Object and multiple inheritance for interfaces

[jpn]

Multiple inheritance of QObject is not supported, check this post.

Here's a little example what can be done with QMetaObject.
Let's take a look at a classical C++ interface approach first. Here we define an interface:

class SomeInterface
{
public:
	virtual ~SomeInterface() {}
 
	virtual void operation() = 0;
};

class SomeInterface
{
public:
	virtual ~SomeInterface() {}

	virtual void operation() = 0;
};

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And then implement the interface in various classes:

class SomeWidget : public QWidget, public SomeInterface
{
public:
	SomeWidget(QWidget* parent = 0);
 
	// SomeInterface
	void operation();
};
 
class SomeObject : public QObject, public SomeInterface
{
public:
	SomeObject(QWidget* parent = 0);
 
	// SomeInterface
	void operation();
};

class SomeWidget : public QWidget, public SomeInterface
{
public:
	SomeWidget(QWidget* parent = 0);

	// SomeInterface
	void operation();
};

class SomeObject : public QObject, public SomeInterface
{
public:
	SomeObject(QWidget* parent = 0);

	// SomeInterface
	void operation();
};

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Ok, so far so good. Then we have a plain QObject pointer in hand which we need then cast to the appropriate type to be able to call SomeInterface methods:

// QObject* object
SomeInterface* some = dynamic_cast<SomeInterface*>(object);
if (some)
{
	some->operation();
}

// QObject* object
SomeInterface* some = dynamic_cast<SomeInterface*>(object);
if (some)
{
	some->operation();
}

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Not very handy is it? Well, Qt at least has a more convenient way for solving such..

We can get away without defining any interface at all by declaring all the functions we had in the interface as slots:

class SomeWidget : public QWidget
{
	Q_OBJECT
 
public:
	SomeWidget(QWidget* parent = 0);
 
public slots:
	// no more "SomeInterface", just slots
	void operation();
};

class SomeWidget : public QWidget
{
	Q_OBJECT

public:
	SomeWidget(QWidget* parent = 0);

public slots:
	// no more "SomeInterface", just slots
	void operation();
};

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And then we can simply call the methods through QMetaObject:

// QObject* object
bool ok = QMetaObject::invokeMethod(object, "operation");

// QObject* object
bool ok = QMetaObject::invokeMethod(object, "operation");

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QMetaObject::invokeMethod() even supports return value and arguments.. Not bad huh?

Thanks bhughes @ #qt @ freenode!