LNK2019 on constructor with QGraphicsView subclass - c++

I may be missing something obvious, but I can't get rid of a linking error in my Qt user interface.
I've isolated the part that's causing trouble. Basically, I'm implementing a subclass of QGraphicsView to display an interactive overhead map. For some reason, I can't get the constructor to be resolved.
OverheadMap.h :
#ifndef OVERHEADMAP_H
#define OVERHEADMAP_H
#include <QGraphicsView>
class OverheadMap : public QGraphicsView {
Q_OBJECT
public:
OverheadMap();
};
#endif // OVERHEADMAP_H
OverheadMap.cpp :
#include "OverheadMap.h"
OverheadMap::OverheadMap() {
// Body
}
main.cpp :
#include "OverheadMap.h"
int main(int argc, char *argv[])
{
OverheadMap *map = new OverheadMap();
}
LNK2019 :
main.obj:-1: error: LNK2019: unresolved external symbol "public: __thiscall OverheadMap::OverheadMap(void)" (??0OverheadMap##QAE#XZ) referenced in function _main
I can without any trouble use QtCreator's auto-completion with OverheadMap, and I have done a similar subclass implementation of a QFrame that's working, so I doubt there's a syntax error here.
What am I missing?
Thanks.

This code works just fine for me.
So, the solution is that you have to re-run qmake because based on your comments, you modified the project structure without telling that to qmake.

You need to call your base class's constructor
OverheadMap::OverheadMap() : QGraphicsView()
{
// Body
}

Related

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.

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.

Linker error with Qt Signal/Slot example

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

Why does this Simple Qt Application not link

I tried to write a simple Qt application like this:
main.cpp:
#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();
}
But when I tried to compile and link it with Qt Creator 2.3.1 (Qt 4.7.4) I get 3 "unresolved external symbol" errors:
main.obj:-1: error:
LNK2001: unresolved external symbol
""public: virtual struct QMetaObject const * __thiscall MyApp::metaObject(void)const "
(?metaObject#MyApp##UBEPBUQMetaObject##XZ)".
main.obj:-1: error:
LNK2001: unresolved external symbol
""public: virtual void * __thiscall MyApp::qt_metacast(char const*)"
(?qt_metacast#MyApp##UAEPAXPBD#Z)".
main.obj:-1: error:
LNK2001: unresolved external symbol
""public: virtual int __thiscall MyApp::qt_metacall(enum QMetaObject::Call,int,void * *)"
(?qt_metacall#MyApp##UAEHW4Call#QMetaObject##HPAPAX#Z)".
I think they are somehow related to the MetaObjectCompiler of Qt, but I can't figure out a solution.
I know it's not considered good programming style in c++ to put declarations and definitions in one file, but that's not the point here. In my opinion it should be possible since there is nothing syntactically wrong here.
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.
I think you need to moc the file and include the resulting main.moc at the bottom.
I think this has something to do with QMake. It's not that the executable app can't see the exported DLL class. It's that the obj file for the class doesn't exist. Running QMake from the QT Creator Build menu and then building seems to work.
Why does this Simple Qt Application not link
I've just met the same problem, and it has been solved by changing the Character set of my header from Unicode to ANSI.

C++ LNK2019 error with constructors and destructors in derived classes

I have two classes, one inherited from the other. When I compile, I get the following errors:
Entity.obj : error LNK2019: unresolved external symbol "public: __thiscall Utility::Parsables::Base::Base(void)" (??0Base#Parsables#Utility##QAE#XZ) referenced in function "public: __thiscall Utility::Parsables::Entity::Entity(void)" (??0Entity#Parsables#Utility##QAE#XZ)
Entity.obj : error LNK2019: unresolved external symbol "public: virtual __thiscall Utility::Parsables::Base::~Base(void)" (??1Base#Parsables#Utility##UAE#XZ) referenced in function "public: virtual __thiscall Utility::Parsables::Entity::~Entity(void)" (??1Entity#Parsables#Utility##UAE#XZ)
D:\Programming\Projects\Caffeine\Debug\Caffeine.exe : fatal error LNK1120: 2 unresolved externals
I really can't figure out what's going on.. can anyone see what I'm doing wrong? I'm using Visual C++ Express 2008. Here are the files..
"include/Utility/Parsables/Base.hpp"
#ifndef CAFFEINE_UTILITY_PARSABLES_BASE_HPP
#define CAFFEINE_UTILITY_PARSABLES_BASE_HPP
namespace Utility
{
namespace Parsables
{
class Base
{
public:
Base( void );
virtual ~Base( void );
};
}
}
#endif //CAFFEINE_UTILITY_PARSABLES_BASE_HPP
"src/Utility/Parsables/Base.cpp"
#include "Utility/Parsables/Base.hpp"
namespace Utility
{
namespace Parsables
{
Base::Base( void )
{
}
Base::~Base( void )
{
}
}
}
"include/Utility/Parsables/Entity.hpp"
#ifndef CAFFEINE_UTILITY_PARSABLES_ENTITY_HPP
#define CAFFEINE_UTILITY_PARSABLES_ENTITY_HPP
#include "Utility/Parsables/Base.hpp"
namespace Utility
{
namespace Parsables
{
class Entity : public Base
{
public:
Entity( void );
virtual ~Entity( void );
};
}
}
#endif //CAFFEINE_UTILITY_PARSABLES_ENTITY_HPP
"src/Utility/Parsables/Entity.cpp"
#include "Utility/Parsables/Entity.hpp"
namespace Utility
{
namespace Parsables
{
Entity::Entity( void )
{
}
Entity::~Entity( void )
{
}
}
}
The relevant bit is this:
unresolved external symbol "public: __thiscall Utility::Parsables::Base::Base(void)"
You need to provide a definition for Base::Base and Base::~Base. A declaration is not good enough. Even if you have nothing to do in either function, you need to leave an empty function body, because C++ actually requires the function to exist. C++ puts things like virtual table maintenance inside your constructors and destructors, so they must be defined even if you don't need to do anything there -- C++ has to do things in there.
Are you sure Base.cpp is being included in the build?
Just encountered this exact same error today in Visual Studio 2015. Unfortunately the accepted answer didn't worked (as well as answers from many same questions). The thing that worked for me was right click on the base class cpp file, exclude and then include it again. I think somehow VS got confused while moving file around and renames and it just silently refused to compile it even though it was marked as "Included In project" = true in property editor as well as listed in vcproj file in group. This is horrible error and ended up spending good hour on it.
Either your base.cpp is not being compiled/linked or you have a misspelling in it