c++ - Putting class declaration in .cpp file

Question

Is it possible to have class declaration and implementation in same .cpp file?

I want to do some unit-testing with help of mock object. Here is some example of my test:

// Some includes removed

#include "abstractconnection.h"

class ConnectionMockup : public AbstractConnection
{
    Q_OBJECT
public:
    explicit ConnectionMockup(QObject *parent = 0);

    bool isReady() const;
    void sendMessage(const QString &message);

    void test_send_message(const QString &message);

    bool ready;
    QStringList messages;
};

ConnectionMockup::ConnectionMockup(QObject *parent)
    : AbstractConnection(parent)
{
    ready = true;
}

bool ConnectionMockup::isReady() const
{
    return ready;
}

void ConnectionMockup::sendMessage(const QString &message)
{
    messages.append(message);
}

void ConnectionMockup::test_send_message(const QString &message)
{
    emit messageRecieved(message);
}

TestEmcProgram::TestEmcProgram(QObject *parent) :
    QObject(parent)
{
}

void TestEmcProgram::open()
{
    ConnectionMockup mockup;
    EmcProgram program(&mockup);
    QCOMPARE(...
...
...

As you can see, the class ConnectionMockup is only used by class TestConnection, and I don't need it anywhere else. So, when I try to compile this program, I get following error:

> testemcprogram.o: In function  
> `ConnectionMockup':  
> /home/sasa/Desktop/QtPro/FocoKernel-build-desktop/../FocoKernel/testemcprogram.cpp:29:  
> undefined reference to `vtable for  
> ConnectionMockup'  
> /home/sasa/Desktop/QtPro/FocoKernel-build-desktop/../FocoKernel/testemcprogram.cpp:29:  
> undefined reference to `vtable for  
> ConnectionMockup' testemcprogram.o: In  
> function `~ConnectionMockup':  
> /home/sasa/Desktop/QtPro/FocoKernel-build-desktop/../FocoKernel/testemcprogram.cpp:14:  
> undefined reference to `vtable for  
> ConnectionMockup'

Is it possible to leave declaration here, or I must create header file and move declaration in to that file?

EDIT: Since Mr. Jerry Coffin (thank you Mr. Coffin) suggested that I may not have some virtual functions implemented, I will put here declaration of AbstractConnection so we could review that possibility:

#include <QObject>

class AbstractConnection : public QObject
{
    Q_OBJECT
public:
    explicit AbstractConnection(QObject *parent = 0);
    virtual ~AbstractConnection();

    virtual bool isReady() const = 0;

signals:
    void messageRecieved(const QString &message);

public slots:
    virtual void sendMessage(const QString &message) = 0;

};

SOLUTION: Thanks to @JCooper, @iammilind and @Jerry Coffin we have the solution. After removing destructor from AbstractConnection (since it actually does nothing) and removing Q_OBJECT from ConnectionMockup it works.

Answer 1

The Q_OBJECT macro declares a set of meta-object member functions. The MOC build tool is responsible for parsing .h files and defining these function declarations. Note that it does not parse .cpp files. In your case, the vtable could not be found because the MOC tool did not parse your .cpp file. The solution is to move your class definition inside a header file and add the header to your .pro file. A second solution - a bit "hacky" - is to do the following:

#include <QObject>
#include <QtDebug>

class Counter : public QObject
{
  Q_OBJECT

public:
  Counter() { value = 0; }
  int getValue() const { qDebug() << "getValue()"; return value; }

public slots:
  void setValue(int value);

signals:
  void valueChanged(int newValue);

private:
  int value;
};

#include "main.moc"

void Counter::setValue(int value)
{
  qDebug() << "setValue()";
  if (this->value != value) {
    this->value = value;
    emit valueChanged(value);
  }
}

int main()
{
  Counter a, b;

  QObject::connect(
    &a, &Counter::valueChanged,
    &b, &Counter::setValue);

  a.setValue(12);
  b.setValue(48);

  return 0;
}

Notice the `#include "myfile.moc" under the class definition.

This works because qmake will invoke the MOC tool on any files with a #include directive. Thus, MOC will parse the .cpp file and generate the meta-object function definitions, resolving your linker error.