www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - pointer to this

reply Carotinho <carotinobg yahoo.it> writes:
Hi!
I don't even know how to name this post...:)
I have a list of classes, and I want a message from a class to be broadcast
to all the other classes in the list, except the one which sent it. I've
managed to obtain this using a delegate to a function in the container
class, which then "spread the words".
But my problem is to recognize who sent the message. I'm using an unique
number for each class added to the list, and this is used to recognize the
class, but I know this is not an elegant way.
Hence, I'm wondering if there's a way to obtain a pointer to the calling
class (the "this" in the title) in some implicit manner, or I have to give
to the delegate some sort of this flavoured variable,in which case,
which?:)

I hope I've been clear in the explanation of my problem!:)

thanks in advance!

Carotinho
Jan 07 2005
parent reply Ilya Minkov <minkov cs.tum.edu> writes:
Carotinho wrote:

 Hi!
 I don't even know how to name this post...:)
 I have a list of classes, and I want a message from a class to be broadcast
 to all the other classes in the list, except the one which sent it. I've
 managed to obtain this using a delegate to a function in the container
 class, which then "spread the words".


So, you have a list of... my guess would be Instances, that is objects?

Class is a sort of prototype to the objects, or class instances. But you 
can have classes stored in the collectiom too, for example by holding 
their classinfo. I would rather believe you don't, so disregard this 
paragraph if unclear. :>

So, you call the function in the container object which then calls a 
messaging function in each of the objects it holds?


 But my problem is to recognize who sent the message. I'm using an unique
 number for each class added to the list, and this is used to recognize the
 class, but I know this is not an elegant way.


Why don't you sort of include "this" (of type "YourClass") into the 
message instead of that serial number? Then, each recieving object can 
compare "this" in the message with its actual "this" by doind something 
like "if (this === message.sendingObjectsThis) return; //It's mine, i 
don't want it!". A further step would be to do this check in the sending 
function, a la "if (someObject !== message.sendingObjectsThis) 
someObject.sendMeMessage(message);


 Hence, I'm wondering if there's a way to obtain a pointer to the calling
 class (the "this" in the title) in some implicit manner, or I have to give
 to the delegate some sort of this flavoured variable,in which case,
 which?:)


The delegate gets the message, which includes the reference to the 
object. I think you need not include the sendingObjectsThis literally 
into the message, just pass it along with it, if you are deciding which 
objects need to recieve the message directly in the delegate.


 I hope I've been clear in the explanation of my problem!:)


If my answer answeres your question then yes. But frankly... well, you 
know...

There is a myriad of things you can do. You can store objects in 
containers, compare them, etc, this is all done by reference, so the 
separate instances are treated as different. If you want do 
differentiate by type instead of instance, every instance carries a 
reference to its type description, it is called classinfo and is of type 
ClassInfo and is a class.

I may have mixed up your mind but i suggest you take a look at object.d 
in Phobos. Look for definitions of classes Object and ClassInfo and 
things may become clear again.

-i.
Jan 07 2005
parent reply Carotinho <carotinobg yahoo.it> writes:
Hi Ilya!

Ilya Minkov wrote:


 So, you have a list of... my guess would be Instances, that is objects?
 
 Class is a sort of prototype to the objects, or class instances. But you
 can have classes stored in the collectiom too, for example by holding
 their classinfo. I would rather believe you don't, so disregard this
 paragraph if unclear. :>


No, no, it's clear, I have a list of objects, instanciated class:) I
understand the difference, this is an error of mine due to my poor
knowledge of right terms:)


 So, you call the function in the container object which then calls a
 messaging function in each of the objects it holds?


The path is that a object say something to the container, and then the
container passes the messages to all the others object of the list


 Why don't you sort of include "this" (of type "YourClass") into the
 message instead of that serial number? Then, each recieving object can
 compare "this" in the message with its actual "this" by doind something
 like "if (this === message.sendingObjectsThis) return; //It's mine, i
 don't want it!". A further step would be to do this check in the sending
 function, a la "if (someObject !== message.sendingObjectsThis)
 someObject.sendMeMessage(message);


Ok I got the sense. It's precisely what I had in mind. But I don't know what
the === is. Well, I've tried and everything's working right (^_^), but
what's the name of that === operator?:)


 The delegate gets the message, which includes the reference to the
 object. I think you need not include the sendingObjectsThis literally
 into the message, just pass it along with it, if you are deciding which
 objects need to recieve the message directly in the delegate.


I can't get the point of this... do you mean that a objects should pass as
parameters this and the message like this:
void delegate(object, int) dg; => dg(this, message1)
?
I'm doing like this and it works, but maybe you mean that when the delegate
gets the message, this message includes somehow automagically the "this
pointer" of the caller?



 If my answer answeres your question then yes. But frankly... well, you
 know...


Yes you answered well:)
 

 There is a myriad of things you can do. You can store objects in
 containers, compare them, etc, this is all done by reference, so the
 separate instances are treated as different. If you want do
 differentiate by type instead of instance, every instance carries a
 reference to its type description, it is called classinfo and is of type
 ClassInfo and is a class.


this is another interesting opening... like having a race of class which
can't understand the language of other classes... cool!:)


 
 I may have mixed up your mind but i suggest you take a look at object.d
 in Phobos. Look for definitions of classes Object and ClassInfo and
 things may become clear again.


I'm already doing it:)

A lot of thanks for your patience and understanding, and for solving my
doubts:)

Carotinho
Jan 07 2005
parent reply Ilya Minkov <minkov cs.tum.edu> writes:
Carotinho wrote:


 Ok I got the sense. It's precisely what I had in mind. But I don't know what
 the === is. Well, I've tried and everything's working right (^_^), but
 what's the name of that === operator?:)


The documentation, the section on operators explains it. === and !== 
operators are meaningful only on classes. Class instance is internally a 
reference (pointer) to a structure which holds the object fields and the 
pointer to class-related data. == and != operators on classes compare 
classes by value, by relying on opCmp overload. By default, opCmp is 
defined to be pointer comparison for classes, however be aware that 
should the left-hand-side object be null, == will segfault while trying 
to call opCmp for the object. === and !== compare object pointers 
directly, without going through a virtual function call.


 I can't get the point of this... do you mean that a objects should pass as
 parameters this and the message like this:
 void delegate(object, int) dg; => dg(this, message1)
 ?
 I'm doing like this and it works, but maybe you mean that when the delegate
 gets the message, this message includes somehow automagically the "this
 pointer" of the caller?


Yes, that's how i would do it. But if message was not an integer but a 
struct, it might make sense to include this into the message. :>


ClassInfo and is a class.




Argh, not class but class instance, which is shared among all instances 
of the class it describes. Nontheless, you got it right.


 A lot of thanks for your patience and understanding, and for solving my
 doubts:)


Glad to be halpful.

-3yE (the evil twin of eye)
Jan 08 2005
parent reply Carotinho <carotinobg yahoo.it> writes:
Ilya Minkov wrote:


 The documentation, the section on operators explains it. 


Do you mean the section about operator overload? === it's in there, but it's
the kind of page I don't read since the need of overloading operators isn't
included in my usual and simple programming needs:)
But now I've understood what you're talking about. 

Thanks again:)

Carotinho
Jan 08 2005
parent Ilya Minkov <minkov cs.tum.edu> writes:
Carotinho wrote:

 Do you mean the section about operator overload? === it's in there, but it's
 the kind of page I don't read since the need of overloading operators isn't
 included in my usual and simple programming needs:)
 But now I've understood what you're talking about. 


Perhaps the section of the documentation is somewhat misnamed.

The documentation for D is very short, and not particularly well 
organized. Due to its shortness, it is hoped for, and in fact expected, 
that the user reads it completely. You will definately find important 
warnings and relevant techniques even in chapters you find irrelevant 
for your current task. The documentation does not repeat itself, thus 
placing a burden at occasional reader but relieving a reader who wants 
to read it entirely.

Also wiki4d FAQs (http://www.prowiki.org/wiki4d/wiki.cgi) are very helpful.

-eye
Jan 08 2005