Undefined Symbol missing vtable

Question

I am a beginner in qt and was attempting to get an online tutorial example running. I got CMake to build the project and I did not use .pro file or .qml file or qmake. while there seem to be some linking errors while compiling.

Undefined symbols for architecture x86_64:
  "Counter::valueChanged(int)", referenced from:
      Counter::setValue(int) in test_1.cpp.o
  "vtable for Counter", referenced from:
      Counter::Counter() in test_1.cpp.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I tried all the solutions in the similar questions asked on Stackflow but they seem not to work since most of them suggested to re-run the Qmake , which I did not use.

Here are my source files

#include <QObject>
class Counter : public QObject
{
Q_OBJECT
public:
    Counter() { m_value = 0; }
    ~Counter(){ delete(this); }
    int value() const { return m_value; }
public slots:
    void setValue(int value)
    {
        if (value != m_value) {

            m_value = value;

            emit valueChanged(value);

        }
    }
signals:
    void valueChanged(int newValue);
private:
    int m_value;
};

int main(){
    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
}

and the CmakeList.txt

cmake_minimum_required(VERSION 3.15)
project(chart_system)

set(CMAKE_CXX_STANDARD 11)

add_executable(chart_system test_1.cpp)


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


find_package(Qt5Core)
find_package(Qt5Widgets)
target_link_libraries(
        ${PROJECT_NAME}
        Qt5::Core
        Qt5::Gui
        Qt5::Widgets
        Qt5::Charts
        )
qt5_use_modules(${PROJECT_NAME} Core Widgets OpenGL Xml Gui Charts)

The two files above are all the files I got in the project. Save me out please😢

Answer 1

First of all I recommend you look for a more tutorial updated or better use the official Qt documentation:

• https://doc.qt.io/qt-5/signalsandslots.html
• https://wiki.qt.io/New_Signal_Slot_Syntax
• https://doc.qt.io/qt-5/moc.html
• https://doc.qt.io/qt-5/cmake-manual.html

In your case you have several errors:

• You do not need to delete the object itself in the destroyer, the objective of the destroyer is to free up resources handled by the class.

• Do not use the old connection style because they hide errors, better use the new connection style.

• If in the main file you create a class that implements signals, slots, etc. then you must include "name_of_file.moc" since this file that is created by the MOC is necessary to compile.

• For the signals and slots functions you must create an eventloop, that is, create a QXApplication and call exec(), in your case, just QCoreApplication.

• You are using very old CMake instructions for Qt, I recommend you check the links above.

test_1.cpp

#include <QCoreApplication>
#include <QObject>
#include <QDebug>

class Counter : public QObject
{
Q_OBJECT
public:
    Counter(QObject *parent=nullptr): QObject(parent) { m_value = 0; }
    ~Counter(){}
    int value() const { return m_value; }
public slots:
    void setValue(int value)
    {
        if (value != m_value) {
            m_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, &Counter::valueChanged, &b, &Counter::setValue);
    QObject::connect(&b, &Counter::valueChanged, [](int value){
        qDebug() << value;
    });
    a.setValue(12);
    return app.exec();
}

#include "test_1.moc"

CMakeLists.txt

cmake_minimum_required(VERSION 3.1.0)

project(chart_system)

set(CMAKE_CXX_STANDARD 11)

set(CMAKE_AUTOMOC ON)

if(CMAKE_VERSION VERSION_LESS "3.7.0")
    set(CMAKE_INCLUDE_CURRENT_DIR ON)
endif()


find_package(Qt5 COMPONENTS Core REQUIRED)

add_executable(chart_system test_1.cpp)

target_link_libraries(
    ${PROJECT_NAME}
    Qt5::Core
)