From jurojon at gmail.com  Thu Jun 12 04:18:37 2008
From: jurojon at gmail.com (Juro Jon)
Date: Thu, 12 Jun 2008 01:18:37 -0700 (PDT)
Subject: [Mono-gc-list]  GC and multiple inheritance as in C++
Message-ID: <17769541.post@talk.nabble.com>


Hello, I have a question concerning implementation of multiple inheritance in
CIL. For a C++ MI object, a pointer can points to inside the object body
instead of the its head. eg.
struct A : B, C { };  C* c = new A;
While Boehm GC detects this kind of reference, is it a supported feature of
Mono? If not, what is the suggested way to implement it (eg. what is the
C++/CLI way) ?

Thanks you
-- 
View this message in context: http://www.nabble.com/GC-and-multiple-inheritance-as-in-C%2B%2B-tp17769541p17769541.html
Sent from the Mono - Garbage Collection mailing list archive at Nabble.com.


From jurojon at gmail.com  Thu Jun 12 21:36:22 2008
From: jurojon at gmail.com (Juro Jon)
Date: Fri, 13 Jun 2008 11:36:22 +1000
Subject: [Mono-gc-list] GC and multiple inheritance as in C++
In-Reply-To: <jeske@willow=01kzmmXNFEDjCiDq>
References: <17769541.post@talk.nabble.com> <jeske@willow=01kzmmXNFEDjCiDq>
Message-ID: <3ca2f2b20806121836p29a4b5dfv27b186dd54a9a52d@mail.gmail.com>

I don't know much of CLI specification so I may be missing something
trivial. Java GC assumes that all pointers point to some "header" to
identity the memory object. Since Java is single inheritance a pointer
always points to the head of a full object, but for C++ multiple inheritance
it may be pointing to an embedded part and need to be adjusted to find the
full object. Are you saying that the CLI GC or typesystem has some way to do
this adjustment?

Thanks

On Fri, Jun 13, 2008 at 3:50 AM, David Jeske <jeske at willowmail.com> wrote:

> AFAIK, you don't need C++ MI to have a pointer to something inside an
> object
> body. This can happen if you take a ref of any instance data in C#.
>
> http://www.java2s.com/Code/CSharp/Language-Basics/CRefandOutParameters.htm
>
> -- Juro Jon wrote:
> > Hello, I have a question concerning implementation of multiple
> inheritance in
> > CIL. For a C++ MI object, a pointer can points to inside the object body
> > instead of the its head. eg.
> > struct A : B, C { };  C* c = new A;
> > While Boehm GC detects this kind of reference, is it a supported feature
> of
> > Mono? If not, what is the suggested way to implement it (eg. what is the
> > C++/CLI way) ?
>
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From jasonw at develop.com  Fri Jun 13 14:46:43 2008
From: jasonw at develop.com (Jason Whittington)
Date: Fri, 13 Jun 2008 13:46:43 -0500
Subject: [Mono-gc-list] GC and multiple inheritance as in C++
In-Reply-To: <3ca2f2b20806121836p29a4b5dfv27b186dd54a9a52d@mail.gmail.com>
References: <17769541.post@talk.nabble.com> <jeske@willow=01kzmmXNFEDjCiDq>
	<3ca2f2b20806121836p29a4b5dfv27b186dd54a9a52d@mail.gmail.com>
Message-ID: <005501c8cd85$d4172450$7c456cf0$@com>

I believe the answer here is: "no".   MI languages have to find a way to
fake it.

 

Microsoft's C++/CLI fakes it by not allowing MI for managed types.   MI
classes are represented as value types in the CLI and thus don't behave with
"true" polymorphism.  They do in the sense that C++ code compiled together
works correctly, but to other languages the inheritance is opaque - calling
GetType().BaseType will just yield System.ValueType.

 

The CLR port of Eiffel fakes it by using a [somewhat complex and arcane]
scheme where inheritance in Eiffel is modeled with interfaces in .NET.

 

Jason

 

 

From: mono-gc-list-bounces at lists.ximian.com
[mailto:mono-gc-list-bounces at lists.ximian.com] On Behalf Of Juro Jon
Sent: Thursday, June 12, 2008 8:36 PM
To: mono-gc-list at lists.ximian.com
Subject: Re: [Mono-gc-list] GC and multiple inheritance as in C++

 

I don't know much of CLI specification so I may be missing something
trivial. Java GC assumes that all pointers point to some "header" to
identity the memory object. Since Java is single inheritance a pointer
always points to the head of a full object, but for C++ multiple inheritance
it may be pointing to an embedded part and need to be adjusted to find the
full object. Are you saying that the CLI GC or typesystem has some way to do
this adjustment?

Thanks

On Fri, Jun 13, 2008 at 3:50 AM, David Jeske <jeske at willowmail.com> wrote:

AFAIK, you don't need C++ MI to have a pointer to something inside an object
body. This can happen if you take a ref of any instance data in C#.

http://www.java2s.com/Code/CSharp/Language-Basics/CRefandOutParameters.htm


-- Juro Jon wrote:
> Hello, I have a question concerning implementation of multiple inheritance
in
> CIL. For a C++ MI object, a pointer can points to inside the object body
> instead of the its head. eg.
> struct A : B, C { };  C* c = new A;
> While Boehm GC detects this kind of reference, is it a supported feature
of
> Mono? If not, what is the suggested way to implement it (eg. what is the
> C++/CLI way) ?

 

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From jeske at willowmail.com  Thu Jun 12 13:50:56 2008
From: jeske at willowmail.com (David Jeske)
Date: Thu, 12 Jun 2008 17:50:56 -0000
Subject: [Mono-gc-list] GC and multiple inheritance as in C++
In-Reply-To: <17769541.post@talk.nabble.com>
References: <17769541.post@talk.nabble.com>
	<willow-jeske-01kz[TRQFEDjCgLc-01kz[TRNFEDjCWyR>
Message-ID: <jeske@willow=01kzmmXNFEDjCiDq>

AFAIK, you don't need C++ MI to have a pointer to something inside an object
body. This can happen if you take a ref of any instance data in C#.

http://www.java2s.com/Code/CSharp/Language-Basics/CRefandOutParameters.htm

-- Juro Jon wrote:
> Hello, I have a question concerning implementation of multiple inheritance in
> CIL. For a C++ MI object, a pointer can points to inside the object body
> instead of the its head. eg.
> struct A : B, C { };  C* c = new A;
> While Boehm GC detects this kind of reference, is it a supported feature of
> Mono? If not, what is the suggested way to implement it (eg. what is the
> C++/CLI way) ?

From jeske at willowmail.com  Thu Jun 12 21:40:42 2008
From: jeske at willowmail.com (David Jeske)
Date: Fri, 13 Jun 2008 01:40:42 -0000
Subject: [Mono-gc-list] GC and multiple inheritance as in C++
In-Reply-To: <3ca2f2b20806121836p29a4b5dfv27b186dd54a9a52d@mail.gmail.com>
References: <3ca2f2b20806121836p29a4b5dfv27b186dd54a9a52d@mail.gmail.com>
	<willow-jeske-01kz[TRQFEDjCgLc-01kzxDb[FEDjCbxm>
Message-ID: <jeske@willow=01kzyO7kFEDjCX2N>

I think my optimism may have got the better of me. The CLR has a mechanism to
handle internal references, but I don't believe this answers your question or
solves your issue. The CLI GC handles interior references to objects even in C#
for "byref" paramaters, but only for interior references on the stack. Here are
some potentially useful references:

(see the bottom of this for comments on Rotor)
http://blogs.msdn.com/joelpob/archive/2004/02/26/80776.aspx
http://ksrenevasan.blogspot.com/2005/08/incomplete-rotor-gc-notes.html

Partition I, section 12.4.1.5.2, it talks about byref and lifetimes.
Partition II, section 12, it talks about interfaces and mentions MI.

----

However, it occurs to me that in C++ MI you may need interior pointers that are
not on the stack. Your concern may be well-founded, as the snippets I can find
only talk about Rotor tracing interior references which are on the stack.

Perhaps someone with more knowledge can offer a more complete (or accurate)
answer.

-- Juro Jon wrote:
> I don't know much of CLI specification so I may be missing something
> trivial. Java GC assumes that all pointers point to some "header" to
> identity the memory object. Since Java is single inheritance a pointer
> always points to the head of a full object, but for C++ multiple inheritance
> it may be pointing to an embedded part and need to be adjusted to find the
> full object. Are you saying that the CLI GC or typesystem has some way to do
> this adjustment?