Linker error with Qt Signal/Slot example - c++

I have a Qt application with multiple classes that use signals and slots and it compiles just fine. However, when I make a custom class inside the main CPP (main.cpp) file, I get a linker error.
Here is the code I use:
class Counter : public QObject
{
Q_OBJECT
public:
Counter() { m_value = 0; }
int value() const { return m_value; }
public slots:
void setValue(int value)
{
if(value!=m_value)
{
m_value = value;
qDebug() << "Value " << value;
emit valueChanged(value);
}
}
signals:
void valueChanged(int newValue);
private:
int m_value;
};
int main(int argc, char *argv[])
{
QCoreApplication app(argc, argv);
Counter a, b;
QObject::connect(&a, SIGNAL(valueChanged(int)), &b, SLOT(setValue(int)));
a.setValue(12); // a.value() == 12, b.value() == 12
b.setValue(48); // a.value() == 12, b.value() == 48
QTimer::singleShot(0, &app, SLOT(quit()));
return app.exec();
}
Here are the errors:
Error 4 error LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __thiscall Counter::metaObject(void)const " (?metaObject#Counter##UBEPBUQMetaObject##XZ)
Error 5 error LNK2001: unresolved external symbol "public: virtual void * __thiscall Counter::qt_metacast(char const *)" (?qt_metacast#Counter##UAEPAXPBD#Z)
Error 6 error LNK2001: unresolved external symbol "public: virtual int __thiscall Counter::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall#Counter##UAEHW4Call#QMetaObject##HPAPAX#Z)
Error 7 error LNK2019: unresolved external symbol "protected: void __thiscall Counter::valueChanged(int)" (?valueChanged#Counter##IAEXH#Z) referenced in function "public: void __thiscall Counter::setValue(int)" (?setValue#Counter##QAEXH#Z)
This linker error does not occur when I place the counter in a separate header file. What's the reason for this behavior?

I'm assuming you're working with qmake.
The moc is made to run on header files automatically by default, because that's where classes are declared in general. Notice that this rule is defined in the makefile, you can manually run moc on a source file.
You have to inform qmake that the file contains a class. To do this, put #include "filename.moc" after the declaration of Counter. You can see more details here (QtCentre) or here (doc).
If you're working with another tool than qmake, say CMake, you have to specify a rule to force the moc to parse the .cpp files (the simplest is to process them all). For files that do not contain a Qt object class, moc will generate an empty file.
However, even if this class is made to be 'private', I advise you to declare it in a header (for example counter_private.h). For example, Qt source is using this trick.

It looks like you have only one code file. If you use the default way to create a Qt project build (qmake && make or QtCreator), the MOC only scans *.h files. If you have all your code in one main.cpp the MOC will not create any code, but that's needed for Signal/Slots to work.
The simplest way to make this specific example working would be adding a line "#include "main.moc"" at the end of your main.cpp. This dependency will be detected by qmake and the needed Makefile targets will be created.
The cutest way would be the clean one: One class - One header and one implementation file.

They moc/uic custom build commands are done on the header file, so it compiles when put in a seperate header/source file and not when put in the same source file

Related

LNK2001 Error even though .cpp is being compiled

I'm working on a project where I'm using Bullet3 to add a Physics Simulation Component-System to EntityX.
I'm getting link errors when compiling code, and I can't quite figure out why. Both my header PhysicsSystem.h and the source PhysicsSystem.cpp are in the project, and I can see them both being linked as a .obj in the intermediate files.
The header file PhysicsSystem.h:
class PhysicsSystem : public entityx::System<PhysicsSystem>, public entityx::Receiver<PhysicsSystem> {
public:
void configure(entityx::EntityManager &entities, entityx::EventManager &events) override;
void update(entityx::EntityManager& entities, entityx::EventManager& events, entityx::TimeDelta dt) override;
void receive(const entityx::ComponentAddedEvent<RigidBody>& event);
void setGravity(ci::vec3 gravity);
btDiscreteDynamicsWorld* getWorld();
private:
btDefaultCollisionConfiguration* mCollisionConfiguration;
btCollisionDispatcher* mDispatcher;
btBroadphaseInterface* mOverlappingPairCache;
btSequentialImpulseConstraintSolver* mBulletSolver;
btDiscreteDynamicsWorld* mDynamicsWorld;
};
and PhysicsSystem.cpp:
void PhysicsSystem::configure(entityx::EntityManager &entities, entityx::EventManager &events) {
///collision configuration contains default setup for memory, collision setup. Advanced users can create their own configuration.
mCollisionConfiguration = new btDefaultCollisionConfiguration();
///use the default collision dispatcher. For parallel processing you can use a diffent dispatcher (see Extras/BulletMultiThreaded)
mDispatcher = new btCollisionDispatcher(mCollisionConfiguration);
///btDbvtBroadphase is a good general purpose broadphase. You can also try out btAxis3Sweep.
mOverlappingPairCache = new btDbvtBroadphase();
///the default constraint solver. For parallel processing you can use a different solver (see Extras/BulletMultiThreaded)
mBulletSolver = new btSequentialImpulseConstraintSolver();
mDynamicsWorld = new btDiscreteDynamicsWorld(mDispatcher, mOverlappingPairCache, mBulletSolver, mCollisionConfiguration);
mDynamicsWorld->setGravity(btVector3(0.0, 0.0, 0.0));
events.subscribe<entityx::ComponentAddedEvent<RigidBody>>(*this);
}
void PhysicsSystem::update(entityx::EntityManager& entities, entityx::EventManager& events, entityx::TimeDelta dt) {
mDynamicsWorld->stepSimulation(static_cast<float>(dt), 10);
}
void PhysicsSystem::receive(const entityx::ComponentAddedEvent<RigidBody>& event) {
mDynamicsWorld->addRigidBody(event.component->getRigidBody());
}
void PhysicsSystem::setGravity(ci::vec3 gravity) {
if (mDynamicsWorld != nullptr) {
mDynamicsWorld->setGravity(btVector3(gravity.x, gravity.y, gravity.z));
}
else {
std::printf("PhysicsSystem ERROR -- must configure() system before you can set parameters.\n");
}
}
btDiscreteDynamicsWorld* PhysicsSystem::getWorld() {
return mDynamicsWorld;
}
When I try to compile I get the errors:
Error LNK2001 unresolved external symbol "public: virtual void __cdecl PhysicsSystem::configure(class entityx::EntityManager &,class entityx::EventManager &)" (?configure#PhysicsSystem#ecs#sitara##UEAAXAEAVEntityManager#entityx##AEAVEventManager#5##Z) PhysicsSystemExample C:\Project\physics\examples\PhysicsSystemExample\vc2013\PhysicsSystemExampleApp.obj 1
Error LNK2001 unresolved external symbol "public: virtual void __cdecl PhysicsSystem::update(class entityx::EntityManager &,class entityx::EventManager &,double)" (?update#PhysicsSystem#ecs#sitara##UEAAXAEAVEntityManager#entityx##AEAVEventManager#5#N#Z) PhysicsSystemExample C:\Project\physics\examples\PhysicsSystemExample\vc2013\PhysicsSystemExampleApp.obj 1
Error LNK2019 unresolved external symbol "public: void __cdecl PhysicsSystem::setGravity(struct glm::tvec3<float,0>)" (?setGravity#PhysicsSystem#ecs#sitara##QEAAXU?$tvec3#M$0A##glm###Z) referenced in function "public: virtual void __cdecl PhysicsSystemExampleApp::setup(void)" (?setup#PhysicsSystemExampleApp##UEAAXXZ) PhysicsSystemExample C:\Project\physics\examples\PhysicsSystemExample\vc2013\PhysicsSystemExampleApp.obj 1
Relatedly, I get a link warning 4042:
Severity Code Description Project File Line Suppression State
Warning LNK4042 object specified more than once; extras ignored PhysicsSystemExample C:\Project\physics\examples\PhysicsSystemExample\vc2013\build\x64\Debug\intermediate\PhysicsSystem.obj 1
But I only have the file added once, and can't find the file specified a second time. Why would the file be specified more than once?
What other steps can I take to troubleshoot why I'm having a linkage problem?

Define a QObject derived class inside an anonymous namespace?

I am working with Qt 5.7 (C++).
Inside the cpp file of one class I am using an anonymous namespace to create a class (some utility) that I will use only in that file.
However, I got Linking errors if the utility-class is derived from a Qt class. I think that the problem is at the Q_OBJECT macro, if I don't add it I don't get the errors. But in any Qt derived class is imperative/recommended to have the Q_OBJECT macro.
How can I avoid this isue?
Is there any other approach to have a utility-class with file-scope?
Simple example to show errors: the class CMyClass uses a utility class (named CUtility) that derives from QWidget.
Thank you.
CMyClass.h
class CMyClass
{
public:
CMyClass();
void someMethod();
};
CMyClass.cpp
#include <QtWidgets>
#include "CMyClass.h"
namespace
{
class CUtility : public QWidget
{
Q_OBJECT
public:
CUtility(QWidget *p_parent = 0) : QWidget(p_parent){qDebug() << "CUtility constructor";}
void utilityMethod() {qDebug() << "This is CUtility::utilityMethod()";}
};
}
CMyClass::CMyClass()
{
qDebug() << "CMyClass constructor.";
}
void CMyClass::someMethod()
{
qDebug() << "This is CMyClass::someMethod().";
CUtility p_myUtil;
p_myUtil.utilityMethod();
}
The errors are:
LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __cdecl `anonymous namespace'::CUtility::metaObject(void)const " (?metaObject#CUtility#?A0x27a8253c##UEBAPEBUQMetaObject##XZ)
LNK2001: unresolved external symbol "public: virtual void * __cdecl `anonymous namespace'::CUtility::qt_metacast(char const *)" (?qt_metacast#CUtility#?A0x27a8253c##UEAAPEAXPEBD#Z) sin resolver
LNK2001: unresolved external symbol "public: virtual int __cdecl `anonymous namespace'::CUtility::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall#CUtility#?A0x27a8253c##UEAAHW4Call#QMetaObject##HPEAPEAX#Z) sin resolver
This has nothing to do with anonymous namespaces at all. They are a non sequitur, in fact.
Recall that moc generates the implementations of a few methods, including signals, and some static data. For this to work, the class declaration must be visible to moc output. It is visible at the end of the .cpp file.
Thus, to have a Q_OBJECT class inside a foo.cpp file, you must #include "foo.moc" at the end of that file. Then just re-build if using cmake, or, for qmake, re-run qmake first and then build the project. That's all.
In the complete example below, the Utility class can be in the anonymous namespace, but doesn't have to be. The anonymous namespace isn't "really" a namespace: it has a special meaning that limits the scope of the contained identifiers to the translation unit. It's like static, except it can be also applied to types, not only functions and variables.
// main.cpp
#include <QObject>
namespace {
class Utility : public QObject {
Q_OBJECT
public:
Utility(QObject *parent = {});
};
}
Utility::Utility(QObject *parent) : QObject(parent) {}
int main() {
Utility utility;
}
#include "main.moc"
It doesn't work with the Q_OBJECT macro because the macro add members to your class, members which are defined in the C++ code generated by the moc (generally in moc_CMyClass.cpp making it incompatible with a file-scope).
One possible solution is to skip the Q_OBJECT macro, it is not mandatory and you may not need it. The drawback is that you will lose introspection information about your class and cannot declare signals and slots.
The other solution is, as suggested by #KubaOber, to include the generated cpp file at the end of your own copy file. In this case qmake will detect it and will not compile the moc
cpp file on its own.

Error: LNK2001: unresolved external symbol "private: static class

This forum contains many examples of such situation, but in my case static variables are defined correctly, however I still get that error. So this issue is not duplicate of previous and above link does not answer the question. Suggested 21 answers post does not have solution Simon gave me here, please unmark this as "duplicate".
Seems I've declared all correctly, check this:
.h file:
class ValueSetsModelsContainer : public QObject
{
Q_OBJECT
public:
static void DLLEXPORT loadAllergiesValueSets(MPTDatabase *db);
static void DLLEXPORT loadProceduresValueSets(MPTDatabase *db);
// Models access functions
static QStandardItemModel *drugsModel();
static QStandardItemModel *substanceModel();
static QStandardItemModel *reactionsModel();
private:
static QStandardItemModel *myDrugsModel, *mySubstanceModel, *myReactionsModel;
};
.cpp:
QStandardItemModel *ValueSetsModelsContainer::myDrugsModel = 0;
QStandardItemModel *ValueSetsModelsContainer::mySubstanceModel = 0;
QStandardItemModel *ValueSetsModelsContainer::myReactionsModel = 0;
QStandardItemModel *ValueSetsModelsContainer::drugsModel()
{
return ValueSetsModelsContainer::myDrugsModel;
}
QStandardItemModel *ValueSetsModelsContainer::substanceModel()
{
return ValueSetsModelsContainer::mySubstanceModel;
}
QStandardItemModel *ValueSetsModelsContainer::reactionsModel()
{
return ValueSetsModelsContainer::myReactionsModel;
}
So static variables are defined in cpp, however I still get linking error in another module which calls ValueSetsModelsContainer methods:
allergiesdialog.obj:-1: error: LNK2001: unresolved external symbol
"private: static class QStandardItemModel *
ValueSetsModelsContainer::myDrugsModel"
(?myDrugsModel#ValueSetsModelsContainer##0PAVQStandardItemModel##A)
allergiesdialog.obj:-1: error: LNK2001: unresolved external symbol "private: static class QStandardItemModel *
ValueSetsModelsContainer::mySubstanceModel"
(?mySubstanceModel#ValueSetsModelsContainer##0PAVQStandardItemModel##A)
allergiesdialog.obj:-1: error: LNK2001: unresolved external symbol "private: static class QStandardItemModel *
ValueSetsModelsContainer::myReactionsModel"
(?myReactionsModel#ValueSetsModelsContainer##0PAVQStandardItemModel##A)
Where the problem could be?
From your link commands it turned out that you are linking together objects into a DLL and then in a second step link the DLL with your final binary. This might be caused by a subdirs template in your project settings.
Whenever you want to have a method of a DLL available from outside, you need to make it available via __declspec( dllexport ). I guess this is done in your custom precompiler constant DLLEXPORT.
Now try this in your .h file:
static DLLEXPORT QStandardItemModel *drugsModel();
static DLLEXPORT QStandardItemModel *substanceModel();
static DLLEXPORT QStandardItemModel *reactionsModel();
to make those methods available from outside the DLL.
By the way: I don't think it makes sense here to have an intermediate dynamic library (DLL) if you are just linking stuff from your own project and don't need to make it available to someone else. Consider using a static library instead by setting TEMPLATE = lib and CONFIG += staticlib in the .pro file where ValueSetsModelsContainer is in. But this is another topic and another question.

Qt+VS2010 - Unresolvable external symbol LNK2001

All I have is a main.cpp:
#include "myclass.h"
#include <QApplication>
#include <QTextEdit>
#include <QtGui>
class Notepad : public QWidget {
Q_OBJECT
public:
Notepad();
private slots:
void quit();
private:
QTextEdit *textEdit;
QPushButton *quitButton;
};
Notepad::Notepad()
{
textEdit = new QTextEdit();
quitButton = new QPushButton(tr("Quit"));
connect(quitButton, SIGNAL(clicked()), this, SLOT(quit()));
QVBoxLayout *layout = new QVBoxLayout;
layout->addWidget(textEdit);
layout->addWidget(quitButton);
setLayout(layout);
setWindowTitle(tr("Notepad"));
}
void Notepad::quit()
{
QMessageBox messageBox;
messageBox.setWindowTitle(tr("Notepad"));
messageBox.setText(tr("Do you really want to quit?"));
messageBox.setStandardButtons(QMessageBox::Yes | QMessageBox::No);
messageBox.setDefaultButton(QMessageBox::No);
if(messageBox.exec() == QMessageBox::Yes)
qApp->quit();
}
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
return app.exec();
}
Error info:
1>main.obj : error LNK2001: Unresolvable external symbol "public: virtual struct QMetaObject const * __thiscall Notepad::metaObject(void)const " (?metaObject#Notepad##UBEPBUQMetaObject##XZ)
1>main.obj : error LNK2001: Unresolvable external symbol "public: virtual void * __thiscall Notepad::qt_metacast(char const *)" (?qt_metacast#Notepad##UAEPAXPBD#Z)
1>main.obj : error LNK2001: Unresolvable external symbol "public: virtual int __thiscall Notepad::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall#Notepad##UAEHW4Call#QMetaObject##HPAPAX#Z)
1>main.obj : error LNK2001: Unresolvable external symbol "public: static struct QMetaObject const Notepad::staticMetaObject" (?staticMetaObject#Notepad##2UQMetaObject##B)
I'm new to VS and Qt, Hope to find a solution to this, thanks.
The problem is likely that you have Q_OBJECT in your cpp file
class Notepad : public QWidget {
Q_OBJECT
To make it work you need to manually add moc step for your cpp to compilation process. Or move it to the header file, where it's done automatically
You use Q_OBJECT in your Notepad class, so you should moc it, compile the moc output and link with resulting .obj file.
Usually, classes are defined in header files, so you just run moc on the Notepad.h, make it generate moc_Notepad.cpp and compile the latter. In your case you should run moc on your main.cpp, generate something like main.moc and at the bottom of main.cpp add #include "main.moc".
I found my solution for this (FINALLY!) and thought I'd post it here.
I found this page: http://msdn.microsoft.com/en-us/library/aa267384%28v=vs.60%29.aspx
I'll paste the key piece here incase it disappears:
To use this run-time library
Single-threaded (libc.lib)
Multithreaded (libcmt.lib)
Multithreaded using DLL (msvcrt.lib)
Debug Single-threaded (libcd.lib)
Debug Multithreaded (libcmtd.lib)
Debug Multithreaded using DLL (msvcrtd.lib)
What this tells you is the runtile library you need, and which ones you don't.
So when I set release mode, Multi-threaded DLL (/MD), it didn't work for me, the issue was it had in the linker (under Input) the wrong values for the /NODEFAULTLIB, it had 3 entries, one of them being the one I was trying to use the msvcrt.lib. As soon as I changed it to: (leaving all the other values that weren't NODEFAULTLIB items)
/NODEFAULTLIB:libc.lib
/NODEFAULTLIB:libcmt.lib
/NODEFAULTLIB:libcd.lib
/NODEFAULTLIB:libcmtd.lib
/NODEFAULTLIB:msvcrtd.lib
(notice the absence of the one I'm interested it the non debug multithreaded dll lib msvcrt.lib)
It worked 110%!!!
Please like if you find useful, stack wasn't letting me post for a bit, hopefully it will let me post this one.

Qt: Linker error with QVERIFY (Qt Unit Tests)

Basically I am following the basic example here. My .pro file contains QT += core network qtestlib. [Solved] testlib instead of typo qtestlib
When I include QVERIFY, It get the following linker error:
testwaypointlist.obj:-1: error: LNK2019: unresolved external symbol "bool __cdecl
QTest::qVerify(bool,char const *,char const *,char const *,int)"
(?qVerify#QTest##YA_N_NPBD11H#Z) referenced in function "private: void __thiscall
TestWaypointList::fillWaypoints(void)" (?fillWaypoints#TestWaypointList##AAEXXZ)
What files do I miss to link? Without QVERIFY the linker error disappears.
Header file:
#include <QObject>
#include <QtTest/QtTest>
#include "waypointlist.h"
//
// Testing the waypoint list
//
class TestWaypointList : public QObject
{
Q_OBJECT
private:
WaypointList _waypointList;
public:
explicit TestWaypointList(QObject *parent = 0);
private slots:
void fillWaypoints();
};
cpp:
//
// Fill the waypoint list
//
void TestWaypointList::fillWaypoints()
{
_waypointList = WaypointList();
.....
for (int i=0; i < TESTWPLISTCOUNT; i++) {
.....
TestWaypointList::_waypointList.updateOrAppend(id, timeframe);
}
QVERIFY(TestWaypointList::_waypointList.count() == 1); // causing the linker error
}
In your .pro file, you need to change QT += qtestlib to QT += testlib (note the absence of a "q").
Of note, you used to be able to do this: CONFIG += qtestlib, but according to the comment on this page, this is no longer the recommended way of linking to the test library.