Unresolved external symbol "public: virtual struct QMetaObject const * __thiscall Parent

Question

I inherited a class from QObject :

class Parent: public QObject
{
    Q_OBJECT
    QObject* cl;

public:
    Parent(QObject *parent=0):QObject(parent) {
        cl = NULL;
    }

    QObject* getCl() const {
        return cl;
    }
    void setCl(QObject *obj) {
        cl = obj;
    }
};

But when I write :

Parent ev;

I get the following error:

main.obj:-1: error: LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __thiscall Parent::metaObject(void)const " (?metaObject@Parent@@UBEPBUQMetaObject@@XZ)

main.obj:-1: error: LNK2001: unresolved external symbol "public: virtual void * __thiscall Parent::qt_metacast(char const *)" (?qt_metacast@Parent@@UAEPAXPBD@Z)

main.obj:-1: error: LNK2001: unresolved external symbol "public: virtual int __thiscall Parent::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@Parent@@UAEHW4Call@QMetaObject@@HPAPAX@Z)

Answer 1

You should delete the debug folder of your application and run it again to correct this problem.

Answer 2

If you're using Visual Studio, delete the line Q_OBJECT from the header file, save the file, put Q_OBJECT back into the header file, save the file again. This should generate the moc_* file and should build and link correctly.

Answer 3

I noticed some answers are based on Visual Studio.

This answer is based on Qt Creator.

Unlike the name suggest, Rebuild Project will not wipe out everything and build from scratch. If you recently added QObject (and/or Q_OBJECT) to your class, you'll have to run qmake again, e.g.

1. Clean Project
2. Run qmake
3. Build Project

This is because, by default, qmake only runs when you do significant changes to your solution like adding new source files or modify the .pro file. If you make edits to an existing file, it doesn't know it needs to run qmake.

As a fall back, to brute force Qt to build everything from scratch, delete the Debug or Release folder.

Answer 4

So the issue was I needed the Qt MOC compiler to compile my .h file. This is required for any classes that extend QObject or one of its children. The fix involed (for me) right-clicking on the header file, choosing Properties, and setting the Item Type to "Qt MOC Input", then hitting "Compile" on the header, and then adding the resulting moc_myfilename.cpp file to my project.

Answer 5

I added cpp/ui files to my project manually, but forgot to add the header file explicitly as header file. Now when compiling I got a similar error message as above and the moc_*.cpp file(s) were not generated in the debug (or release) directory of the build. That was not such an obvious mistake, qmake did not complain and other than the linker message I got no errors.

So if anyone encounters the same problem again (or makes the same copy & pase mistake): make sure the header files have also been added to your project file

Answer 6

If your moc files are generated in the visual studio project try to include them into project if they are not included into project then rebuild.

Answer 7

I had the same problem in Visual Studio, and solved it by taking the following steps:

1. Right-click header file in solution Explorer
2. Properties
3. Change "Item Type" to "Custom Build Tool"

Then in the Custom Build Tool configuration:

1. Go to General

2. set "Command Line" to:

   "$(QTDIR)\bin\moc.exe" "%(FullPath)" -o ".\GeneratedFiles\$(ConfigurationName)\moc_%(Filename).cpp" "-fStdAfx.h" "-f../../../src/FileName.h" -DUNICODE -DWIN32 -DWIN64 -DQT_DLL -DQT_NO_DEBUG -DNDEBUG -DQT_CORE_LIB -DQT_GUI_LIB -DQT_WIDGETS_LIB -DQT_NETWORK_LIB -DWIN32_LEAN_AND_MEAN -DDIS_VERSION=7 -D_MATH_DEFINES_DEFINED "-I.\SFML_STATIC" "-I.\GeneratedFiles" "-I." "-I$(QTDIR)\include" "-I.\GeneratedFiles\$(ConfigurationName)." "-I$(QTDIR)\include\QtCore" "-I$(QTDIR)\include\QtGui" "-I$(QTDIR)\include\QtNetwork"

3. set "Outputs" to:

   .\GeneratedFiles\$(ConfigurationName)\moc_%(Filename).cpp

4. set "Additional Dependencies" to:
   $(QTDIR)\bin\moc.exe;%(FullPath)

---

Your exact values may be different. They are usually applied via a Qt plugin.

Answer 8

I had this problem with Visual Studio 2012 when I had a Q_OBJECT class definition in my cpp file. Moving the class definition to the header file resolved the issue.

It looks like it should be possible to support Q_OBJECT class in cpp file by adding the cpp file to moc but I did not try this.

Answer 9

I use CMake to manage Qt projects and the new Q_OBJECT needs to be added under the QT4_WRAP_CPP call. This will generate the moc_*.cxx for inclusion in the project and clean up the unresolved externals.

Answer 10

My problem was that one of my files that used a Qt macro didn't get moc'ed. I found out, that the Qt Plugin for Visual Studio doesn't recognize the Q_NAMESPACE macro and therefore doesn't add the file to the moc'ing list.

So I used the solution from this answer to add the file to the mic'ing list:

You should find a .h file which has successfully generated "moc_*", and copy all the contents in "Custom Build Tool -> General" to the new .h file setting page.

Be careful with the different options for Debug and Release-Mode.

After that, build your project.

Build it once each in Debug and Release-Mode

Finally, add the generated "moc_*" file to your project.

Now, "moc_filename.cpp" should be in Generated Files\Debug and Generated Files\Release.

Right click on each of them and change thair properties:

• The file in Debug: Change configuration to Release and then change General->Excluded from build to yes.
• The file in Release: Change configuration to Debug and then change General->Excluded from build to yes.

Answer 11

Faced this issue in case of chained CMake targets. Turned out I had to enable CMAKE_AUTOMOC even in targets that didn't use Qt directly (transitively). Also turned out that CMAKE_AUTOMOC can't be used w/o find_package(QtX) in same CMakeLists.txt or CMakeLists.txt of parent.

Answer 12

For Visual Studio 2022 (with Qt VS Tools extention)

In Solution Explorer right click on the affected header file and select Properties.
Under "Configurations Properties -> General -> Item Type" select the option "Qt Meta-Object Compiler (moc)". Then rebuild the project.

Answer 13

In my case (using QtAdd-in with VS2012 and Qt v4.8.4) none of the above suggestions worked. For some reason VS could not generate proper moc files (build output: No relevant classes found. No output generated.) and when I compiled relevant headers by hand (setting qt moc as a compiler and clicking 'Compile') it produced empty moc file.

What did work was to compile all necessary mocs from command line (moc -o moc_SomeClass.cpp SomeClass.h) and then replace the wrong ones in GeneratedFiles folder.

This is only workaround (and not a convenient one for a big project) to have your project build succesfully, but does not really explain strange VS/QtAdd-in behaviour.

Answer 14

Using QtAdd-in with VS2010 i realized the moc_*.cpp files were updated in the GeneratedFiles/Debug folder although i was in release mode. Copying the files into the Release folder worked for me.

Answer 15

I know that this is a very old question, but it seems to be still interesting (I've been here at least 4 or 5 times in the last months) and seems like I found another reason for which is possible to get this error.

In my case in the header file I wrongly typed:

#include "MyClass.h""

Only after inspecting the whole output I found out that at that line the compiler was emitting a warning.

Now that I removed the additional quotation mark my QObject compiles perfectly!

Answer 16

I've encountered this problem with the use of a "private class" in Qt when employing the "PIMPL" (private implementation) programming pattern. Qt uses this model all through out their source code. I have come to really like it myself.

This technique involves the use of a "private" forward-declared class in a public header file, which will be be used by the "public" class (i.e. it's "parent"). The parent then has a pointer to an instance of the private class as a data member.

The "private" class is defined entirely within the cpp file for the public one. There is NO header file for the private class.

All of the "dirty work" is done with that private class. This hides all of the implementation of your public class including every other private member typically (both data and functions).

I highly recommend learning about the PIMPL pattern - especially if you are going ever read the internal Qt source.

Without explaining that coding style further, here's the point as it relates to this question... To get the Q_OBJECT macro to work inside the cpp for the "private" class to be QObject which can use signals/slot etc., you needed to explicitly include the .moc to the public class inside the cpp:

#include "MyPublicClass.moc"

You can ignore any IDE warnings about this line.

I'm not sure if it matters exactly off hand, but that inclusion I always see done AFTER the private class definition, rather than at the top of the cpp (like includes are typically placed). So, the cpp layout goes like this:

1. "Normal" includes are defined.
2. The private class is defined.
3. The moc for the public class is #included.
4. The public class implementation is defined.

Answer 17

Visual Studio 2017.

I've added file to already set up Qt project and got this error. How I fixed it:

Right click on the header in Solution Explorer Properties... -> Configuration Properties -> General -> Item Type Change from C/C++ Header to Qt Meta-Object Compiler (moc)

voila :)

Answer 18

This happened to me recently when switching from MingW to MSVC. I had a prototyped class/struct listed as a class, and MingW didn't mind.

MSVC definitely sees a difference between class and struct when prototyping is concerned.

Hope that helps someone else one day.

Answer 19

In my case, none of the above worked but it was totally my mistake.

I had overrided virtual functions in .h file (declared them) but had never defined them in .cpp :)

Answer 20

Either answer works for me in the VS 2013 environment. I eventually solve the problem by removing the .h/.cpp from project, and adding it back.

Answer 21

I am working in VS2015 with an integrated Perforce p4v client. In my case Perforce tried to add moc file to a depo, when I reverted this operation, Perforce removed this moc file from project and deleted it. The file was recreated after the next compilation, but it wasn't included in project, I have to add it manually to Generated files, when I finally understood what was the problem.

Answer 22

I have the same problem, my solution was the encoding( my file with "UTF16LE BOM" can't generate with moc.exe ) , y create another file with ASCII enconding and it work.

HxD HexEditor can help you to see the codification.

Answer 23

For me, this is the cause: some header or source file not included in QT's project file

Answer 24

In my case I had the .h and the .cpp file for the problematic QObject ancestor in subfolders of the project. When I moved them next to the CMakeLists.txt (project root folder) it linked successfully. I'm probably missing some CMake command to include mocs fot files in subdirectories.

Answer 25

when I removed Q_OBJECT it works fine.

I'm using Clion + CMake + MSVC/14.31.31103.

Is it because Q_OBJECT no longer needed after these?

set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)
set(CMAKE_AUTOUIC ON)

Answer 26

In Qt Creator

It turned out I had this error because I added Q_OBJECT afterwards in the .h file.

I managed to fix it after removing and adding the file back into the project.

Happy coding...

Answer 27

If your Parent is in a library, you should use Q_DECL_EXPORT to build your library project with the Parent.h file and use Q_DECL_IMPORT to build your main project with the Parent.h file. You can define LIBRARY_MACRO in your library project and undefine LIBRARY_MACRO in your main project to switch Q_DECL_EXPORT/Q_DECL_IMPORT

#if defined(LIBRARY_MACRO)
#  define QSS_EXPORT Q_DECL_EXPORT
#else
#  define QSS_EXPORT Q_DECL_IMPORT
#endif
class QSS_EXPORT Parent: public QObject
{
    Q_OBJECT
    QObject* cl;

public:
    Parent(QObject *parent=0):QObject(parent) {
        cl = NULL;
    }

    QObject* getCl() const {
        return cl;
    }
    void setCl(QObject *obj) {
        cl = obj;
    }
};

Answer 28

I solved my problem by adding this to my header file :

#ifndef MYCLASSNAME_H
#define MYCLASSNAME_H

... // all the header file content.

#endif