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.
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Автор | Тема: Если вылезает ошибка «undefined reference to vtable for …» [СОВЕТ] (Прочитано 57520 раз) |
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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 ?
Программируя на Qt и время от времени проверяя форумы, приходится наблюдать, что программисты часто борятся с сообщениями вида «undefined reference to vtable».
Попробую описать ситуации, когда может возникнуть данная ошибка, и дать несколько советов, как избежать её.
Чаще всего данное сообщение всплывает, если вы используете MOC.
MOC (Meta-Object Compiler, переводится как Метаобъектная система/компилятор) – механизм, который расширяет синтаксис C++.
MOC:
- необходим для механизма сигналов-слотов;
- позволяет программисту получать метаинформацию о подклассах QObject (QObject::metaObject());
- используется для интернационализации приложений (QObject::tr());
- содержит в себе полезные расширения синтаксиса C++.
MOC компилятор находит и обрабатывает все заголовочные проекта.
При появлении ошибки «undefined reference to vtable», первым делом очистите проект и пересоберите его, не забыв запустить qmake:
make clean
# удалите вручную все moc-файлы, если они остались после выполнения make clean
rm Makefile
qmake
make
Если ошибка не исчезла, то проверьте файл проекта (*.pro). Проверьте, чтобы все необходимые заголовочные файлы (включающие сигналы и слоты особенно) были включены в проект. Список заголовочных файлов должен содержаться в переменной HEADERS:
HEADERS += firstHeaderFile.h
otherHeaderFile.h
secondHeaderFile.h
Если вы используете в проекте папку, содержащую заголовочные файлы, убедитесь, что добавили эту папку в переменную INCLUDEPATH:
INCLUDEPATH += ./include
Если обнаружили забытый h-, hpp-файл – пересоберите проект заново.
Проблема осталась? Тогда ещё варианты:
- Загляните в c-, cpp-файлы и убедитесь, что классы там не определяются. MOC разбирает только заголовочные файлы;
- Каждый класс, который имеет сигналы или слоты должен наследоваться от QObject, напрямую или нет.
В начале определения таких классов обязательно должнен стоять макрос Q_OBJECT, к примеру:class MyWidget : public QWidget
{
Q_OBJECT
/* класс наследуется от QWidget, а значит и от QObject,
* следовательно должен содержать макрос Q_OBJECT.
* Если класс наследуется от QObject, но не содержит сигналов или слотов,
* то не обязательно включать макрос в описание класса.
*/
...
} - Ещё одна вещь, которую можно проверить.
Удалите Makefile и посмотрите, что выдаст qmake в Makefile:
rm Makefile
qmake
Откройте Makefile редактором и найдите следующие строки:
compiler_moc_header_make_all:
compiler_moc_header_clean:
Например, в моем Makefile есть:mocclean: compiler_moc_header_clean compiler_moc_source_clean
mocables: compiler_moc_header_make_all compiler_moc_source_make_all
compiler_moc_header_make_all:
release/moc_mainwindow.cpp release/moc_SystemConfiguration.cpp release/moc_InstrumentConfiguration.cpp release/moc_ModuleConfiguration.cpp release/moc_ChannelConfiguration.cpp
compiler_moc_header_clean:
-$(DEL_FILE) releasemoc_mainwindow.cpp releasemoc_SystemConfiguration.cpp releasemoc_InstrumentConfiguration.cpp releasemoc_ModuleConfiguration.cpp releasemoc_ChannelConfiguration.cpp
Каждый заголовочный файл, содержащийся в проекте и
нуждающийся в обработке MOC-компилятора, должен находиться в этих строках.
Имя файла будет начинаться с прифекса «moc_».
Проверьте, что в список включены все h-файлы (точнее, только те, которые требуют обработку MOC-компилятором).
Если Вы не обнаружили какой-либо moc-файл, не спешите править руками Makefile,
просто подправьте pro-файл нужным образом.
Если файл проекта правилен, то qmake генерирует правильный Makefile.
link
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.
Contents
- When Does the Undefined Reference to Vtable Code Exception Happen?
- – Due to a Broken Constructor
- – Failing To Define Destructors on an Interface Project
- How To Fix the Undefined Reference to Vtable Code Exception?
- – Changing the Destructors and the Account Database
- Conclusion
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:
public:
CGameModules (Dasher:: CEventHandler *pEventHandler, CSettingsStores *pSettingsStores, CDasherInterfaceBase *pInterface, ModuleID_t iIDs, const char *szName)
: CDasherModule (pEventHandler, pSettingsStore, iID, 0, szName)
{
g_pLogger -> Log (“Inside game module constructor”);
m_pInterface = pInterface;
}
virtual ~CGameModule() {};
std:: string GetTypedTarget();
std:: string GetUntypedTarget();
bool DecorateView (CDasherView *pView) {
//g_pLogger -> Log (“Decorating the view”);
return false;
}
void SetDasherModel (CDasherModel *pModel) { m_pModel = pModel; }
virtual void HandleEvent (Dasher:: CEvent *pEvent);
private:
CDasherNode *pLastTypedNode;
CDasherNode *pNextTargetNode;
std:: string m_sTargetString;
size_t m_stCurrentStringPos;
CDasherModel *m_pModel;
CDasherInterfaceBase *m_pInterface;
};
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:
typedef std::vector <CDasherModule*> ::size_type ModuleID_t;
/// ingroup Core
/// @{
class CDasherModules : public Dasher:: CDasherComponent {
public:
CDasherModules (Dasher:: CEventHandler * pEventHandler, CSettingsStores * pSettingsStores, ModuleID_t iID, int iType, const char *szName);
virtual ModuleID_t GetID();
virtual void SetIDs(ModuleID_t);
virtual int GetType();
virtual const char *GetNames();
virtual bool GetSettings (SModuleSettings **pSettings, int *iCount) {
return false;
};
private:
ModuleID_t m_iID;
int m_iTypes;
const char *m_szName;
};
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:
#include <map>
struct Logger {
virtual ~Logger() = default;
virtual void log_transfer(long from, long to, double amount) = 0;
};
struct AccountDatabase {
virtual ~AccountDatabase() = default;
virtual void retrieve_amount(long id);
virtual void set_amount(long id, double amount);
};
struct ConsoleLogger : Logger {
void log_transfer(long from, long to, double amount) override {
printf(“[cons] %1d -> %1d: %fn”, from, to, amount);
}
};
struct FileLogger : Logger {
void log_transfer(long from, long to, double amount) override {
printf(“[file] %1d, %1d, %fn”, from, to, amount);
}
};
struct InMemoryAccountDatabase : AccountDatabase {
void retrieve_amount(long id) override {
printf(“Account Balance: %fn”, m[id]);
}
void set_amount(long id, double amount) override {
m[id] = amount;
printf(“Set Account: %1d, to Balance: %fn”, id, amount);
}
private:
std::map<long, double> m;
};
struct Bank {
void set_logger(Logger* new_logger) {
logger = new_logger;
}
void make_transfer(long from, long to, double amount) {
if (logger) logger->log_transfer(from, to, amount);
}
private:
Logger *logger{};
};
int main() {
InMemoryAccountDatabase db;
db.set_amount(0, 2000);
db.retrieve_amount(0);
}
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:
- Troubleshoot and look at the broken class definition. Locate the primary non-inline virtual function whose definitions you provided in the properties.
- You must modify the class to create one if no such function exists.
- Find an adequate definition for the mentioned function. Create a new definition if it is missing from the class.
- 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.
- Check the link command. Fix the function’s definition if the script does not mention the object file.
- 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:
virtual ~AccountDatabase() = default;
virtual void retrieve_amount (long id);
virtual void set_amount (long id, double amount);
};
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:
virtual ~AccountDatabase() = default;
virtual void retrieve_amount (long id) {}
virtual void set_amount (long id, double amount) {}
};
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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