Hi!

I've run into a big problem in my project, it's not really a problem with D, more a problem with ... you'll see.

Anyway: I've got a couple of classes in my project which represent type information (like MIDI notes, audio output, sample data, etc. - it'll be an audio app written in D). Basically it's a reflection system with some extras that are specific to this project.

Now, the idea is that every of these type classes has a static field which holds a single instance, like that:

' class TypeFoo : TypeBase
' {
'     static TypeBase instance = null;
'     static this() { instance = new TypeFoo(); }
' }

I've got a static opCall to return that instance:

' static TypeBase opCall() { return instance; }

Now everywhere I need to call an overloaded constructor or tell some object which type of other objects it can process, I just can call

' TypeFoo()

and get my type class.

Next is, I've got a namespace- or registry-like tree system, where all objects are stored into. That's where I store the information of "I can process objects of type X" too. It's somehow like:

' getObject("foo").setType(TypeFoo());
' getObject("foo").addCanProcess(TypeBar());

Now the thing is that the type classes need to identify themselves to the rest of the system as type classes. Therefore I've got a TypeType class. Which also resides in that tree.

Up until now I set up this information on my type classes via a "construct"-function, which was called at a point where the class instances already exist. But I'm a bit fed up of having to maintain two pieces of code (class definition and the construct-function), so I decided to make the type classes able to set up themselves. And here the trouble starts.

So: In the TypeType static c'tor, I make a new instance of that class. Then the normal c'tor is called, trying to setup itself in the tree system, telling everybody that it is indead a type. So ... it needs the very instance of the TypeType class that is being constructed at that very moment ... so: access violation.

' class TypeType : TypeBase
' {
'     TypeBase instance = null;
'	static this() { instance = new TypeType(); }
'     static TypeBase opCall() { return instance; }
'     this()
'     {
'          addObject("typetype");
'          getObject("typetype").setType(TypeType()); // crashes: access violation
'     }
' }

It's a bit of a chicken and egg problem. I can't new an instance of that class unless there's already one instance there. Is it possible to solve that somehow? Something like an "after_this()" or the like? I guess I've outsmarted myself there and I have no idea what to do about it besides reverting to the old system with that stupid, ugly construct function (shudder).

Any ideas would be really appreciated.

-Mike

-- 
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mike wrote:
> Hi!
> 
> I've run into a big problem in my project, it's not really a problem with D, more a problem with ... you'll see.
> 
> Anyway: I've got a couple of classes in my project which represent type information (like MIDI notes, audio output, sample data, etc. - it'll be an audio app written in D). Basically it's a reflection system with some extras that are specific to this project.
> 
> Now, the idea is that every of these type classes has a static field which holds a single instance, like that:
> 
> ' class TypeFoo : TypeBase
> ' {
> '     static TypeBase instance = null;
> '     static this() { instance = new TypeFoo(); }
> ' }
> 
> I've got a static opCall to return that instance:
> 
> ' static TypeBase opCall() { return instance; }
> 
> Now everywhere I need to call an overloaded constructor or tell some object which type of other objects it can process, I just can call
> 
> ' TypeFoo()
> 
> and get my type class.
> 
> Next is, I've got a namespace- or registry-like tree system, where all objects are stored into. That's where I store the information of "I can process objects of type X" too. It's somehow like:
> 
> ' getObject("foo").setType(TypeFoo());
> ' getObject("foo").addCanProcess(TypeBar());
> 
> Now the thing is that the type classes need to identify themselves to the rest of the system as type classes. Therefore I've got a TypeType class. Which also resides in that tree.
> 
> Up until now I set up this information on my type classes via a "construct"-function, which was called at a point where the class instances already exist. But I'm a bit fed up of having to maintain two pieces of code (class definition and the construct-function), so I decided to make the type classes able to set up themselves. And here the trouble starts.
> 
> So: In the TypeType static c'tor, I make a new instance of that class. Then the normal c'tor is called, trying to setup itself in the tree system, telling everybody that it is indead a type. So ... it needs the very instance of the TypeType class that is being constructed at that very moment ... so: access violation.
> 
> ' class TypeType : TypeBase
> ' {
> '     TypeBase instance = null;
> '    static this() { instance = new TypeType(); }
> '     static TypeBase opCall() { return instance; }
> '     this()
> '     {
> '          addObject("typetype");
> '          getObject("typetype").setType(TypeType()); // crashes: access violation
> '     }
> ' }
> 
> It's a bit of a chicken and egg problem. I can't new an instance of that class unless there's already one instance there. Is it possible to solve that somehow? Something like an "after_this()" or the like? I guess I've outsmarted myself there and I have no idea what to do about it besides reverting to the old system with that stupid, ugly construct function (shudder).
> 
> Any ideas would be really appreciated.
> 
> -Mike
> 
> --Erstellt mit Operas revolutionärem E-Mail-Modul: http://www.opera.com/mail/

Have you tried placing code after "instance = new TypeType()"?

Like this:

' class TypeType : TypeBase
' {
'     TypeBase instance = null;
'     static this() {
'         instance = new TypeType();
'         // I don't know whether addObject is a function
'         // or a member of TypeType, but you get the point
'         instance.addObject("typetype");
'         instance.getObject("typetype").setType(instance);
'     }
'     static TypeBase opCall() { return instance; }
'     this()
'     {
'     }
' }

Hope that helps (and works).
Ary

mike wrote:
> [snip]
> 
> So: In the TypeType static c'tor, I make a new instance of that class.  Then the normal c'tor is called, trying to setup itself in the tree  system, telling everybody that it is indead a type. So ... it needs the  very instance of the TypeType class that is being constructed at that very  moment ... so: access violation.
> 
> ' class TypeType : TypeBase
> ' {
> '     TypeBase instance = null;
> '    static this() { instance = new TypeType(); }
> '     static TypeBase opCall() { return instance; }
> '     this()
> '     {
> '          addObject("typetype");
> '          getObject("typetype").setType(TypeType()); // crashes: access  violation
> '     }
> ' }
> 
> It's a bit of a chicken and egg problem. I can't new an instance of that  class unless there's already one instance there. Is it possible to solve  that somehow? Something like an "after_this()" or the like? I guess I've  outsmarted myself there and I have no idea what to do about it besides  reverting to the old system with that stupid, ugly construct function  (shudder).
> 
> Any ideas would be really appreciated.
> 
> -Mike
> 

Create two constructors for your TypeType classes.

One private c'tor with a dummy parameter to differentiate it from the normal public c'tor.

Call that private c'tor in your static c'tor; the private c'tor should obviously _not_ call into your registration system.

The normal public c'tor should call the registration system, just like in your code above.

class TypeType : TypeBase {
    TypeBase instance = null;

    static this() {
        instance = new TypeType(true);
        addObject("typetype");
        addObject("typetype").setType(instance);
    }
    static TypeBase opCall() { return instance; }

    public this() {
        addObject("typetype");
        getObject("typetype").setType(TypeType());
    }

    private this(bool dummy) {
        // Just a dummy to suppress infinite recursion in constructor
        // calls...
    }
}

I'm curious as to the purpose of the multiple registration of the "typetype" named instance in your registry system.  I hope you get the basic idea of what I'm proposing so you can fix these issues yourself. :)

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James Dunne

Thanks, it works! :-)

I still need one little workaround, but that's ok.

Funny thing is: Thinking about it a little bit more I got that idea:

' class TypeFoo : TypeBase
' {
'     public:
'         static this() { instance = new Bar(); }
'
'     private:
'         void setup() { /* add object, setup */ }
'         TypeBase instance;
'
'     unittest
'     {
'         instance.setup();
'     }
' }

since the unittest will be executed after ALL static this()'s are done, and it's the only piece of code that is tied to a class and called automatically (besides the constructors) - but I don't want to be that evil :-)

Another question:

Is there a way to prevent a derived class from calling super()? I solved this currently with an empty this(int) and calling super(0), but is there another way?

-Mike


Am 10.08.2006, 14:14 Uhr, schrieb Ary Manzana <asterite@gmail.com>:

> Have you tried placing code after "instance = new TypeType()"?
>
> Like this:
>
> ' class TypeType : TypeBase
> ' {
> '     TypeBase instance = null;
> '     static this() {
> '         instance = new TypeType();
> '         // I don't know whether addObject is a function
> '         // or a member of TypeType, but you get the point
> '         instance.addObject("typetype");
> '         instance.getObject("typetype").setType(instance);
> '     }
> '     static TypeBase opCall() { return instance; }
> '     this()
> '     {
> '     }
> ' }
>
> Hope that helps (and works).
> Ary



-- 
Erstellt mit Operas revolutionärem E-Mail-Modul: http://www.opera.com/mail/

mike wrote:
> Thanks, it works! :-)
> 
> I still need one little workaround, but that's ok.
> 
> Funny thing is: Thinking about it a little bit more I got that idea:
> 
> ' class TypeFoo : TypeBase
> ' {
> '     public:
> '         static this() { instance = new Bar(); }
> '
> '     private:
> '         void setup() { /* add object, setup */ }
> '         TypeBase instance;
> '
> '     unittest
> '     {
> '         instance.setup();
> '     }
> ' }
> 
> since the unittest will be executed after ALL static this()'s are done, and it's the only piece of code that is tied to a class and called automatically (besides the constructors) - but I don't want to be that evil :-)

Watch out!! Unit tests are only for debug purposes, once you compile your code with the "release" flag, no unit test will ever be invoked.

> 
> Another question:
> 
> Is there a way to prevent a derived class from calling super()? I solved this currently with an empty this(int) and calling super(0), but is there another way?

I believe not, and it's because of the OOP philosophy... But you can always make a protected constructor for your derived classes, although this increases code dependency between classes, and I don't recommend it.

> 
> -Mike
> 
> 
> Am 10.08.2006, 14:14 Uhr, schrieb Ary Manzana <asterite@gmail.com>:
> 
>> Have you tried placing code after "instance = new TypeType()"?
>>
>> Like this:
>>
>> ' class TypeType : TypeBase
>> ' {
>> '     TypeBase instance = null;
>> '     static this() {
>> '         instance = new TypeType();
>> '         // I don't know whether addObject is a function
>> '         // or a member of TypeType, but you get the point
>> '         instance.addObject("typetype");
>> '         instance.getObject("typetype").setType(instance);
>> '     }
>> '     static TypeBase opCall() { return instance; }
>> '     this()
>> '     {
>> '     }
>> ' }
>>
>> Hope that helps (and works).
>> Ary
> 
> 
> 
> --Erstellt mit Operas revolutionärem E-Mail-Modul: http://www.opera.com/mail/