Undefined reference to vtable for qt ошибка

I’ve seen a lot of ways to solve the problem, but no explanation for why it happens, so here goes.

When the compiler sees a class with virtual functions (directly declared or inherited), it must generate a vtable for that class. Since classes are generally defined in headers (and thus appear in multiple translation units), the question is where to place the vtable.

In general, the problem can be solved by generating the vtable in every TU* where the class is defined, and then let the linker eliminate duplicates. Since class definitions are required to be the same on every occurrence by the ODR**, this is safe. However, it also slows down compilation, bloats object files, and requires the linker to do more work.

As an optimization, therefore, compilers will, when possible, choose a specific TU to put the vtable in. In the common C++ ABI***, this TU is the one where the key function of the class is implemented in, where the key function is the first virtual member function that is declared in the class, but not defined.

In the case of Qt classes, they usually start with the Q_OBJECT macro, and this macro contains the declaration

virtual const QMetaObject *metaObject() const;

which, since it is the first virtual function in the macro, will generally be the first virtual function of the class and thus its key function. The compiler will therefore not emit the vtable in most TUs, only the one that implements metaObject. And this function’s implementation is written automatically by moc when it processes the header. Thus, you need to have moc process your header to generate a new .cpp file, and then include the .cpp file in your compilation.

So when you have a new header that defines a QObject-derived class, you need to rerun qmake so that it updates your makefiles to run moc on the new header and compile the resulting .cpp file.

* TU: Translation Unit. A term of art in C and C++, it refers to a single source file plus all the header files transitively included from it. Basically, the stuff the compiler sees when it works on a single source file.

** ODR: One Definition Rule. A set of rules in the C++ standard that define what happens when things (functions, classes, etc.) are defined multiple times in different translation units.

*** ABI: Application Binary Interface. A description of the way the code is organized when compiled, necessary for object files to be linked together. The Common C++ ABI is a specification that compilers for Linux generally follow so that they can interoperate.

Есть пример кода из книги Шлее о Qt 5.3. Он разбросан на хедер Progress.h, где лежит определение класса Progress, Progress.cpp, где лежат определения методов этого класса, и основной файл main.cpp. Я закинул все в один файл main.cpp.

// Как бы файл Progress.h
#include <QApplication>
#include <QtWidgets>
#include <QProgressBar>
#include <QPushButton>

// ======================================================================
class Progress : public QWidget {
    Q_OBJECT
private:
    QProgressBar* m_pprb;
    int           m_nStep;

public:
    Progress(QWidget* pobj = 0);

public slots:
    void slotStep ();
    void slotReset();
};

// ----------------------------------------------------------------------
Progress::Progress(QWidget* pwgt/*= 0*/)
    : QWidget(pwgt)
    , m_nStep(0)
{
    m_pprb = new QProgressBar;
    m_pprb->setRange(0, 5);
    m_pprb->setMinimumWidth(200);
    m_pprb->setAlignment(Qt::AlignCenter);

    QPushButton* pcmdStep  = new QPushButton("&Step");
    QPushButton* pcmdReset = new QPushButton("&Reset");

    QObject::connect(pcmdStep, SIGNAL(clicked()), SLOT(slotStep()));
    QObject::connect(pcmdReset, SIGNAL(clicked()), SLOT(slotReset()));

    //Layout setup
    QHBoxLayout* phbxLayout = new QHBoxLayout;
    phbxLayout->addWidget(m_pprb);
    phbxLayout->addWidget(pcmdStep);
    phbxLayout->addWidget(pcmdReset);
    setLayout(phbxLayout);
}
// Как бы файл Progress.cpp
// ----------------------------------------------------------------------
void Progress::slotStep()
{
    m_pprb->setValue(++m_nStep);
}

// ----------------------------------------------------------------------
void Progress::slotReset()
{
    m_nStep = 0;
    m_pprb->reset();
}
// Как бы файл main()
// ----------------------------------------------------------------------
int main (int argc, char** argv)
{
    QApplication app(argc, argv);
    Progress     progress;

    progress.show();

    return app.exec();
}

В этом случае файл не компилируется, вылетают ошибки:

main.o: In function `Progress::Progress(QWidget*)':
/mnt/data-disk/MEGA/Programming/C++/build-untitled1-Desktop-Debug/../untitled1/main.cpp:24: undefined reference to `vtable for Progress'
/mnt/data-disk/MEGA/Programming/C++/build-untitled1-Desktop-Debug/../untitled1/main.cpp:24: undefined reference to `vtable for Progress'
main.o: In function `Progress::~Progress()':
/mnt/data-disk/MEGA/Programming/C++/build-untitled1-Desktop-Debug/../untitled1/main.cpp:7: undefined reference to `vtable for Progress'
/mnt/data-disk/MEGA/Programming/C++/build-untitled1-Desktop-Debug/../untitled1/main.cpp:7: undefined reference to `vtable for Progress'

Подскажите пожалуйста, почему так?

Hi everybody, I got a big problem … I want to connect a PushButton with a slot to save some parameters but I have an error when I make the project…

So my H file is :
@#ifndef CONFIGRS232_H
#define CONFIGRS232_H

#include <QtGui>
#include <QWidget>

class ConfigRS232: public QWidget
{
Q_OBJECT

QPushButton *saveConfigButton;

public:
ConfigRS232(QWidget *parent = 0);

private:
int _portCom;
int _baudRate;
int _dataLength;
int _parity;
int _stopBit;

QComboBox *portCom;
QComboBox *baudRate;
QComboBox *dataLength;
QComboBox *parity;
QComboBox *stopBit;

public slots:
void SaveConfigParameters();
};

#endif // CONFIGRS232_H@

And my .cpp file is
@#include «configrs232.h»

#include <iostream>
using namespace std;

ConfigRS232::ConfigRS232(QWidget *parent)
: QWidget(parent)
{
QGroupBox *rs232ConfigGroup = new QGroupBox(tr(«RS232 Configuration»));

QLabel *pComLabel = new QLabel(tr("Com Port #"));
portCom = new QComboBox;
QLabel *pBaudLabel = new QLabel(tr("Bite Rate (bps):"));
baudRate = new QComboBox;
QLabel *pDataLabel = new QLabel(tr("Data Length:"));
dataLength = new QComboBox;
QLabel *pParLabel = new QLabel(tr("Parity:"));
parity = new QComboBox;
QLabel *pStopLabel = new QLabel(tr("Stop Bit:"));
stopBit = new QComboBox;

portCom->addItem("1",1);
portCom->addItem("2",2);
portCom->addItem("3",3);
portCom->addItem("4",4);
portCom->addItem("5",5);
portCom->addItem("6",6);
portCom->addItem("7",7);
portCom->addItem("8",8);

baudRate->addItem("4800",4800);
baudRate->addItem("9600",9600);
baudRate->addItem("19200",19200);
baudRate->addItem("38400",38400);
baudRate->addItem("57600",57600);

dataLength->addItem("5",5);
dataLength->addItem("6",6);
dataLength->addItem("7",7);
dataLength->addItem("8",8);

parity->addItem("None","None");
parity->addItem("Even","Even");
parity->addItem("Odd","Odd");

stopBit->addItem("1",1);
stopBit->addItem("2",2);

saveConfigButton = new QPushButton(tr("Save Configuration"));

QGridLayout *updateLayout = new QGridLayout;
updateLayout->addWidget(pComLabel,0,0);
updateLayout->addWidget(portCom,0,1);
updateLayout->addWidget(pBaudLabel,1,0);
updateLayout->addWidget(baudRate,1,1);
updateLayout->addWidget(pDataLabel,2,0);
updateLayout->addWidget(dataLength,2,1);
updateLayout->addWidget(pParLabel,3,0);
updateLayout->addWidget(parity,3,1);
updateLayout->addWidget(pStopLabel,4,0);
updateLayout->addWidget(stopBit,4,1);
rs232ConfigGroup->setLayout(updateLayout);

QVBoxLayout *mainLayout = new QVBoxLayout;
mainLayout->addWidget(rs232ConfigGroup);
mainLayout->addWidget(rs232ConfigGroup);
mainLayout->addSpacing(12);
mainLayout->addWidget(saveConfigButton);
mainLayout->addStretch(1);
setLayout(mainLayout);

connect(saveConfigButton, SIGNAL(clicked()), this, SLOT(SaveConfigParameters())); // here is my problem

}

void ConfigRS232::SaveConfigParameters()
{
_portCom = portCom->currentIndex();
_baudRate = baudRate->currentIndex();
_dataLength = dataLength->currentIndex();
_parity = parity->currentIndex();
_stopBit = stopBit->currentIndex();

cout << endl << "Port Com # : " << _portCom << endl;

}@

Just for more precisions, I tried without instanciate Q_OBJECT in the .h file but it doesn’t connect my pushbutton with my slot (SaveConfigParameters) …

Somebody have an idea ?

Библиотеки Qt комплектуются программой qmake, которая выполняет все необходимые подготовительные действия, чтобы компиллятор смог нормально собрать проект, в том же MinGW и nmake нет поддрежки ни signals ни slots, однако компиллят.

В общем при возникновении проблемы с vtable чаще всего помогает «Сборка — Очистить все», «Сборка — Запустить qmake»
Ну и дальше уже собирать проект
moc-файл вручную не сделать, точнее можно, но тогда надо внутренности Qt знать от и до, АБСОЛЮТНО

Добавлено через 55 секунд
То что разбиение помогло — это из-за того, что изменилась конфигурация проекта и qmake был вызван автоматически

The undefined reference to vtable for constructor code exception obliterates your programming experience when the cpp file is not in the main makefile. In addition, your program will likely experience this linker error log when one of your classes derives definitions from other classes with inadequate values and properties. Finally, the system can display the undefined reference to vtable cmake bug when one or more classes get broken reports and types.

Taking that into consideration, we wrote the ultimate debugging article to help you overcome the undefined reference using real-life examples and scripts that do not affect other functions and commands.

When Does the Undefined Reference to Vtable Code Exception Happen?

The undefined reference to vtable for destructor error log happens when the cpp file is not in the primary makefile. Moreover, this mistake can obliterate your programming experience when one of your app’s classes derives definitions from other classes with inadequate values. These culprits raise identical warnings and invalid messages.

Therefore, this undefined reference to vtable for qobject is almost inevitable when your project’s main makefile misses some files and scripts. In addition, although this mistake is typical with complex applications and programs, it can affect any project regardless of elements, functions, and commands.

Therefore, we will help you remake the undefined reference to vtable for base class bug using several syntaxes launching basic properties, which should help you understand the error’s exact cause.

You should not worry if the functions in your document differ because the debugging techniques and bug’s root remain identical for all operating systems and applications.

On the flip side, the undefined reference to `typeinfo for mistake is inevitable when the classes in your script derive other classes with broken values or inputs. Although this sounds harmless, calling classes improperly can severely affect your programming experience or project, especially with advanced properties and inputs.

Moreover, we confirmed the undefined reference to vtable for enemy instance when your elements create a vtable reference for the iBase where the document cannot find any compiled objects of iBase to look up. So, the vtable symbol may be undefined because the class is missing its key function, which is a vital indicator when applying the solutions.

– Due to a Broken Constructor

This article’s first broken instance creates a basic application with a broken constructor, which launches the error log and terminates the program.

As a result, we will show you the related source code and the code snippet that inherits the constructors. Although these syntaxes only include several elements and commands, your system fails to render the values and displays a warning.

The following example provides the basic source code:

After providing the inherited code snippet, we can only finish recreating the code exception. We must note that each document is unique and launches different values, so your project likely has other inputs that keep the solutions the same.

You can learn about the inherited code snippet below:

This section completes the first broken instance, but it is one of many such cases.

– Failing To Define Destructors on an Interface Project

Your system encounters a similar undefined reference exception when failing to define destructors on an interface project. As a result, the machine displays a warning confirming the inconsistencies and halting further procedures.

We will provide you with the script causing this error log that renders several destructors with straightforward values and inputs.

You can learn more about this code snippet in the following example:

Although we could include other elements and commands, we kept the example as short as possible. You can use this code snippet to pinpoint the failed commands.

How To Fix the Undefined Reference to Vtable Code Exception?

You can fix the vtable undefined reference code exception by modifying the classes if the snippet misses a function you indicated in the definition. In addition, you must repeat this debugging approach for all virtual and non-virtual functions to repair the program and prevent further complications.



This section confirms overcoming the broken exception is relatively straightforward after changing the classes and their values. In addition, this approach works for all projects and applications because the responsible type is identical.

Next, check the link command carrying the object file with the function’s definition to clarify the inconsistencies. Finally, we will summarize the debugging technique in a few standard steps.

The following numbered list explains the first approach:

1. Troubleshoot and look at the broken class definition. Locate the primary non-inline virtual function whose definitions you provided in the properties.
2. You must modify the class to create one if no such function exists.
3. Find an adequate definition for the mentioned function. Create a new definition if it is missing from the class.
4. Ensure the function definition has a qualified name if the definition is outside the class. The following code line exemplifies the class name: ClassName:: function_name.
5. Check the link command. Fix the function’s definition if the script does not mention the object file.
6. Repeat the preliminary steps for each virtual and non-virtual function until the system removes the mistake. Repeat the debugging approach for all static data members if the error log persists.

Your program should no longer experience this code exception after fixing the function definitions and non-virtual functions.

– Changing the Destructors and the Account Database

Another excellent debugging approach suggests changing the destructors and the account database to overcome the mistake. As a result, you will clear the inconsistencies and repair the virtual values confusing your program and displaying the error log.

Although the destructors and account database values are unique for each document, the technique repairs all code snippets regardless of purpose. We will exemplify the code differences by listing the incorrect and fixed syntaxes to help you understand the changes.

The following example provides the broken destructors:

Although this example appears functional without apparent obstacles, your system throws a warning confirming the undefined reference. Hence, we suggest placing curly braces after the voids to indicate the purpose and fix the issue. Still, remember to add an opening and a closing brace after each command.

You can learn about the changed inputs in the following example:

This section confirms overcoming the undefined reference bug is simple after adding the necessary inputs and properties. The code alteration seems insignificant, but it significantly affects your code.

Conclusion

The undefined reference to vtable code exception obliterates your programming experience when the cpp file is not in the main makefile. However, we fixed the issue, so let’s remember the vital points:

• The bug happens when one of your app’s classes derives definitions from other classes with inadequate values
• We reproduced the mistake when failing to define destructors on an interface project
• The first debugging approach suggests modifying the classes if the snippet misses a function you indicated in the definition
• You can fix the system by adding curly braces after the voids

Your application or project should be error-free after implementing the solutions and techniques shared in this article. We encourage you to open and repair your project because the keys are accessible.

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