C++ – Q_OBJECT linker error!

clinker-errorsqobjectqtqt4

I am receiving the following linker error when I build my application.

HIMyClass.obj:: error:
unresolved external symbol "public:
virtual struct QMetaObject const *
__thiscall CHIMyClass::metaObject(void)const
"
(?metaObject@CHIMyClass@@UBEPBUQMetaObject@@XZ)
File not found : HIMyClass.obj

HIMyClass.obj:: error:
unresolved external symbol "public:
virtual void * __thiscall
CHIMyClass::qt_metacast(char
const *)"
(?qt_metacast@CHIMyClass@@UAEPAXPBD@Z) File not found : HIMyClass.obj

HIMyClass.obj:: error:
unresolved external symbol "public:
virtual int __thiscall
CHIMyClass::qt_metacall(enum
QMetaObject::Call,int,void * *)"
(?qt_metacall@CHIMyClass@@UAEHW4Call@QMetaObject@@HPAPAX@Z) File not found : HIMyClass.obj

My class declaration is like

class CHIMyClass:public QDialog
{
   Q_OBJECT

   ....

};

When I comment Q_OBJECT the linker error goes off (and obviously I am not able to use signals and slots). I am using Qt Creator as IDE and Qt 4.5.3. When I give Rebuild All it's definite that QMake will be called. I guess that, its the generation of moc_* files is where the problem lies. I am using Windows XP and cl as the compiler.

What might be the reason behind this linker error?

Best Answer

Such errors usually mean that you haven't added the header of your class to "HEADERS" variable in pro file (meta object compiler generates moc_ files only for headers listed in this variable). Remember to run qmake after you change .pro file!

Related Solutions

C++ – Why does this Simple Qt Application not link

Use the code below, and make sure to run qmake (Build > Run qmake) before building.

#include <QApplication>

class MyApp : public QApplication {
  Q_OBJECT
public:
  MyApp(int argc, char* argv[]);
};

MyApp::MyApp(int argc, char* argv[]) :
  QApplication(argc,argv) {
}

int main(int argc, char* argv[]) {
  MyApp app(argc,argv);
  return app.exec();
}

#include "main.moc"

Explanation: When you include the Q_OBJECT macro, this signals Qt to do a bunch of stuff that is not standard C++, such as signals and slots. It does this by running moc, which in large part is a code generator. Running qmake creates the metadata so that when your project is built, it knows which files to moc, etc.

C++ – Link error while compiling qt example on visual studio

As you do not have Qt Integration, I checked one of my Qt project for the settings.

On the property page of FindDialog.h, change Item Type from C/C++ header to Custom Build Tool. When press Apply, a new Custom Build Tool Item would appear on the left. Expand it, in General you will find Command Line, enter

"$(QTDIR)\bin\moc.exe"   -DUNICODE -DWIN32 -DQT_THREAD_SUPPORT -DQT_NO_DEBUG -DNDEBUG -DQT_CORE_LIB -DQT_GUI_LIB -DQT_DLL  "-I.\GeneratedFiles" "-I$(QTDIR)\include" "-I.\GeneratedFiles\$(ConfigurationName)\." "-I." "-I$(QTDIR)\include\QtCore" "-I$(QTDIR)\include\QtGui" ".\FindDialog.h" -o ".\GeneratedFiles\$(ConfigurationName)\moc_$(InputName).cpp"

This is my command line for a release build. You should change it according to your configuration. Remember to added the generated cpp to your project.

Under Custom Build Tool, there's an item called Outputs, which should be

".\GeneratedFiles\$(ConfigurationName)\moc_FindDialog.cpp"

As I may be still missing something, the following is the section for this file in vc's project file:

        <File
            RelativePath=".\FindDialog.h"
            >
            <FileConfiguration
                Name="Release|Win32"
                >
                <Tool
                    Name="VCCustomBuildTool"
                    Description="Moc&apos;ing FindDialog.h..."
                    CommandLine="&quot;$(QTDIR)\bin\moc.exe&quot;   -DUNICODE -DWIN32 -DQT_THREAD_SUPPORT -DQT_NO_DEBUG -DNDEBUG -DQT_CORE_LIB -DQT_GUI_LIB -DQT_DLL  &quot;-I.\GeneratedFiles&quot; &quot;-I$(QTDIR)\include&quot; &quot;-I.\GeneratedFiles\$(ConfigurationName)\.&quot; &quot;-I.&quot; &quot;-I$(QTDIR)\include\QtCore&quot; &quot;-I$(QTDIR)\include\QtGui&quot; &quot;.\FindDialog.h&quot; -o &quot;.\GeneratedFiles\$(ConfigurationName)\moc_$(InputName).cpp&quot;&#x0D;&#x0A;"
                    AdditionalDependencies="&quot;$(QTDIR)\bin\moc.exe&quot;;.\FindDialog.h"
                    Outputs="&quot;.\GeneratedFiles\$(ConfigurationName)\moc_FindDialog.cpp&quot;"
                />
            </FileConfiguration>
            <FileConfiguration
                Name="Debug|Win32"
                >
                <Tool
                    Name="VCCustomBuildTool"
                    Description="Moc&apos;ing FindDialog.h..."
                    CommandLine="&quot;$(QTDIR)\bin\moc.exe&quot;   -DUNICODE -DWIN32 -DQT_THREAD_SUPPORT -DQT_CORE_LIB -DQT_GUI_LIB  &quot;-I.\GeneratedFiles&quot; &quot;-I$(QTDIR)\include&quot; &quot;-I.\GeneratedFiles\$(ConfigurationName)\.&quot; &quot;-I.&quot; &quot;-I$(QTDIR)\include\QtCore&quot; &quot;-I$(QTDIR)\include\QtGui&quot; &quot;.\FindDialog.h&quot; -o &quot;.\GeneratedFiles\$(ConfigurationName)\moc_$(InputName).cpp&quot;&#x0D;&#x0A;"
                    AdditionalDependencies="&quot;$(QTDIR)\bin\moc.exe&quot;;.\FindDialog.h"
                    Outputs="&quot;.\GeneratedFiles\$(ConfigurationName)\moc_FindDialog.cpp&quot;"
                />
            </FileConfiguration>
        </File>

Note this is from a vcproj for VS2008.

Best Answer

Related Solutions

C++ – Why does this Simple Qt Application not link

C++ – Link error while compiling qt example on visual studio

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