDesignerExportWidget>
#include "CEditer.h"
#pragma comment(lib,"cediterplugin.lib")
namespace Ui {
    class CEditerPanelForm;
}

class QDESIGNER_WIDGET_EXPORT CEditerPanel : public QWidget
{
    Q_OBJECT
public:
    CEditerPanel();
    XXX
     .
     .
     .
private:
    Ui::CEditerPanelForm *ui;
};

"ui_CEditerPanelForm.h"

#include "CEditer.h"

QT_BEGIN_NAMESPACE

class Ui_CEditerPanelForm
{
public:
    QHBoxLayout *horizontalLayout_3;
    CEditer *editer;
    void setupUi(QWidget *CEditerPanelForm)
{
...
}

namespace Ui {
    class CEditerPanelForm: public Ui_CEditerPanelForm {};
} // namespace Ui

.pro file segment as follows:

QT_DESIGNER_PLUGIN_PATH = $$[QT_INSTALL_PLUGINS]/designer
LIBS += \
        -L$${QT_DESIGNER_PLUGIN_PATH}  \
        -lcediterplugin

When I compiled the program,I got the following link error:

ceditpanelplugin.obj : error LNK2019: unresolved external symbol "public: static struct QMetaObject const CEditer::staticMetaObject" (?staticMetaObject@CEditer@@2UQMetaObject@@B)，referenced in function "public: static class QString __cdecl CEditer::tr(char const *,char const *,int)" (?tr@CEditer@@SA?AVQString@@PBD0H@Z) 
ceditpanel.obj : error LNK2001: unresolved external symbol  "public: static struct QMetaObject const CEditer::staticMetaObject" (?staticMetaObject@CEditer@@2UQMetaObject@@B)
moc_ceditpanel.obj : error LNK2001: unresolved external symbol  "public: static struct QMetaObject const CEditer::staticMetaObject" (?staticMetaObject@CEditer@@2UQMetaObject@@B)

I guess maybe the problem is dllexport or dllimport settings,But I don't know how to solve the error.

someone can help me?

m.sue

@A.Michael

Hi,

this error usually occurs if CEditor has no Q_OBJECT macro.

-Michael.

A.Michael

@m.sue thank you

But there are Q_OBJECT macro in my code.

mrjj

@A.Michael
Hi
have you check that $$[QT_INSTALL_PLUGINS] actually maps to the place where the .lib and dll are ?

message({$$[QT_INSTALL_PLUGINS])

for me it gives
Project MESSAGE: {C:/Qt/5.8/msvc2015/plugins}

But the path where my Designer plugin needs to be is
C:/Qt/Tools/QtCreator/bin/plugins/designer

Your setup might vary of course but do check what the variable contains.

A.Michael

@mrjj Thank you for your answer.

There's nothing wrong with my .pro file. I'm pretty sure.

Everything is OK when I compiled a EXE file with cediterplugin.dll and cediterplugin.lib.

The link error 2019 occurs only when I created a custom control that uses another custom control.

The only difference between a exe program and a custom control plugin is the QDESIGNER_WIDGET_EXPORT .

QDESIGNER_WIDGET_EXPORT is a null value, when compiling a exe program

so, I guess it might be the QDESIGNER_WIDGET_EXPORT setting problem.

SGaist

Hi,

Did you follow the Splitting up the plugin recommendation in the custom widget for Designer documentation ?

A.Michael

@SGaist thanks for reminding me

I have two custom widgets, A(a.dll,a.lib) and B(b.dll,b.lib) .

linking a custom widget(A) into another custom widget(B) has problems.

Unable to generate a custom widget library file(b.dll and b.lib)

SGaist

Are you linking all dependencies correctly ?

A.Michael

This post is deleted!

A.Michael

@SGaist I don't know what's wrong with it.

The mistake is easy to reproduce.

Two custom designer widgets,examples:

The First custom designer widget code as follows:
[ FirstCustomWidget.pro]
CONFIG += plugin release
TARGET = $$qtLibraryTarget(firstcustomwidgetplugin)
TEMPLATE = lib

HEADERS = firstcustomwidgetplugin.h
SOURCES = firstcustomwidgetplugin.cpp
RESOURCES = icons.qrc
LIBS += -L.

greaterThan(QT_MAJOR_VERSION, 4) {
QT += designer
} else {
CONFIG += designer
}

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

include(firstcustomwidget.pri)

lib_out_path = $${PWD}/../../LnkTester/build/lib
include_out_path = $${PWD}/../../LnkTester/build/include

$$[QMAKE_SPEC] {
CONFIG(release, debug|release) {
QMAKE_POST_LINK += copy /y release\.dll "$${target.path}" &
QMAKE_POST_LINK += copy /y release\.dll "$${lib_out_path}" &
QMAKE_POST_LINK += copy /y release\*.lib "$${lib_out_path}"
}
}

[FirstCustomWidget.h]
#ifndef FIRSTCUSTOMWIDGET_H
#define FIRSTCUSTOMWIDGET_H

#include <QWidget>
#include <QLabel>
#include <QPixmap>
#include <QtUiPlugin/QDesignerExportWidget>

class QDESIGNER_WIDGET_EXPORT FirstCustomWidget : public QWidget
{
Q_OBJECT
public:
FirstCustomWidget(QWidget * parent = 0);
private:
QLabel * _label;
};
#endif

[ FirstCustomWidget.cpp]
#include "firstcustomwidget.h"

FirstCustomWidget::FirstCustomWidget(QWidget * parent) :
QWidget(parent), _label(new QLabel(this))
{
}

[firstcustomwidgetplugin.h] [firstcustomwidgetplugin.cpp]
These two files are generic

The Second custom designer widget code as follows:
[SecondCustomWidget.pro]
CONFIG += plugin release
TARGET = $$qtLibraryTarget(secondcustomwidgetplugin)
TEMPLATE = lib

HEADERS = secondcustomwidgetplugin.h
SOURCES = secondcustomwidgetplugin.cpp
RESOURCES = icons.qrc
LIBS += -L.

greaterThan(QT_MAJOR_VERSION, 4) {
QT += designer
} else {
CONFIG += designer
}

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

include(secondcustomwidget.pri)

lib_out_path = $${PWD}/../../LnkTester/build/lib

INCLUDEPATH += $${PWD}/../../LnkTester/FirstCustomWidget

$$[QMAKE_SPEC] {
CONFIG(release, debug|release) {
LIBS += -L$${lib_out_path} -lfirstcustomwidgetplugin
}
}

[secondcustomwidget.h]
#ifndef SECONDCUSTOMWIDGET_H
#define SECONDCUSTOMWIDGET_H

#include <QWidget>
#include <QtUiPlugin/QDesignerExportWidget>
#include "firstcustomwidget.h"

class QDESIGNER_WIDGET_EXPORT SecondCustomWidget : public QWidget
{
Q_OBJECT
public:
SecondCustomWidget(QWidget * parent = 0);
~SecondCustomWidget();
private:
FirstCustomWidget *_cwidget;
};

#endif

[secondcustomwidget.cpp]
#include "secondcustomwidget.h"

SecondCustomWidget::SecondCustomWidget(QWidget *parent) :
QWidget(parent),_cwidget(new FirstCustomWidget(this))
{
}

SecondCustomWidget::~SecondCustomWidget()
{
delete _cwidget;
}

[secondcustomwidgetplugin.h] [secondcustomwidgetplugin.cpp]
These two files are generic

the lnk error is as follows:
Generating Code...
link /NOLOGO /DYNAMICBASE /NXCOMPAT /INCREMENTAL:NO /DLL /SUBSYSTEM:WINDOWS /OUT:release\secondcustomwidgetplugin.dll @E:\TEMP\nm4C1A.tmp
Creating library release\secondcustomwidgetplugin.lib and object release\secondcustomwidgetplugin.exp
secondcustomwidgetplugin.obj : error LNK2019: unresolved external symbol "public: static struct QMetaObject const FirstCustomWidget::staticMetaObject" (?staticMetaObject@FirstCustomWidget@@2UQMetaObject@@B) referenced in function "public: static class QString __cdecl FirstCustomWidget::tr(char const *,char const *,int)" (?tr@FirstCustomWidget@@SA?AVQString@@PBD0H@Z)
secondcustomwidget.obj : error LNK2001: unresolved external symbol "public: static struct QMetaObject const FirstCustomWidget::staticMetaObject" (?staticMetaObject@FirstCustomWidget@@2UQMetaObject@@B)
moc_secondcustomwidget.obj : error LNK2001: unresolved external symbol "public: static struct QMetaObject const FirstCustomWidget::staticMetaObject" (?staticMetaObject@FirstCustomWidget@@2UQMetaObject@@B)
release\secondcustomwidgetplugin.dll : fatal error LNK1120: 1 unresolved externals

SGaist

So you are not doing the splitting suggested in the documentation I linked to ?

A.Michael

@SGaist Thank you very much.

I solved the problem.The staticlib option must be used.

"CONFIG += staticlib"