Material for my blog post "Faking QObject derived classes (part 1)".

_Faking QObject derived classes (part 1).txt

Material for my blog post 'Faking QObject derived classes (part 1)'.

current_solution.cpp

/*
 * I have the impression that this idea is also the answer described in [1].
 *
 * References:
 *   [1] http://stackoverflow.com/questions/22693729/closest-solution-to-multiple-inheritance-through-qobject-subclasses?rq=1
 */

#include <QObject>
#include <iostream>

class QObjectInterface {
public:
    virtual ~QObjectInterface() {}
    virtual QString objectName() = 0;
    virtual void setObjectName(const QString & name) = 0;
};

class FooInterface : public QObjectInterface {
public:
    virtual ~FooInterface() {}
    virtual void doFooStuff() = 0;
};

class Foo : public QObject, public FooInterface {
public:
    QString objectName() { return QObject::objectName(); }
    void setObjectName(const QString& name) { QObject::setObjectName(name); }

    void doFooStuff() { std::cout << "Foo::doFooStuff()" << std::endl; }
};

class FakeFoo : public QObject, public FooInterface {
public:
    QString objectName() { return QObject::objectName(); }
    void setObjectName(const QString& name) { QObject::setObjectName(name); }

    void doFooStuff() { std::cout << "Foo::doFakeFooStuff()" << std::endl; }
};

int main()
{
    FooInterface* fakeFoo = new FakeFoo();
    fakeFoo->setObjectName("fake foo");
    std::cout << fakeFoo->objectName().toStdString() << std::endl;
    fakeFoo->doFooStuff();

    FooInterface* realFoo = new Foo();
    realFoo->setObjectName("real foo");
    std::cout << realFoo->objectName().toStdString() << std::endl;
    realFoo->doFooStuff();
}

current_solution.pro

TEMPLATE = app

TARGET = current_solution

CONFIG += console

SOURCES = current_solution.cpp

failed_attempt.cpp

#include <QObject>
#include <iostream>

class FooInterface
{
public:
    virtual ~FooInterface() {}
    virtual void doFooStuff() = 0;
};

class Foo : public QObject, public FooInterface {
public:
    void doFooStuff() {
        std::cout << "Foo::doFooStuff()" << std::endl;
    }
};

class FakeFoo : public QObject, public FooInterface {
public:
    void doFooStuff() {
        std::cout << "Foo::doFakeFooStuff()" << std::endl;
    }
};


int main()
{
    FooInterface* fakeFoo = new FakeFoo();
    fakeFoo->setObjectName("fake foo");
    std::cout << fakeFoo->objectName().toStdString() << std::endl;
    fakeFoo->doFooStuff();

    FooInterface* realFoo = new Foo();
    realFoo->setObjectName("real foo");
    std::cout << realFoo->objectName().toStdString() << std::endl;
    realFoo->doFooStuff();
}

failed_attempt.pro

TEMPLATE = app

TARGET = failed_attempt

CONFIG += console

SOURCES = failed_attempt.cpp

second_solution.cpp

#include <QObject>
#include <iostream>

class FooInterface : public QObject {

    Q_OBJECT

public:
    virtual ~FooInterface() {}
    virtual void doFooStuff() = 0;
};

class Foo : public FooInterface {

    Q_OBJECT

public:
    void doFooStuff() { std::cout << "Foo::doFooStuff()" << std::endl; }
};

class FakeFoo : public FooInterface {

    Q_OBJECT

public:
    void doFooStuff() { std::cout << "Foo::doFakeFooStuff()" << std::endl; }
};

int main()
{
    FooInterface* fakeFoo = new FakeFoo();
    fakeFoo->setObjectName("fake foo");
    std::cout << fakeFoo->objectName().toStdString() << std::endl;
    fakeFoo->doFooStuff();

    FooInterface* realFoo = new Foo();
    realFoo->setObjectName("real foo");
    std::cout << realFoo->objectName().toStdString() << std::endl;
    realFoo->doFooStuff();
}

second_solution.pro

TEMPLATE = app

TARGET = second_solution

CONFIG += console

SOURCES = second_solution.cpp

starting_point.cpp

#include <QObject>
#include <iostream>

class Foo : public QObject {
public:
    void doFooStuff() {
        std::cout << "Foo::doFooStuff()" << std::endl;
    }
};

int main()
{
    Foo foo;
    foo.setObjectName("foo");
    std::cout << foo.objectName().toStdString() << std::endl;
    foo.doFooStuff();
}

starting_point.pro

TEMPLATE = app

TARGET = starting_point

CONFIG += console

SOURCES = starting_point.cpp

third_solution.cpp

/*
 * When building this solution, I get the errors:
 * third_solution.cpp:29: Warning: Class FooBar inherits from two QObject subclasses FooInterface and BarInterface. This is not supported!
 * third_solution.cpp:38: Warning: Class FakeFooBar inherits from two QObject subclasses FooInterface and BarInterface. This is not supported!
 *
 * References:
 *   [1] http://stackoverflow.com/questions/8578657/qobject-multiple-inheritance
 *   [2] http://doc.qt.io/qt-4.8/moc.html#multiple-inheritance-requires-qobject-to-be-first
 *   [3] http://stackoverflow.com/questions/22693729/closest-solution-to-multiple-inheritance-through-qobject-subclasses?rq=1
 */

#include <QObject>
#include <iostream>

class FooInterface : public virtual QObject {

    Q_OBJECT

public:
    virtual ~FooInterface() {}
    virtual void doFooStuff() = 0;
};

class BarInterface : public virtual QObject {

    Q_OBJECT

public:
    virtual ~BarInterface() {}
    virtual void doBarStuff() = 0;
};

class FooBar : public FooInterface, public BarInterface {

    Q_OBJECT

public:
    void doFooStuff() { std::cout << "FooBar::doFooStuff()" << std::endl; }
    void doBarStuff() { std::cout << "FooBar::doBarStuff()" << std::endl; }
};

class FakeFooBar : public FooInterface, public BarInterface {

    Q_OBJECT

public:
    void doFooStuff() { std::cout << "FooBar::doFakeFooStuff()" << std::endl; }
    void doBarStuff() { std::cout << "FooBar::doFakeBarStuff()" << std::endl; }
};

int main()
{
    // Use real and fake implementation as a FooInterface.

    FooInterface* fakeFooBar = new FakeFooBar();
    fakeFooBar->setObjectName("fake foobar");
    std::cout << fakeFooBar->objectName().toStdString() << std::endl;
    fakeFooBar->doFooStuff();

    FooInterface* realFooBar = new FooBar();
    realFooBar->setObjectName("real foobar");
    std::cout << realFooBar->objectName().toStdString() << std::endl;
    realFooBar->doFooStuff();

    // TODO: can we find an example that demonstrates that virtual inheritance
    // with QObject is not supported?
}

// To force moc-compiler to process this file.
#include "third_solution.moc"

third_solution.pro

TEMPLATE = app

TARGET = third_solution

CONFIG += console

SOURCES = third_solution.cpp