Can D be cute? (Qt)

Jacob Carlborg doob at
Sat May 8 07:36:56 PDT 2010

On 5/8/10 14:01, Justin Johansson wrote:
> bearophile wrote:
>> Justin Johansson:
>>> Whilst QtD looks like a cool project, my question was really about
>>> whether D could achieve the same sorts of things that Qt does with
>>> its meta-object compiler.
>>> In other words, and calling a spade a spade, Qt is a "hack" to make
>>> C++ more pleasant for GUI. Would it be possible for D to achieve
>>> natively the same sorts that Qt does via preprocessing to C++.
>> I suggest you to explain what such things are, and to give some
>> examples too.
>> Bye,
>> bearophile
> Okay, perhaps not everybody here is across Qt.
> Programming in Qt is almost C++ bu not exactly C++ as Qt uses a
> preprocessor to translate "enhanced C++" into "vanilla C++".
> Here is an example that demonstrates that "Qt C++" is not "vanilla C++".
> In Qt, one adorns class declarations with things that are not valid C++.
> So in a class declaration you can say
> class MyWidget
> {
> public:
> // regular C++ stuff, attributes and methods
> public slots:
> // method declarations for intercepting event signals
> };
> You will observe that "public slots" is not valid C++. Qt's meta-object
> compiler does a preprocessing of the MyWidget class so that methods
> declared under the "public slots:" scope are subject to some kind of
> reflection or introspection that enables events (signals) to be delegate
> to the respective methods.
> In doing so, according to my naive understanding, the Qt MOC
> (meta-object compiler) compiles the MyWidget class retaining knowledge
> of compile time analysis of the MyWidget class methods which are
> designated as slots.
> So this is some kind of black magic that the MOC does to Qt source code
> in order to make event routing easier. The concepts of reflection come
> to mind here. In a way the MOC processor must be keeping a lot of the
> static analysis of classes around in similar fashion that Java does.
> Accordingly it is fair to ask why does Qt do this? Obviously Qt
> designers do not feel that vanilla C++ is adequate for cutting GUI apps
> which is why they resort to a preprocessor.
> Now I was thinking about why is C++ deficient for Qt to go to such
> resorts. And if vanilla C++ is deficient for GUI programming, and given
> that D is viewed by many as a "better C++", then can D do natively what
> Qt has to do with a preprocessor?
> I'm not an expert on Qt or its MOC (only 2 weeks into it). Perhaps
> others on this NG might cotton on to what I'm trying to express and
> chime in.
> Does the simple example above help explain what I'm on about?
> Cheers
> Justin Johanssom

I've read the two links bearophile posted and I think that Qt signals 
and slots look similar to C# events. This can easily be implemented 
using delegates. Basically create a new template type (a struct for 
example) called Event. Event contains a data structure (a linked list 
for example) of delegates. Then you have a function that adds new 
delegates to the data structure and finally a function that calls all 
the delegates in the data structure.

This code is basically the same thing as the Counter example on Qt site.

class Counter
	int value_;
	Event!(void, int) valueChanged;

	void value (int value)
		if (value != value_)
			value_ = value;

	int value ()
		return value_;

auto a = new Counter;
auto b = new Counter;

a.valueChanged += &b.value; // I'm hoping this will choose the right 
method based on the signature that is needed

a.value = 12;
b.value = 48;

Am I missing something? This seems pretty simple. If some additional 
support is needed there is always compile time reflection in the form of 
__traits: and runtime 
reflection in the form of flectioned:

/Jacob Carlborg

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