The C++ Object Model: Good? Bad? Ugly? : c++

On Nov 7, 5:03 pm, Jerry Coffin <jcof...@taeus.com> wrote:
> In article <e60dbedd-a21a-4542-9e56-
> 21b46587a...@v16g2000prc.googlegroups.com>, james.ka...@gmail.com
> says...

> [ ... ]

> > I don't think anyone actually knows, since as far as I know,
> > no one has actually implemented it this way. A virtual
> > table look-up involves two indirections---read the vptr,
> > then the vtbl itself; it seems pretty obvious that a hash
> > table implementation can't come close. It might be useful,
> > however, for instrumenting the code, in order to generate
> > various statistics, or even doing some additional error
> > checking on the side (although I'm not sure what).

> > As I said above, my comment was just a nit---it's possible
> > to implement polymorphism with no information about the type
> > in the class object itself. Conceptually, at least; it's
> > probably not practical.

> I can think of a way that might be practical. Allocate objects
> in pools of (say) one megabyte (of virtual address space)
> apiece. All objects in a given pool are of the same type. A
> vtable for that pool of objects is located at the beginning of
> the one megabyte range, so you can get from an object to its
> vtable by anding out the bottom 20 bits of the object's
> address.

> This could waste a little bit of address space if you had more
> than one megabyte of a given type of object, but _could_ also
> be somewhat faster than the usual current implementation -- it
> substitutes a CPU operation (usually quite fast) for a memory
> access (often a lot slower).

It would also be very difficult to make it work with static and
local objects; you'd probably need a fall-back solution for
those.

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

On Nov 7, 2:02 am, Ian Collins <ian-n...@hotmail.com> wrote:
> tonytech08 wrote:
> > On Nov 3, 10:51 pm, Ian Collins <ian-n...@hotmail.com> wrote:
> >> tonytech08 wrote:
> >>> On Nov 3, 4:25 pm, James Kanze <james.ka...@gmail.com> wrote:
> >>>> It restricts the use of OO concepts to classes designed to be
> >>>> used with OO concepts.  
> >>> Not really, since one can have POD classes with methods, just not
> >>> CERTAIN methods (you are suggesting that "classes designed to be used
> >>> with OO concepts" are those heavyweight classes that break PODness,
> >>> right?).
> >> What's a "heavyweight class"?
>
> >>> You seem to be saying that POD classes are not supported or
> >>> at least not encouraged.
> >> Where?  They are supported, a C struct is also a C++ struct.
>
> > "not encouraged" I should have said.
>
> By whom?

Whoever decided to focus on heavyweight objects for OO, "only" in
designing/evolving/implementing C++.

On Nov 7, 4:10 am, James Kanze <james.ka...@gmail.com> wrote:
> On Nov 6, 11:36 pm, tonytech08 <tonytec...@gmail.com> wrote:
>
> > On Nov 4, 3:08 am, James Kanze <james.ka...@gmail.com> wrote:
> > > On Nov 4, 5:51 am, Ian Collins <ian-n...@hotmail.com> wrote:
> > >     [...]
> > > > An instance of a polymorphic class has to contain
> > > > information about its type, so it can't be the same as a
> > > > POD class.  Constructors and (non-virtual) methods are
> > > > irrelevant.
> > > Just a nit, but it's possible to implement polymorphism with
> > > no additional information in the instance itself; you could,
> > > for example, maintain the information is a separate
> > > hashtable which mapped the address to the type information.
> > > What isn't possible is to establish this mapping without
> > > executing some code. I.e. a constructor.
> > And how relatively (relative to the current implementation)
> > slow would that be is the question.
>
> I don't think anyone actually knows, since as far as I know, no
> one has actually implemented it this way.  A virtual table
> look-up involves two indirections---read the vptr, then the vtbl
> itself; it seems pretty obvious that a hash table implementation
> can't come close.  It might be useful, however, for
> instrumenting the code, in order to generate various statistics,
> or even doing some additional error checking on the side
> (although I'm not sure what).
>
> As I said above, my comment was just a nit---it's possible to
> implement polymorphism with no information about the type in the
> class object itself.  Conceptually, at least; it's probably not
> practical.

The only other way to avoid adding a vptr to the data portion that
pops into my head at the time is to drastically depart from "struct is
the object" paradigm and have a "2 pointer thing" be "the object": 1
ptr points to the data, the other to the vtable. Of course that breaks
my ideal "data is the object" view of things.

On Nov 7, 4:40 am, James Kanze <james.ka...@gmail.com> wrote:
> On Nov 6, 11:29 pm, tonytech08 <tonytec...@gmail.com> wrote:
>
>
>
>
>
> > On Nov 4, 3:06 am, James Kanze <james.ka...@gmail.com> wrote:
> > > On Nov 3, 11:55 pm, tonytech08 <tonytec...@gmail.com> wrote:
> > > > On Nov 3, 4:25 pm, James Kanze <james.ka...@gmail.com> wrote:
> > > > > On Nov 3, 8:44 pm, tonytech08 <tonytec...@gmail.com> wrote:
> > > > > > On Nov 1, 3:32 am, James Kanze <james.ka...@gmail.com> wrote:
> > > > > > > On Nov 1, 4:02 am, tonytech08 <tonytec...@gmail.com> wrote:
> > > > > > > > What I like about the C++ object model: that the data
> > > > > > > > portion of the class IS the object (dereferencing an
> > > > > > > > object gets you the data of a POD object).
> > > > > > > No it doesn't.
> > > > > > class SomePODClass
> > > > > > {
> > > > > >   public:
> > > > > >    int first_int;
> > > > > >    int second_int;
> > > > > >    void Func1(){}
> > > > > > };
> > > > > > SomePODClass X;
> > > > > > int first_int_in_obj = *(int*)&X; // this is not pretty, but true
> > > > > But it's only true for POD types,
> > > > Well that's why I added the parenthetical part in my
> > > > original post: to make clear I was referring to what I
> > > > like about the C++ model and wish it wouldn't get
> > > > abberated so quickly by changing the memory representation
> > > > of the data portion. 'POD' was very key for my thought,
> > > > though could have been worded better to show that I guess.
> > > Well, PODs make up part of the C++ object model; one
> > > important aspect of it is that PODs do behave differently
> > > from objects which have non-trivial constructors or
> > > destructors.  The main reasons for this are probably linked
> > > with C compatibility, but it was always intended in C++ that
> > > you could create simple structures which were no more than a
> > > collection of data (with or without member functions), as
> > > well as full OO type objects.  The object model of C++ is
> > > designed to support both.
> > Thanks for reiterating my thought: C++ has more support for OO
> > with "full OO type objects".
>
> More support than what.  

More support for OO with "heavyweight" classes than for POD classes.

> C++ has support for "full OO type
> objects", if that's what you need.  Most of my objects aren't
> "full OO type objects", in the sense that they don't support
> polymorphism.  C++ supports them just as well.

I think I may be OK without polymorphism in "lightweight" classes, but
overloaded constructors sure would be nice. And conversion operators.
Can a POD class derive from a pure abstract base class? That would be
nice also if not.

>
> > > > > and only because of constraints of C compatiblity.  The
> > > > > data portion of the class isn't the object in general.
> > > > I tend to think of the data portion (noun, vs.
> > > > behavior=verb) as "the thing" because that's what get's
> > > > operated on and maybe even directly manipulated.
> > > In C++ (and even in C, for that matter), an object has a
> > > type and an address; the type determines its size, and the
> > > set of legal operations on it.  Since an object is a thing,
> > > in some way, I guess it is a noun, but even a POD struct has
> > > behavior: you can assign it, for example, or access members.
> > > Compared to C, C++ adds the ability for the user to define
> > > additional operations (member functions), and to define
> > > non-trivial initialization and destruction (which forces
> > > significant changes in the object model).  Beyond that, C++
> > > adds support for dynamic typing (which is what one usually
> > > understands with OO).
> > Not sure what your point is. I said that I consider the data
> > portion of an object, "the object".
>
> But that's simply wrong,

No it's not. It's just an abstract way of looking at it. It's hardly a
stretch either, since the C++ object model or at least most
implementations use that as the foundation upon which to implement
polymorphism: tacking a vptr onto "the thing part" (noun) of "the
object".

> at least in the C++ object model.  An
> object has a type.  Otherwise, it's just raw memory.  That's a
> fundamental principle of any typed language.

I could easily go further and say something like "the memory in which
the data portion of an object, is the object". While that may bother
purists, it is a valid abstract way of thinking about it.

>
> > I wasn't trying to be implementation literal about it. Yes,
> > data+behavior= class, but when the implementation starts
> > adding things to the data portion, that defines a different
> > animal than a POD class.
>
> But the implementation *always* adds things to the data portion,
> or controls how the data portion is interpreted.  It defines a
> sign bit in an int, for example (but not in an unsigned int).
> If you want to support signed arithmetic, then you need some way
> of representing the sign.  If you want to support polymorphism,
> then you need some way of representing the type.  I don't see
> your point.  (The point of POD, in the standard, is C
> compatibility; anything in a POD will be interpretable by a C
> compiler, and will be interpreted in the same way as in C++.)

Well maybe I'm breaking new ground then in suggesting that there
should be a duality in the definition of what a class object is. There
are "heavyweight" classes and "lightweight" ones. I use C++ with that
paradigm today, but it could be more effective if there was more
support for "object-ness" with "lightweight" classes. The limitation
appears to be backward compatibity with C. If so, maybe there should
be structs, lightweight classes, heavyweight classes.

Of course I can only ruminate about such things being implemented in C+
+ in the future. I am beginning to investigate either preprocessing
"my language" to C++ or full new language implementation (though the
latter seems only doable if I could hack an existing C++
implementation and supplant it with an improved object model).

>
> > >     [...]
> > > > > It restricts the use of OO concepts to classes designed to
> > > > > be used with OO concepts.
> > > > Not really, since one can have POD classes with methods,
> > > > just not CERTAIN methods (you are suggesting that "classes
> > > > designed to be used with OO concepts" are those
> > > > heavyweight classes that break PODness, right?).
> > > No.  I'm really not suggesting much of anything.  However you
> > > define the concept of OO, the concept only applies to classes
> > > which were designed with it in mind.  C++ doesn't force any
> > > particular OO model, but allows you to chose.  And to have
> > > classes which aren't conform to this model.
> > "Allows you to choose"? "FORCES you to choose" between
> > lightweight (POD) class design with more limited OO and and
> > heavyweight (non-POD) class design with all OO mechanisms
> > allowed but at the expense of losing POD-ness. It's a
> > compromise. I'm not saying it's a bad compromise, but I am
> > wondering if so and what the alternative implementation
> > possibilities are.
>
> Obviously, you have to choose the appropriate semantics for the
> class.  That's part of design, and is inevitable.  So I don't
> see your point; C++ gives you the choice, without forcing you
> into any one particular model.  And there aren't just two
> choices.

The change occurs when you do something to a POD ("lightweight") class
that turns the data portion of the class into something else than just
a data struct, as when a vptr is added. Hence then, you have 2
distinct types of class objects that are dictated by the
implementation of the C++ object model.

>
> > > > You seem to be saying that POD classes are not supported
> > > > or at least not encouraged.
> > > Where do I say that?  POD classes are definitely supported,
> > > and are very useful in certain contexts.  They aren't
> > > appropriate for what most people would understand by OO, but
> > > so what.  Not everything has to be rigorously OO.
> > You seemed to imply that the "supported" ("ecouraged" would
> > probably be a better word to use) paradigms were: A. data
> > structs with non- trivial member functions and built-in
> > "behavior" and B. "full OO type objects".
>
> Not at all.  You define what you need.  

There are the limitations though: you can't have overloaded
constructors, for example, without losing POD-ness. Or conversion
operators (?). Or derivation from "interfaces" (?).

> From a design point of
> view, I find that it rarely makes sense to mix models in a
> single class: either all of the data will be public, or all of
> it will be private.  But the language doesn't require it.
>
> > > > That would be a real downer if true. I'd like to see more
> > > > support in the langauge for POD classes.
> > > Such as?
> > What I call "initializing constructors" for one thing.
> > (Constructors that take arguments to initialize a POD class in
> > various ways).
>
> Well, if there is a non-trivial constructor, the class can't be
> POD, since you need to call the constructor in order to
> initialize it.  

Well maybe then "POD" is the hangup and I should have used
"lightweight" from the beginning. I just want the data portion to
remain intact while having the constructor overloads and such.
Polymorphism I can probably do without, but deriving from interfaces
would be nice if possible.

> Anything else would be a contradiction: are you
> saying you want to provide a constructor for a class, but that
> it won't be called?  

Of course I want it to be called. By "POD-ness" I just meant I want a
struct-like consistency of the object data (with no addition such as a
vptr, for example).

> That doesn't make sense.
>
> > > Part of the motivation for defining POD as a special
> > > category is C compatibility; a POD should be usable in C.
> > > Beyond that, there is a wide range of things you can do.
> > Can't construct conveniently as you can with heavyweight class
> > objects. Why allowing this would break POD-ness escapes me.
>
> Because you can't have a constructor in C, basically.  Because
> the compiler must generate code when the object is created, if
> there is a constructor.  That is what POD-ness is all about; a
> POD object doesn't require any code for it to be constructed.

Then apparently I was using "POD" inappropriately. My concern is the
in-memory representation of the object data.

>
> > Perhaps it would break C-compatibility?
>
> Certainly, since you couldn't instantiate an instance of the
> object in C.

That pesky C-compatibility constraint! A good reason for investigation
of "a better C++" (as of course I am pondering already).

>
> > Maybe defining POD-ness as "C compatibility of structs" is a
> > hindrance, if it is defined something like that.
>
> The next version of the standard does have an additional
> category "layout compatible".  I'm not sure what it buys us,
> however.

Where can I read up on that?

>
> > > > I don't know how much can be implemented before it becomes
> > > > impossible. Certainly initializing constructors can be
> > > > had?
> > > A non-trivial constructor causes changes in the way object
> > > lifetime is defined.  So the results aren't (and can't be) a
> > > POD.  The justification is simple: you can't define an
> > > object with a non-trivial constructor in C.
> > So it is just the backward compatibility with C that prevents
> > having those nifty constructors?
>
> Interface compatibility.  And if you're not interfacing with C,
> you can have them.  The object won't be a POD, but if you're not
> using it to interface with C, who cares?

I don't. I didn't know that that is all I would be "breaking". I was
worried that putting overloaded constructors into "lightweight"
classes may either now or in the future aberrate my "data objects" in
some way. As long as the data portion of the object looks like a
struct of just the data in memory, I'm a happy camper.

>
> > > > Polymorphism not, but only NOT because of the way C++
> > > > implements it?
> > > Polymorphism implies a non-trivial constructor.  Regardless
> > > of how it is implemented, something must occur (some code
> > > must execute) for the raw memory to assume its type.
> > The more obvious issue with the polymorphism implementation is
> > the vptr. But I'm not too worried about not being able to
> > derive from POD- classes (for now anyway).
>
> But you can derive from a POD class.  A pointer to a POD class
> just won't behave polymorphically.

Of course not, because you can't define a virtual function and still
have a POD class. The thing you end up with is a non-POD though
because PODs can't have base classes. Whether I can derive from a POD
and still have one of my "lightweight" classes is a question (?).

On Nov 7, 10:20 pm, tonytech08 <tonytec...@gmail.com> wrote:
> On Nov 7, 4:40 am, James Kanze <james.ka...@gmail.com> wrote:
> > On Nov 6, 11:29 pm, tonytech08 <tonytec...@gmail.com> wrote:
[...]
> > > Thanks for reiterating my thought: C++ has more support
> > > for OO with "full OO type objects".

> > More support than what.  

> More support for OO with "heavyweight" classes than for POD
> classes.

You're not making sense. How does C++ have more support for OO
than for other idioms?

> > C++ has support for "full OO type objects", if that's what
> > you need.  Most of my objects aren't "full OO type objects",
> > in the sense that they don't support polymorphism.  C++
> > supports them just as well.

> I think I may be OK without polymorphism in "lightweight"
> classes, but overloaded constructors sure would be nice. And
> conversion operators. Can a POD class derive from a pure
> abstract base class? That would be nice also if not.

And C++ supports all of that. I fail.to see what you're
complaining about. In C++, a class is as heavyweight or as
lightweight as its designer wishes. It is the class designer
who makes the choice, not the language. More than anything
else, it is this which sets C++ off from other languages.

> > > > > > and only because of constraints of C compatiblity.  The
> > > > > > data portion of the class isn't the object in general.
> > > > > I tend to think of the data portion (noun, vs.
> > > > > behavior=verb) as "the thing" because that's what get's
> > > > > operated on and maybe even directly manipulated.
> > > > In C++ (and even in C, for that matter), an object has a
> > > > type and an address; the type determines its size, and the
> > > > set of legal operations on it.  Since an object is a thing,
> > > > in some way, I guess it is a noun, but even a POD struct has
> > > > behavior: you can assign it, for example, or access members.
> > > > Compared to C, C++ adds the ability for the user to define
> > > > additional operations (member functions), and to define
> > > > non-trivial initialization and destruction (which forces
> > > > significant changes in the object model).  Beyond that, C++
> > > > adds support for dynamic typing (which is what one usually
> > > > understands with OO).
> > > Not sure what your point is. I said that I consider the data
> > > portion of an object, "the object".

> > But that's simply wrong,

> No it's not.

Yes it is. Without behavior, all you have is raw memory. C++
is a typed language, which means that objects do have behavior.
(I'm not talking necessarily of behavior in the OO sense here.
In C as well, object have behavior, and the set of operations on
an int is not the same as the set of operations on a float.)

> It's just an abstract way of looking at it. It's hardly a
> stretch either, since the C++ object model or at least most
> implementations use that as the foundation upon which to
> implement polymorphism: tacking a vptr onto "the thing part"
> (noun) of "the object".

C++ supports dynamic typing, if that's what you mean. In other
words, the type of an object vary at runtime. But I don't see
your point. It is the designer of the class who decides whether
to use dynamic typing or not. The language doesn't impose it.

> > at least in the C++ object model.  An object has a type.
> > Otherwise, it's just raw memory.  That's a fundamental
> > principle of any typed language.

> I could easily go further and say something like "the memory
> in which the data portion of an object, is the object". While
> that may bother purists, it is a valid abstract way of
> thinking about it.

Not in a typed language. If you want raw memory, C++ even
supports that. Any object can be read as an array of unsigned
char. Of course, the representation isn't always defined; are
int's 16, 36, 36, 48 or 64 bits? Are they 2's complement, 1's
complement or signed magnitude?

> > > I wasn't trying to be implementation literal about it.
> > > Yes, data+behavior= class, but when the implementation
> > > starts adding things to the data portion, that defines a
> > > different animal than a POD class.

> > But the implementation *always* adds things to the data
> > portion, or controls how the data portion is interpreted.
> >  It defines a sign bit in an int, for example (but not in an
> > unsigned int). If you want to support signed arithmetic,
> > then you need some way of representing the sign.  If you
> > want to support polymorphism, then you need some way of
> > representing the type.  I don't see your point.  (The point
> > of POD, in the standard, is C compatibility; anything in a
> > POD will be interpretable by a C compiler, and will be
> > interpreted in the same way as in C++.)

> Well maybe I'm breaking new ground then in suggesting that
> there should be a duality in the definition of what a class
> object is. There are "heavyweight" classes and "lightweight"
> ones.

There's no strict binary division. There are a number of
different classifications possible---at the application level,
the distinction between value objects and entity objects is
important, for example (but there are often objects which don't
fit into either category). In many cases, it certainly makes
sense to divide types into categories (two or more); in this
regard, about the only thing particular with "lightweight" and
"heavyweight" is that the names don't really mean anything.

> I use C++ with that paradigm today, but it could be more
> effective if there was more support for "object-ness" with
> "lightweight" classes.

Again: what support do you want? You've yet to point out
anything that isn't supported in C++.

> The limitation appears to be backward compatibity with C. If
> so, maybe there should be structs, lightweight classes,
> heavyweight classes.

And maybe there should be value types and entity types. Or
maybe some other classification is relevant to your application.
The particularity of C++ is that it lets you choose. The
designer is free to develop the categories he wants. (If I'm
not mistaken, in some circles, these type of categories are
called stereotypes.)

> > > >     [...]
> > > > > > It restricts the use of OO concepts to classes designed to
> > > > > > be used with OO concepts.
> > > > > Not really, since one can have POD classes with methods,
> > > > > just not CERTAIN methods (you are suggesting that "classes
> > > > > designed to be used with OO concepts" are those
> > > > > heavyweight classes that break PODness, right?).
> > > > No.  I'm really not suggesting much of anything.  However you
> > > > define the concept of OO, the concept only applies to classes
> > > > which were designed with it in mind.  C++ doesn't force any
> > > > particular OO model, but allows you to chose.  And to have
> > > > classes which aren't conform to this model.
> > > "Allows you to choose"? "FORCES you to choose" between
> > > lightweight (POD) class design with more limited OO and and
> > > heavyweight (non-POD) class design with all OO mechanisms
> > > allowed but at the expense of losing POD-ness. It's a
> > > compromise. I'm not saying it's a bad compromise, but I am
> > > wondering if so and what the alternative implementation
> > > possibilities are.

> > Obviously, you have to choose the appropriate semantics for
> > the class.  That's part of design, and is inevitable.  So I
> > don't see your point; C++ gives you the choice, without
> > forcing you into any one particular model.  And there aren't
> > just two choices.

> The change occurs when you do something to a POD
> ("lightweight") class that turns the data portion of the class
> into something else than just a data struct, as when a vptr is
> added. Hence then, you have 2 distinct types of class objects
> that are dictated by the implementation of the C++ object
> model.

The concept of a POD was introduced mainly for reasons of
interfacing with C. Forget it for the moment. You have as many
types of class objects as the designer wishes. If you want just
a data struct, fine; I use them from time to time (and they
aren't necessarily POD's---it's not rare for my data struct's to
contain an std::string). If you want polymorphism, that's fine
too. If you want something in between, say a value type with
deep copy semantics, no problem.

There is NO restriction in C++ with regards to what you can do.

> > > > > You seem to be saying that POD classes are not supported
> > > > > or at least not encouraged.
> > > > Where do I say that?  POD classes are definitely supported,
> > > > and are very useful in certain contexts.  They aren't
> > > > appropriate for what most people would understand by OO, but
> > > > so what.  Not everything has to be rigorously OO.
> > > You seemed to imply that the "supported" ("ecouraged" would
> > > probably be a better word to use) paradigms were: A. data
> > > structs with non- trivial member functions and built-in
> > > "behavior" and B. "full OO type objects".

> > Not at all.  You define what you need.  

> There are the limitations though: you can't have overloaded
> constructors, for example, without losing POD-ness.

Obviously, given the particular role of PODs. So? What's your
point? There are ony two reasons I know for insisting on
something being a POD: you need to be able to use it from C as
well, or you need static initialization. Both mean that
construction must be trivial.

> Or conversion operators (?).

A conversion operator doesn't affect POD-ness. In fact, POD
structures with a conversion operator are a common idiom for
certain types of initialization.

> Or derivation from "interfaces" (?).

How is a C program going to deal with derivation? For that
matter, an interface supposes virtual functions and dynamic
typing; it's conceptually impossible to create a dynamically
typed object without executing some code.

You do have to know what you want. (And this has nothing to do
with the "design" of the C++ object model; it's more related to
simple possibilties. C++ does try to not impose anything
impossible to implement.)

> > From a design point of view, I find that it rarely makes
> > sense to mix models in a single class: either all of the
> > data will be public, or all of it will be private.  But the
> > language doesn't require it.

> > > > > That would be a real downer if true. I'd like to see more
> > > > > support in the langauge for POD classes.
> > > > Such as?
> > > What I call "initializing constructors" for one thing.
> > > (Constructors that take arguments to initialize a POD class in
> > > various ways).

> > Well, if there is a non-trivial constructor, the class can't
> > be POD, since you need to call the constructor in order to
> > initialize it.  

> Well maybe then "POD" is the hangup and I should have used
> "lightweight" from the beginning. I just want the data portion
> to remain intact while having the constructor overloads and
> such.

I'm not sure what you mean by "the data portion to remain
intact". Taken literally, the data portion had better remain
intact for all types of objects. If you mean contiguous, that's
a different issue: not even POD's are guaranteed to have
contiguous data (since C doesn't guarantee it)---on many
machines (e.g. Sparcs, IBM mainframes...) that would introduce
totally unacceptable performance costs.

If anything, C++ specifies the structure of the data too much.
A compiler is not allowed to reorder data if there is no
intervening change of access, for example. If a programmer
writes:

struct S
{
char c1 ;
int i1 ;
char c2 ;
int i2 ;
} ;

for example, the compiler is not allowed to place the i1 and i2
elements in front of c1 and c2, despite the fact that this would
improve memory use and optimization.

> Polymorphism I can probably do without, but deriving from
> interfaces would be nice if possible.

If you have dynamic typing, some code must be executed when the
object is created; otherwise, there is no way later to know what
the dynamic type is.

> > Anything else would be a contradiction: are you saying you
> > want to provide a constructor for a class, but that it won't
> > be called?  

> Of course I want it to be called. By "POD-ness" I just meant I
> want a struct-like consistency of the object data (with no
> addition such as a vptr, for example).

I don't understand all this business of vptr. Do you want
polymorphism, or not. If you want polymorphism, the compiler
must memorize the type of the object (each object) somewhere,
when the object is created; C++ doesn't require it to be in the
object itself, but in practice, this is by far the most
effective solution. If you don't want polymorphism, and don't
declare any virtual functions, then the compiler doesn't have to
memorize the type of the object, and none that I know of do.

[...]
> > Because you can't have a constructor in C, basically.
> >  Because the compiler must generate code when the object is
> > created, if there is a constructor.  That is what POD-ness
> > is all about; a POD object doesn't require any code for it
> > to be constructed.

> Then apparently I was using "POD" inappropriately. My concern
> is the in-memory representation of the object data.

Which is implementation defined in C++, just as it was in C.
With some constraints; the compiler can insert padding (and all
do in some cases), but it cannot reorder non-static data members
unless there is an intervening change in access control. That,
and the fact that a class cannot have a size of 0, are about the
only restraints. C (and C++ for PODs) also have a constraint
that the first data element must be at the start of the object;
the compiler may not introduce padding before the first element.
If I'm not mistaken, the next version of the standard extends
this constraint to "standard-layout classes"; i.e. to classes
that have no virtual functions and no virtual bases, no changes
in access control, and a few other minor restrictions (but which
may have non-trivial constructors). This new rule, however,
does nothing but describe current practice.

[...]
> > > Maybe defining POD-ness as "C compatibility of structs" is
> > > a hindrance, if it is defined something like that.

> > The next version of the standard does have an additional
> > category "layout compatible".  I'm not sure what it buys us,
> > however.

> Where can I read up on that?

In the current draft. I think
http://www.open-std.org/jtc1/sc22/wg...2008/n2798.pdf
should get it (or something close---the draft is still
evolving).

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

On Nov 8, 3:12 am, James Kanze <james.ka...@gmail.com> wrote:
> On Nov 7, 10:20 pm, tonytech08 <tonytec...@gmail.com> wrote:
>
> > On Nov 7, 4:40 am, James Kanze <james.ka...@gmail.com> wrote:
> > > On Nov 6, 11:29 pm, tonytech08 <tonytec...@gmail.com> wrote:
>     [...]
> > > > Thanks for reiterating my thought: C++ has more support
> > > > for OO with "full OO type objects".
> > > More support than what.  
> > More support for OO with "heavyweight" classes than for POD
> > classes.
>
> You're not making sense.  How does C++ have more support for OO
> than for other idioms?

Why are you asking that when I said nothing of the sort? I said that
once you put a vptr into the data portion of an object (for example),
it's a different animal than a class without a vptr (for example!). I
distinguished these fundamentally different animals by calling them
"heavyweight" and "lightweight" classes/object (and apparently wrongly
POD-classes wrongly).

Moreso, I was concerned that other things one can do with a class,
such as defining overloaded constructors, may make code fragile
against some future or other current implementation of the language.
Who's to say (not me) that someone won't make a compiler that tacks on
or into a class object some other "hidden" ptr or something to
implement "overloaded constructors"? I don't care if code is generated
but I do care if the compiler starts aberrating the data portion.

>
> > > C++ has support for "full OO type objects", if that's what
> > > you need.  Most of my objects aren't "full OO type objects",
> > > in the sense that they don't support polymorphism.  C++
> > > supports them just as well.
> > I think I may be OK without polymorphism in "lightweight"
> > classes, but overloaded constructors sure would be nice. And
> > conversion operators.  Can a POD class derive from a pure
> > abstract base class? That would be nice also if not.
>
> And C++ supports all of that.  

But am I guaranteed that my a class will stay lightweight if I do that
or is it implementation defined?

> I fail.to see what you're
> complaining about.  In C++, a class is as heavyweight or as
> lightweight as its designer wishes.  It is the class designer
> who makes the choice, not the language.  

That's not the case with polymorphism for example: the vptr in the
data portion is not free. It changes the size of the object. I want to
know where that line of crossover is or how to keep clear of it
anyway.

> More than anything
> else, it is this which sets C++ off from other languages.
>
> > > > > > > and only because of constraints of C compatiblity.  The
> > > > > > > data portion of the class isn't the object in general.
> > > > > > I tend to think of the data portion (noun, vs.
> > > > > > behavior=verb) as "the thing" because that's what get's
> > > > > > operated on and maybe even directly manipulated.
> > > > > In C++ (and even in C, for that matter), an object has a
> > > > > type and an address; the type determines its size, and the
> > > > > set of legal operations on it.  Since an object is a thing,
> > > > > in some way, I guess it is a noun, but even a POD struct has
> > > > > behavior: you can assign it, for example, or access members.
> > > > > Compared to C, C++ adds the ability for the user to define
> > > > > additional operations (member functions), and to define
> > > > > non-trivial initialization and destruction (which forces
> > > > > significant changes in the object model).  Beyond that, C++
> > > > > adds support for dynamic typing (which is what one usually
> > > > > understands with OO).
> > > > Not sure what your point is. I said that I consider the data
> > > > portion of an object, "the object".
> > > But that's simply wrong,
> > No it's not.
>
> Yes it is.  Without behavior, all you have is raw memory.  C++
> is a typed language, which means that objects do have behavior.
> (I'm not talking necessarily of behavior in the OO sense here.
> In C as well, object have behavior, and the set of operations on
> an int is not the same as the set of operations on a float.)

Context matters. You may like to design thinking of objects FIRST as
the set of methods that work on the data. I, OTOH, prefer to think in
terms of the data FIRST (and more importantly because that's what's
going to end up somewhere external to the program).

>
> > It's just an abstract way of looking at it. It's hardly a
> > stretch either, since the C++ object model or at least most
> > implementations use that as the foundation upon which to
> > implement polymorphism: tacking a vptr onto "the thing part"
> > (noun) of "the object".
>
> C++ supports dynamic typing, if that's what you mean.  In other
> words, the type of an object vary at runtime.  But I don't see
> your point.  It is the designer of the class who decides whether
> to use dynamic typing or not.  The language doesn't impose it.

It imposes "a penalty" the second you introduce the vptr. The class
becomes fundamentally and categorically different in a major way.
(Read: turns a lightweight class into a heavyweight one).

>
> > > at least in the C++ object model.  An object has a type.
> > > Otherwise, it's just raw memory.  That's a fundamental
> > > principle of any typed language.
> > I could easily go further and say something like "the memory
> > in which the data portion of an object, is the object". While
> > that may bother purists, it is a valid abstract way of
> > thinking about it.
>
> Not in a typed language.  If you want raw memory, C++ even
> supports that.  Any object can be read as an array of unsigned
> char.  Of course, the representation isn't always defined; are
> int's 16, 36, 36, 48 or 64 bits? Are they 2's complement, 1's
> complement or signed magnitude?

I agree that there are other hindrances to having an elegant
programming model. Sigh. That's not to say that one can't get around
them to a large degree. (Not the least of which is: define your
platform as narrowly as possible).

>
> > > > I wasn't trying to be implementation literal about it.
> > > > Yes, data+behavior= class, but when the implementation
> > > > starts adding things to the data portion, that defines a
> > > > different animal than a POD class.
> > > But the implementation *always* adds things to the data
> > > portion, or controls how the data portion is interpreted.
> > >  It defines a sign bit in an int, for example (but not in an
> > > unsigned int).  If you want to support signed arithmetic,
> > > then you need some way of representing the sign.  If you
> > > want to support polymorphism, then you need some way of
> > > representing the type.  I don't see your point.  (The point
> > > of POD, in the standard, is C compatibility; anything in a
> > > POD will be interpretable by a C compiler, and will be
> > > interpreted in the same way as in C++.)
> > Well maybe I'm breaking new ground then in suggesting that
> > there should be a duality in the definition of what a class
> > object is. There are "heavyweight" classes and "lightweight"
> > ones.
>
> There's no strict binary division.  

A class with a vptr is fundamentally different than one without, for
example.

> There are a number of
> different classifications possible

The only ones I'm considering in this thread's topic though is the
lightweight/heavyweight ones.

> ---at the application level,
> the distinction between value objects and entity objects is
> important, for example (but there are often objects which don't
> fit into either category).  In many cases, it certainly makes
> sense to divide types into categories (two or more); in this
> regard, about the only thing particular with "lightweight" and
> "heavyweight" is that the names don't really mean anything.

I've described it at least a dozen times now, so if you don't get it,
then I'm out of ways to describe it.

>
> > I use C++ with that paradigm today, but it could be more
> > effective if there was more support for "object-ness" with
> > "lightweight" classes.
>
> Again: what support do you want?  You've yet to point out
> anything that isn't supported in C++.

(Deriving from interface classes and maintaining the size of the
implementation (derived) class would be nice (but maybe impossible?)).

I am just trying to understand where the line of demarcation is
between lightweight and heavyweight classes is and how that can
potentially change in the future and hence break code.


>
> > The limitation appears to be backward compatibity with C. If
> > so, maybe there should be structs, lightweight classes,
> > heavyweight classes.
>
> And maybe there should be value types and entity types.  Or
> maybe some other classification is relevant to your application.
> The particularity of C++ is that it lets you choose.  The
> designer is free to develop the categories he wants.  (If I'm
> not mistaken, in some circles, these type of categories are
> called stereotypes.)

I'm only talking about the two categories based upon the C++
mechanisms that change the data portion of the object. Deriving a
simple struct from a pure abstract base class will get you a beast
that is the size of the struct plus the size of a vptr. IOW: an
aberrated struct or heavyweight object. Call it what you want, it's
still fundamentally different.

>
> > > > >     [...]
> > > > > > > It restricts the use of OO concepts to classes designed to
> > > > > > > be used with OO concepts.
> > > > > > Not really, since one can have POD classes with methods,
> > > > > > just not CERTAIN methods (you are suggesting that "classes
> > > > > > designed to be used with OO concepts" are those
> > > > > > heavyweight classes that break PODness, right?).
> > > > > No.  I'm really not suggesting much of anything.  However you
> > > > > define the concept of OO, the concept only applies to classes
> > > > > which were designed with it in mind.  C++ doesn't force any
> > > > > particular OO model, but allows you to chose.  And to have
> > > > > classes which aren't conform to this model.
> > > > "Allows you to choose"? "FORCES you to choose" between
> > > > lightweight (POD) class design with more limited OO and and
> > > > heavyweight (non-POD) class design with all OO mechanisms
> > > > allowed but at the expense of losing POD-ness. It's a
> > > > compromise. I'm not saying it's a bad compromise, but I am
> > > > wondering if so and what the alternative implementation
> > > > possibilities are.
> > > Obviously, you have to choose the appropriate semantics for
> > > the class.  That's part of design, and is inevitable.  So I
> > > don't see your point; C++ gives you the choice, without
> > > forcing you into any one particular model.  And there aren't
> > > just two choices.
> > The change occurs when you do something to a POD
> > ("lightweight") class that turns the data portion of the class
> > into something else than just a data struct, as when a vptr is
> > added. Hence then, you have 2 distinct types of class objects
> > that are dictated by the implementation of the C++ object
> > model.
>
> The concept of a POD was introduced mainly for reasons of
> interfacing with C.  Forget it for the moment.  You have as many
> types of class objects as the designer wishes.  If you want just
> a data struct, fine; I use them from time to time (and they
> aren't necessarily POD's---it's not rare for my data struct's to
> contain an std::string).  If you want polymorphism, that's fine
> too.  If you want something in between, say a value type with
> deep copy semantics, no problem.
>
> There is NO restriction in C++ with regards to what you can do.

Yes there is if you don't want the size of your struct to be it's size
plus the size of a vptr. If maintaining that size is what you want,
then you can't have polymophism. Hence, restriction.

>
> > > > > > You seem to be saying that POD classes are not supported
> > > > > > or at least not encouraged.
> > > > > Where do I say that?  POD classes are definitely supported,
> > > > > and are very useful in certain contexts.  They aren't
> > > > > appropriate for what most people would understand by OO, but
> > > > > so what.  Not everything has to be rigorously OO.
> > > > You seemed to imply that the "supported" ("ecouraged" would
> > > > probably be a better word to use) paradigms were: A. data
> > > > structs with non- trivial member functions and built-in
> > > > "behavior" and B. "full OO type objects".
> > > Not at all.  You define what you need.  
> > There are the limitations though: you can't have overloaded
> > constructors, for example, without losing POD-ness.
>
> Obviously, given the particular role of PODs.  So?  What's your
> point?  

My point is that I'm worried about defining some overloaded
constructors and then finding (now or in the future) that my class
object is not "struct-like" anymore (read, has some bizarre
representation in memory).

> There are ony two reasons I know for insisting on
> something being a POD: you need to be able to use it from C as
> well, or you need static initialization.  Both mean that
> construction must be trivial.
>
> > Or conversion operators (?).
>
> A conversion operator doesn't affect POD-ness.  In fact, POD
> structures with a conversion operator are a common idiom for
> certain types of initialization.

Well that's good to know.

>
> > Or derivation from "interfaces" (?).
>
> How is a C program going to deal with derivation?  For that
> matter, an interface supposes virtual functions and dynamic
> typing; it's conceptually impossible to create a dynamically
> typed object without executing some code.

Code generation/execution is not what I'm worried about.

>
> You do have to know what you want.  

I do.

> (And this has nothing to do
> with the "design" of the C++ object model;

It does.

> it's more related to
> simple possibilties.  C++ does try to not impose anything
> impossible to implement.)

Base upon your comments, maybe polymorphism IS the only thing that
changes a class from lightweight to heavyweight. I'm not sure that
that can be relied upon with future implementations or even different
implementations of the language, for I think that the mechanisms are
mostly implementation defined.

>
> > > From a design point of view, I find that it rarely makes
> > > sense to mix models in a single class: either all of the
> > > data will be public, or all of it will be private.  But the
> > > language doesn't require it.
> > > > > > That would be a real downer if true. I'd like to see more
> > > > > > support in the langauge for POD classes.
> > > > > Such as?
> > > > What I call "initializing constructors" for one thing.
> > > > (Constructors that take arguments to initialize a POD class in
> > > > various ways).
> > > Well, if there is a non-trivial constructor, the class can't
> > > be POD, since you need to call the constructor in order to
> > > initialize it.  
> > Well maybe then "POD" is the hangup and I should have used
> > "lightweight" from the beginning. I just want the data portion
> > to remain intact while having the constructor overloads and
> > such.
>
> I'm not sure what you mean by "the data portion to remain
> intact".  

Derive a class and you have compiler baggage attached to the data
portion. If I ever instantiate a class object that has overloaded
constructors and find that the size of the object is different from
the expected size of all the data members (please don't bring up
padding and alignment etc), I'm going to be unhappy.

> Taken literally, the data portion had better remain
> intact for all types of objects.  If you mean contiguous, that's
> a different issue: not even POD's are guaranteed to have
> contiguous data (since C doesn't guarantee it)---on many
> machines (e.g. Sparcs, IBM mainframes...) that would introduce
> totally unacceptable performance costs.

If a platform is so brain-damaged that I can't do things to have a
high degree of confidence that the size of a struct is what I expect
it to be, then I won't be targeting that platform.
Other people can program "the exotics".

>
> If anything, C++ specifies the structure of the data too much.
> A compiler is not allowed to reorder data if there is no
> intervening change of access, for example.  If a programmer
> writes:
>
>     struct S
>     {
>         char c1 ;
>         int  i1 ;
>         char c2 ;
>         int  i2 ;
>     } ;
>
> for example, the compiler is not allowed to place the i1 and i2
> elements in front of c1 and c2, despite the fact that this would
> improve memory use and optimization.

And I think I have control over most of those things on a given
platform. Which is all fine with me, as long as I HAVE that control
(via compiler pragmas or switches or careful coding or whatever).

>
> > Polymorphism I can probably do without, but deriving from
> > interfaces would be nice if possible.
>
> If you have dynamic typing, some code must be executed when the
> object is created; otherwise, there is no way later to know what
> the dynamic type is.

Again, I'm not worried about code generation/execution.

>
> > > Anything else would be a contradiction: are you saying you
> > > want to provide a constructor for a class, but that it won't
> > > be called?  
> > Of course I want it to be called. By "POD-ness" I just meant I
> > want a struct-like consistency of the object data (with no
> > addition such as a vptr, for example).
>
> I don't understand all this business of vptr.  Do you want
> polymorphism, or not.  

Yes, but without the vptr please (coffee without cream please).

> If you want polymorphism, the compiler
> must memorize the type of the object (each object) somewhere,
> when the object is created; C++ doesn't require it to be in the
> object itself, but in practice, this is by far the most
> effective solution.

But what if just pure ABC derived classes were handled differently?
Then maybe the situation would be less bad.

> If you don't want polymorphism, and don't
> declare any virtual functions, then the compiler doesn't have to
> memorize the type of the object, and none that I know of do.
>
>     [...]
>
> > > Because you can't have a constructor in C, basically.
> > >  Because the compiler must generate code when the object is
> > > created, if there is a constructor.  That is what POD-ness
> > > is all about; a POD object doesn't require any code for it
> > > to be constructed.
> > Then apparently I was using "POD" inappropriately. My concern
> > is the in-memory representation of the object data.
>
> Which is implementation defined in C++, just as it was in C.
> With some constraints; the compiler can insert padding (and all
> do in some cases), but it cannot reorder non-static data members
> unless there is an intervening change in access control.

Well there's another example then of heavyweightness: sprinkle in
"public" and "private" in the wrong places and the compiler may
reorder data members. (I had a feeling there was more than the vptr
example).

> That,
> and the fact that a class cannot have a size of 0, are about the
> only restraints.  C (and C++ for PODs) also have a constraint
> that the first data element must be at the start of the object;
> the compiler may not introduce padding before the first element.

So you are saying that a non-POD does not have to have the first data
element at the start of the object. Example number 3 of
heavyweightness. (NOW we're getting somewhere!).
So "losing POD-ness" IS still "bad" and my assumed implication of that
and use of "POD-ness" seems to have been correct.

> If I'm not mistaken, the next version of the standard extends
> this constraint to "standard-layout classes"; i.e. to classes
> that have no virtual functions and no virtual bases, no changes
> in access control, and a few other minor restrictions (but which
> may have non-trivial constructors).  This new rule, however,
> does nothing but describe current practice.

So in the future I will be able to have overloaded constructors (I'm
not sure what exactly a "trivial" constructor is, but I assumed that
an overloaded one is not trivial) and still have lightweight classes,
good. That threat of a compiler not putting data at the front of non-
PODs is a real killer.

>
>     [...]
>
> > > > Maybe defining POD-ness as "C compatibility of structs" is
> > > > a hindrance, if it is defined something like that.
> > > The next version of the standard does have an additional
> > > category "layout compatible".  I'm not sure what it buys us,
> > > however.
> > Where can I read up on that?
>
> In the current draft.  I thinkhttp://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2798.pdf
> should get it (or something close---the draft is still
> evolving).

Downloaded it. Thx for the link.

Tony

On Nov 8, 1:08 pm, tonytech08 <tonytec...@gmail.com> wrote:
> On Nov 8, 3:12 am, James Kanze <james.ka...@gmail.com> wrote:

> > On Nov 7, 10:20 pm, tonytech08 <tonytec...@gmail.com> wrote:

> > > On Nov 7, 4:40 am, James Kanze <james.ka...@gmail.com> wrote:
> > > > On Nov 6, 11:29 pm, tonytech08 <tonytec...@gmail.com> wrote:
> >     [...]
> > > > > Thanks for reiterating my thought: C++ has more support
> > > > > for OO with "full OO type objects".
> > > > More support than what.  
> > > More support for OO with "heavyweight" classes than for
> > > POD classes.

> > You're not making sense.  How does C++ have more support for
> > OO than for other idioms?

> Why are you asking that when I said nothing of the sort? I
> said that once you put a vptr into the data portion of an
> object (for example), it's a different animal than a class
> without a vptr (for example!). I distinguished these
> fundamentally different animals by calling them "heavyweight"
> and "lightweight" classes/object (and apparently wrongly
> POD-classes wrongly).

Well, OO is often used to signify the presence of polymorphism,
and the only time you'll get a vptr is if the class is
polymorphic.

> Moreso, I was concerned that other things one can do with a
> class, such as defining overloaded constructors, may make code
> fragile against some future or other current implementation of
> the language. Who's to say (not me) that someone won't make a
> compiler that tacks on or into a class object some other
> "hidden" ptr or something to implement "overloaded
> constructors"? I don't care if code is generated but I do care
> if the compiler starts aberrating the data portion.

Well, that's C++. And C. And Fortran, and just about every
other language I'm aware of. The only language I know which
specifies the exact format of any types is Java, and it only
does so for the built-in types.

So what's your point. Data layout is implementation defined,
period. That was the case in C, and C++ didn't introduce any
additional restrictions.

> > > > C++ has support for "full OO type objects", if that's what
> > > > you need.  Most of my objects aren't "full OO type objects",
> > > > in the sense that they don't support polymorphism.  C++
> > > > supports them just as well.
> > > I think I may be OK without polymorphism in "lightweight"
> > > classes, but overloaded constructors sure would be nice. And
> > > conversion operators.  Can a POD class derive from a pure
> > > abstract base class? That would be nice also if not.

> > And C++ supports all of that.  
>
> But am I guaranteed that my a class will stay lightweight if I
> do that or is it implementation defined?

Everything is implementation defined. C++ inherits this from C,
and it was pretty much standard practice at the time C was
invented.

[...]
> > > It's just an abstract way of looking at it. It's hardly a
> > > stretch either, since the C++ object model or at least
> > > most implementations use that as the foundation upon which
> > > to implement polymorphism: tacking a vptr onto "the thing
> > > part" (noun) of "the object".

> > C++ supports dynamic typing, if that's what you mean.  In
> > other words, the type of an object vary at runtime.  But I
> > don't see your point.  It is the designer of the class who
> > decides whether to use dynamic typing or not.  The language
> > doesn't impose it.

> It imposes "a penalty" the second you introduce the vptr. The
> class becomes fundamentally and categorically different in a
> major way. (Read: turns a lightweight class into a
> heavyweight one).

You're throwing around meaningless adjectives again. The
compiler has to implement dynamic typing somehow. You don't pay
for it unless you use it, and using a vptr is about the cheapest
implementation known.

[...]
> I agree that there are other hindrances to having an elegant
> programming model. Sigh. That's not to say that one can't get
> around them to a large degree. (Not the least of which is:
> define your platform as narrowly as possible).

That's a route C and C++ intentionally don't take. If there
exists a platform on which the language is not implementable,
it's pretty much considered a defect in the language.

> > > > > I wasn't trying to be implementation literal about it.
> > > > > Yes, data+behavior= class, but when the implementation
> > > > > starts adding things to the data portion, that defines a
> > > > > different animal than a POD class.
> > > > But the implementation *always* adds things to the data
> > > > portion, or controls how the data portion is interpreted.
> > > >  It defines a sign bit in an int, for example (but not in an
> > > > unsigned int).  If you want to support signed arithmetic,
> > > > then you need some way of representing the sign.  If you
> > > > want to support polymorphism, then you need some way of
> > > > representing the type.  I don't see your point.  (The point
> > > > of POD, in the standard, is C compatibility; anything in a
> > > > POD will be interpretable by a C compiler, and will be
> > > > interpreted in the same way as in C++.)
> > > Well maybe I'm breaking new ground then in suggesting that
> > > there should be a duality in the definition of what a class
> > > object is. There are "heavyweight" classes and "lightweight"
> > > ones.

> > There's no strict binary division.  

> A class with a vptr is fundamentally different than one
> without, for example.

And a class with private data members is fundamentally different
from one with public data members. And a class with user
defined constructors is fundamentally different from one
without.

> > There are a number of different classifications possible

> The only ones I'm considering in this thread's topic though is
> the lightweight/heavyweight ones.

Without defining it or showing its relevance to anything at all.

> > > I use C++ with that paradigm today, but it could be more
> > > effective if there was more support for "object-ness" with
> > > "lightweight" classes.

> > Again: what support do you want?  You've yet to point out
> > anything that isn't supported in C++.

> (Deriving from interface classes and maintaining the size of
> the implementation (derived) class would be nice (but maybe
> impossible?)).

Not necessarily impossible, but it would make the cost of
resolving a virtual function call significantly higher. And
what does it buy you? You say it would be "nice", but you don't
explain why; I don't see any real advantage.

> I am just trying to understand where the line of demarcation
> is between lightweight and heavyweight classes is and how that
> can potentially change in the future and hence break code.

There is no line of demarcation because there isn't really such
a distinction. It's whatever you want it to mean, which puts it
where ever you want.

> > > The limitation appears to be backward compatibity with C.
> > > If so, maybe there should be structs, lightweight classes,
> > > heavyweight classes.

> > And maybe there should be value types and entity types.  Or
> > maybe some other classification is relevant to your
> > application. The particularity of C++ is that it lets you
> > choose.  The designer is free to develop the categories he
> > wants.  (If I'm not mistaken, in some circles, these type of
> > categories are called stereotypes.)

> I'm only talking about the two categories based upon the C++
> mechanisms that change the data portion of the object.

Which in turn depends on the implementation, just as it did in
C.

Why do you care about the layout of the data anyway? There's
nothing you can really do with it.

> Deriving a simple struct from a pure abstract base class will
> get you a beast that is the size of the struct plus the size
> of a vptr. IOW: an aberrated struct or heavyweight object.
> Call it what you want, it's still fundamentally different.

A C style struct is different from a polymorphic class, yes.
Otherwise, there wouldn't be any point in having polymorphic
classes. The difference isn't any more fundamental than making
the data members private would be, however, or providing a user
defined constructor; in fact, I'd say that both of those were
even more fundamental differences.

> > > > > >     [...]
> > > The change occurs when you do something to a POD
> > > ("lightweight") class that turns the data portion of the
> > > class into something else than just a data struct, as when
> > > a vptr is added. Hence then, you have 2 distinct types of
> > > class objects that are dictated by the implementation of
> > > the C++ object model.

> > The concept of a POD was introduced mainly for reasons of
> > interfacing with C.  Forget it for the moment.  You have as
> > many types of class objects as the designer wishes.  If you
> > want just a data struct, fine; I use them from time to time
> > (and they aren't necessarily POD's---it's not rare for my
> > data struct's to contain an std::string).  If you want
> > polymorphism, that's fine too.  If you want something in
> > between, say a value type with deep copy semantics, no
> > problem.

> > There is NO restriction in C++ with regards to what you can
> > do.

> Yes there is if you don't want the size of your struct to be
> it's size plus the size of a vptr. If maintaining that size is
> what you want, then you can't have polymophism. Hence,
> restriction.

What on earth are you talking about. C++ doesn't guarantee the
size of anything. (Nor does any other language.) If you need
added behavior which requires additional memory, then you need
added behavior which requires additional memory. That's not C++
talking; that's just physical reality.

> > > > > > > You seem to be saying that POD classes are not supported
> > > > > > > or at least not encouraged.
> > > > > > Where do I say that?  POD classes are definitely supported,
> > > > > > and are very useful in certain contexts.  They aren't
> > > > > > appropriate for what most people would understand by OO, but
> > > > > > so what.  Not everything has to be rigorously OO.
> > > > > You seemed to imply that the "supported" ("ecouraged" would
> > > > > probably be a better word to use) paradigms were: A. data
> > > > > structs with non- trivial member functions and built-in
> > > > > "behavior" and B. "full OO type objects".
> > > > Not at all.  You define what you need.  
> > > There are the limitations though: you can't have overloaded
> > > constructors, for example, without losing POD-ness.

> > Obviously, given the particular role of PODs.  So?  What's your
> > point?  

> My point is that I'm worried about defining some overloaded
> constructors and then finding (now or in the future) that my
> class object is not "struct-like" anymore (read, has some
> bizarre representation in memory).

I'm not sure what you mean by "struct-like" or "some bizarre
representation in memory". The representation is whatever the
implementation decides it to be. Both in C and in C++. I've
had the representation of a long change when upgrading a C
compiler. On the platforms I generally work on, the
representation of a pointer depends on compiler options. And on
*ALL* of the platforms I'm familiar with, the layout of a struct
depends on compiler options, both in C and in C++.

The whole point of using a high level language, like C++ (or C,
or even Fortran) is that you're isolated from this
representation.

> > > Or derivation from "interfaces" (?).

> > How is a C program going to deal with derivation?  For that
> > matter, an interface supposes virtual functions and dynamic
> > typing; it's conceptually impossible to create a dynamically
> > typed object without executing some code.

> Code generation/execution is not what I'm worried about.

There's not much point in defining something that can't be
implemented.

[...]
> > I'm not sure what you mean by "the data portion to remain
> > intact".  

> Derive a class and you have compiler baggage attached to the
> data portion.

Or you don't. Even without derivation, you've got "compiler
baggage" attached to the data portion. Both in C and in C++.

> If I ever instantiate a class object that has overloaded
> constructors and find that the size of the object is different
> from the expected size of all the data members (please don't
> bring up padding and alignment etc), I'm going to be unhappy.

Be unhappy. First, there is no "expected" size. The size of an
object varies from implementation to implementation, and depends
on compiler version and options within an implementation. And
second, I've yet to find anything to be gained by changing this.

> > Taken literally, the data portion had better remain intact
> > for all types of objects.  If you mean contiguous, that's a
> > different issue: not even POD's are guaranteed to have
> > contiguous data (since C doesn't guarantee it)---on many
> > machines (e.g. Sparcs, IBM mainframes...) that would
> > introduce totally unacceptable performance costs.

> If a platform is so brain-damaged that I can't do things to
> have a high degree of confidence that the size of a struct is
> what I expect it to be, then I won't be targeting that
> platform. Other people can program "the exotics".

So what do you expect it to be? You can't expect anything,
reasonable.

> > If anything, C++ specifies the structure of the data too
> > much. A compiler is not allowed to reorder data if there is
> > no intervening change of access, for example.  If a
> > programmer writes:

> >     struct S
> >     {
> >         char c1 ;
> >         int  i1 ;
> >         char c2 ;
> >         int  i2 ;
> >     } ;

> > for example, the compiler is not allowed to place the i1 and
> > i2 elements in front of c1 and c2, despite the fact that
> > this would improve memory use and optimization.

> And I think I have control over most of those things on a
> given platform. Which is all fine with me, as long as I HAVE
> that control (via compiler pragmas or switches or careful
> coding or whatever).

And compilers have alway been free (and always will be free) to
provide such controls. I've never found the slightest use for
them, but they're there. The standard intentionally doesn't
specify how to invoke the compiler, or what pragmas are
available, to achieve this, since there's nothing you can really
say which would make sense for all possible platforms.

> > > > Anything else would be a contradiction: are you saying you
> > > > want to provide a constructor for a class, but that it won't
> > > > be called?  
> > > Of course I want it to be called. By "POD-ness" I just meant I
> > > want a struct-like consistency of the object data (with no
> > > addition such as a vptr, for example).

> > I don't understand all this business of vptr.  Do you want
> > polymorphism, or not.  

> Yes, but without the vptr please (coffee without cream
> please).

You mean café au lait without any milk. If you have
polymorphism, it has to be implemented.

> > If you want polymorphism, the compiler must memorize the
> > type of the object (each object) somewhere, when the object
> > is created; C++ doesn't require it to be in the object
> > itself, but in practice, this is by far the most effective
> > solution.

> But what if just pure ABC derived classes were handled
> differently? Then maybe the situation would be less bad.

Propose a solution. If you want polymorphism, the compiler must
maintain information about the dynamic type somehow. Whether
the base class is abstract or not doesn't change anything; an
object must somehow contain additional information. Additional
information means additional bits, which have to be stored
somewhere. The only alternative to storing them in the object
itself is somehow being able to recover them from the address of
the object. A solution which would seem off hand considerably
more expensive in terms of run-time. For practically no
advantage in return.

> >     [...]
> Well there's another example then of heavyweightness: sprinkle
> in "public" and "private" in the wrong places and the compiler
> may reorder data members. (I had a feeling there was more than
> the vptr example).

Yes, because C compatibility is no longer involved. It is
fairly clear that C actually went too far in this regard, and
imposed an unnecessary constraint which had negative effects for
optimization. Since most people would prefer faster programs
with smaller data, C++ did what it could to loosen this
constraint.

Can you give an example of reasonable code where this makes a
difference?

> > That, and the fact that a class cannot have a size of 0, are
> >about the only restraints.  C (and C++ for PODs) also have a
> >constraint that the first data element must be at the start
> >of the object; the compiler may not introduce padding before
> >the first element.

> So you are saying that a non-POD does not have to have the
> first data element at the start of the object.

Obviously. Where do you think that most compilers put the vptr?

> Example number 3 of heavyweightness. (NOW we're getting
> somewhere!). So "losing POD-ness" IS still "bad" and my
> assumed implication of that and use of "POD-ness" seems to
> have been correct.

OK. I'll give you example number 4: the size of a long
typically depends on compiler options (and the default varies),
so putting a long in a class makes it heavyweight. And is "bad"
according to your definitions.

I think you're being emminately silly.

> > If I'm not mistaken, the next version of the standard
> > extends this constraint to "standard-layout classes"; i.e.
> > to classes that have no virtual functions and no virtual
> > bases, no changes in access control, and a few other minor
> > restrictions (but which may have non-trivial constructors).
> >  This new rule, however, does nothing but describe current
> > practice.

> So in the future I will be able to have overloaded
> constructors (I'm not sure what exactly a "trivial"
> constructor is, but I assumed that an overloaded one is not
> trivial) and still have lightweight classes, good. That threat
> of a compiler not putting data at the front of non-PODs is a
> real killer.

I have no idea. I've not bothered really studying this in the
standard, because I don't see any real use for it. (I suspect,
in fact, that it is being introduced more as a means of wording
other requirements more clearly than for any direct advantages
that it may have.)

--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientierter Datenverarbeitung
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34

In article <75591f1c-a0f6-467b-bb7c-
49ea037a2efb@d36g2000prf.googlegroups.com>, james.kanze@gmail.com
says...

[ ... ]

> It would also be very difficult to make it work with static and
> local objects; you'd probably need a fall-back solution for
> those.

Locals would more or less require a fall-back solution. I don't see a
problem with statics. Another difficult one would be obects that are
inside of a struct/class, which requires that they be allocated in the
same order as they're declared. You could still do it, but space wasted
on padding could render it horribly impactical under the wrong
circumstances.

--
Later,
Jerry.

The universe is a figment of its own imagination.

"James Kanze" <james.kanze@gmail.com> ha scritto nel messaggio
news:01e16d1a-8911-450f-88df-

>Why do you care about the layout of the data anyway? There's
>nothing you can really do with it.

if i know how data is written i know how modify it
the same all other languages

if i have
32bits:8bits:64bits
i can write well on each of that

On 2008-11-10 12:40, rio wrote:
> "James Kanze" <james.kanze@gmail.com> ha scritto nel messaggio
> news:01e16d1a-8911-450f-88df-
>
>>Why do you care about the layout of the data anyway? There's
>>nothing you can really do with it.
>
> if i know how data is written i know how modify it
> the same all other languages
>
> if i have
> 32bits:8bits:64bits
> i can write well on each of that

But since neither C, C++, Fortran, or just about any other language
guarantees a particular layout of a struct (or equivalent) you would
still need to use compiler specific methods to ensure that layout, both
in C, C++, Fortran, and just about any other language.

If you want full control of the placement of the bits (without using
platform dependent means) you should allocate a char array and perform
manipulations on that:

unsigned char data[13];

unsigned int* i = &data[0];
unsigned char* c = &data[4];
unsigned long* l = &data[5];

Assuming int, char, and long are 32, 8, and 64 bits respectively, and
that your platform does not require aligned accesses (in which case you
need additional work).

--
Erik Wikström